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postgres/doc/src/sgml/config.sgml
Tom Lane 4cbe3abb31 Fix xreflabel for hot_standby_feedback. 
Rather remarkable that this has been wrong since 9.1 and nobody noticed.
2014-07-19 22:20:29 -04:00

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<!-- doc/src/sgml/config.sgml -->
<chapter id="runtime-config">
<title>Server Configuration</title>
<indexterm>
<primary>configuration</primary>
<secondary>of the server</secondary>
</indexterm>
<para>
There are many configuration parameters that affect the behavior of
the database system. In the first section of this chapter, we
describe how to set configuration parameters. The subsequent sections
discuss each parameter in detail.
</para>
<sect1 id="config-setting">
<title>Setting Parameters</title>
<sect2 id="config-setting-names-values">
<title>Parameter Names and Values</title>
<para>
All parameter names are case-insensitive. Every parameter takes a
value of one of five types: Boolean, integer, floating point,
string or enum. Boolean values can be written as <literal>on</literal>,
<literal>off</literal>, <literal>true</literal>,
<literal>false</literal>, <literal>yes</literal>,
<literal>no</literal>, <literal>1</literal>, <literal>0</literal>
(all case-insensitive) or any unambiguous prefix of these.
</para>
<para>
Some settings specify a memory or time value. Each of these has an
implicit unit, which is either kilobytes, blocks (typically eight
kilobytes), milliseconds, seconds, or minutes. Default units can be
found by referencing <structname>pg_settings</>.<structfield>unit</>.
For convenience,
a different unit can also be specified explicitly. Valid memory units
are <literal>kB</literal> (kilobytes), <literal>MB</literal>
(megabytes), <literal>GB</literal> (gigabytes), and <literal>TB</literal> (terabytes); valid time units
are <literal>ms</literal> (milliseconds), <literal>s</literal>
(seconds), <literal>min</literal> (minutes), <literal>h</literal>
(hours), and <literal>d</literal> (days). Note that the multiplier
for memory units is 1024, not 1000.
</para>
<para>
Parameters of type <quote>enum</> are specified in the same way as string
parameters, but are restricted to a limited set of values. The allowed
values can be found
from <structname>pg_settings</>.<structfield>enumvals</>.
Enum parameter values are case-insensitive.
</para>
</sect2>
<sect2 id="config-setting-configuration-file">
<title>Setting Parameters via the Configuration File</title>
<para>
One way to set these parameters is to edit the file
<filename>postgresql.conf</><indexterm><primary>postgresql.conf</></>,
which is normally kept in the data directory. (A default copy is
installed there when the database cluster directory is
initialized.) An example of what this file might look like is:
<programlisting>
# This is a comment
log_connections = yes
log_destination = 'syslog'
search_path = '"$user", public'
shared_buffers = 128MB
</programlisting>
One parameter is specified per line. The equal sign between name and
value is optional. Whitespace is insignificant and blank lines are
ignored. Hash marks (<literal>#</literal>) designate the remainder of the
line as a comment. Parameter values that are not simple identifiers or
numbers must be single-quoted. To embed a single quote in a parameter
value, write either two quotes (preferred) or backslash-quote.
</para>
<para>
<indexterm>
<primary>SIGHUP</primary>
</indexterm>
The configuration file is reread whenever the main server process
receives a <systemitem>SIGHUP</> signal; this is most easily done by
running <literal>pg_ctl reload</> from the command-line or by calling
the SQL function <function>pg_reload_conf()</function>. The main
server process
also propagates this signal to all currently running server
processes so that existing sessions also get the new
value. Alternatively, you can send the signal to a single server
process directly. Some parameters can only be set at server start;
any changes to their entries in the configuration file will be ignored
until the server is restarted. Invalid parameter settings in the
configuration file are likewise ignored (but logged) during
<systemitem>SIGHUP</> processing.
</para>
</sect2>
<sect2 id="config-setting-other-methods">
<title>Other Ways to Set Parameters</title>
<para>
A second way to set these configuration parameters is to give them
as a command-line option to the <command>postgres</command> command,
such as:
<programlisting>
postgres -c log_connections=yes -c log_destination='syslog'
</programlisting>
Command-line options override any conflicting settings in
<filename>postgresql.conf</filename>. Note that this means you won't
be able to change the value on-the-fly by editing
<filename>postgresql.conf</filename>, so while the command-line
method might be convenient, it can cost you flexibility later.
</para>
<para>
Occasionally it is useful to give a command line option to
one particular session only. The environment variable
<envar>PGOPTIONS</envar> can be used for this purpose on the
client side:
<programlisting>
env PGOPTIONS='-c geqo=off' psql
</programlisting>
(This works for any <application>libpq</>-based client application, not
just <application>psql</application>.) Note that this won't work for
parameters that are fixed when the server is started or that must be
specified in <filename>postgresql.conf</filename>.
</para>
<para>
Furthermore, it is possible to assign a set of parameter settings to
a user or a database. Whenever a session is started, the default
settings for the user and database involved are loaded. The
commands <xref linkend="sql-alterrole">
and <xref linkend="sql-alterdatabase">,
respectively, are used to configure these settings. Per-database
settings override anything received from the
<command>postgres</command> command-line or the configuration
file, and in turn are overridden by per-user settings; both are
overridden by per-session settings.
</para>
<para>
Some parameters can be changed in individual <acronym>SQL</acronym>
sessions with the <xref linkend="SQL-SET">
command, for example:
<screen>
SET ENABLE_SEQSCAN TO OFF;
</screen>
If <command>SET</> is allowed, it overrides all other sources of
values for the parameter. Some parameters cannot be changed via
<command>SET</command>: for example, if they control behavior that
cannot be changed without restarting the entire
<productname>PostgreSQL</productname> server. Also, some parameters
require superuser permission to change via <command>SET</command> or
<command>ALTER</>.
</para>
<para>
Another way to change configuration parameters persistently is by
use of <xref linkend="SQL-ALTERSYSTEM">
command, for example:
<screen>
ALTER SYSTEM SET checkpoint_timeout TO 600;
</screen>
This command will allow users to change values persistently
through SQL command. The values will be effective after reload of server configuration
(<acronym>SIGHUP</>) or server startup. The effect of this command is similar to when
user manually changes values in <filename>postgresql.conf</filename>.
</para>
</sect2>
<sect2 id="config-setting-examining">
<title>Examining Parameter Settings</title>
<para>
The <xref linkend="SQL-SHOW">
command allows inspection of the current values of all parameters.
</para>
<para>
The virtual table <structname>pg_settings</structname> also allows
displaying and updating session run-time parameters; see <xref
linkend="view-pg-settings"> for details and a description of the
different variable types and when they can be changed.
<structname>pg_settings</structname> is equivalent to <command>SHOW</>
and <command>SET</>, but can be more convenient
to use because it can be joined with other tables, or selected from using
any desired selection condition. It also contains more information about
each parameter than is available from <command>SHOW</>.
</para>
</sect2>
<sect2 id="config-includes">
<title>Configuration File Includes</title>
<para>
<indexterm>
<primary><literal>include</></primary>
<secondary>in configuration file</secondary>
</indexterm>
In addition to parameter settings, the <filename>postgresql.conf</>
file can contain <firstterm>include directives</>, which specify
another file to read and process as if it were inserted into the
configuration file at this point. This feature allows a configuration
file to be divided into physically separate parts.
Include directives simply look like:
<programlisting>
include 'filename'
</programlisting>
If the file name is not an absolute path, it is taken as relative to
the directory containing the referencing configuration file.
Inclusions can be nested.
</para>
<para>
<indexterm>
<primary><literal>include_if_exists</></primary>
<secondary>in configuration file</secondary>
</indexterm>
There is also an <literal>include_if_exists</> directive, which acts
the same as the <literal>include</> directive, except for the behavior
when the referenced file does not exist or cannot be read. A regular
<literal>include</> will consider this an error condition, but
<literal>include_if_exists</> merely logs a message and continues
processing the referencing configuration file.
</para>
<para>
<indexterm>
<primary><literal>include_dir</></primary>
<secondary>in configuration file</secondary>
</indexterm>
The <filename>postgresql.conf</> file can also contain
<literal>include_dir</literal> directives, which specify an entire directory
of configuration files to include. It is used similarly:
<programlisting>
include_dir 'directory'
</programlisting>
Non-absolute directory names follow the same rules as single file include
directives: they are relative to the directory containing the referencing
configuration file. Within that directory, only non-directory files whose
names end with the suffix <literal>.conf</literal> will be included. File
names that start with the <literal>.</literal> character are also excluded,
to prevent mistakes as they are hidden on some platforms. Multiple files
within an include directory are processed in file name order. The file names
are ordered by C locale rules, i.e. numbers before letters, and uppercase
letters before lowercase ones.
</para>
<para>
Include files or directories can be used to logically separate portions
of the database configuration, rather than having a single large
<filename>postgresql.conf</> file. Consider a company that has two
database servers, each with a different amount of memory. There are likely
elements of the configuration both will share, for things such as logging.
But memory-related parameters on the server will vary between the two. And
there might be server specific customizations, too. One way to manage this
situation is to break the custom configuration changes for your site into
three files. You could add this to the end of your
<filename>postgresql.conf</> file to include them:
<programlisting>
include 'shared.conf'
include 'memory.conf'
include 'server.conf'
</programlisting>
All systems would have the same <filename>shared.conf</>. Each server
with a particular amount of memory could share the same
<filename>memory.conf</>; you might have one for all servers with 8GB of RAM,
another for those having 16GB. And finally <filename>server.conf</> could
have truly server-specific configuration information in it.
</para>
<para>
Another possibility is to create a configuration file directory and
put this information into files there. For example, a <filename>conf.d</>
directory could be referenced at the end of<filename>postgresql.conf</>:
<programlisting>
include_dir 'conf.d'
</programlisting>
Then you could name the files in the <filename>conf.d</> directory like this:
<programlisting>
00shared.conf
01memory.conf
02server.conf
</programlisting>
This shows a clear order in which these files will be loaded. This is
important because only the last setting encountered when the server is
reading its configuration will be used. Something set in
<filename>conf.d/02server.conf</> in this example would override a value
set in <filename>conf.d/01memory.conf</>.
</para>
<para>
You might instead use this configuration directory approach while naming
these files more descriptively:
<programlisting>
00shared.conf
01memory-8GB.conf
02server-foo.conf
</programlisting>
This sort of arrangement gives a unique name for each configuration file
variation. This can help eliminate ambiguity when several servers have
their configurations all stored in one place, such as in a version
control repository. (Storing database configuration files under version
control is another good practice to consider).
</para>
</sect2>
</sect1>
<sect1 id="runtime-config-file-locations">
<title>File Locations</title>
<para>
In addition to the <filename>postgresql.conf</filename> file
already mentioned, <productname>PostgreSQL</productname> uses
two other manually-edited configuration files, which control
client authentication (their use is discussed in <xref
linkend="client-authentication">). By default, all three
configuration files are stored in the database cluster's data
directory. The parameters described in this section allow the
configuration files to be placed elsewhere. (Doing so can ease
administration. In particular it is often easier to ensure that
the configuration files are properly backed-up when they are
kept separate.)
</para>
<variablelist>
<varlistentry id="guc-data-directory" xreflabel="data_directory">
<term><varname>data_directory</varname> (<type>string</type>)
<indexterm>
<primary><varname>data_directory</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the directory to use for data storage.
This parameter can only be set at server start.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-config-file" xreflabel="config_file">
<term><varname>config_file</varname> (<type>string</type>)
<indexterm>
<primary><varname>config_file</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the main server configuration file
(customarily called <filename>postgresql.conf</>).
This parameter can only be set on the <command>postgres</command> command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-hba-file" xreflabel="hba_file">
<term><varname>hba_file</varname> (<type>string</type>)
<indexterm>
<primary><varname>hba_file</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the configuration file for host-based authentication
(customarily called <filename>pg_hba.conf</>).
This parameter can only be set at server start.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-ident-file" xreflabel="ident_file">
<term><varname>ident_file</varname> (<type>string</type>)
<indexterm>
<primary><varname>ident_file</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the configuration file for
<xref linkend="auth-username-maps"> user name mapping
(customarily called <filename>pg_ident.conf</>).
This parameter can only be set at server start.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-external-pid-file" xreflabel="external_pid_file">
<term><varname>external_pid_file</varname> (<type>string</type>)
<indexterm>
<primary><varname>external_pid_file</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the name of an additional process-ID (PID) file that the
server should create for use by server administration programs.
This parameter can only be set at server start.
</para>
</listitem>
</varlistentry>
</variablelist>
<para>
In a default installation, none of the above parameters are set
explicitly. Instead, the
data directory is specified by the <option>-D</option> command-line
option or the <envar>PGDATA</envar> environment variable, and the
configuration files are all found within the data directory.
</para>
<para>
If you wish to keep the configuration files elsewhere than the
data directory, the <command>postgres</command> <option>-D</option>
command-line option or <envar>PGDATA</envar> environment variable
must point to the directory containing the configuration files,
and the <varname>data_directory</> parameter must be set in
<filename>postgresql.conf</filename> (or on the command line) to show
where the data directory is actually located. Notice that
<varname>data_directory</> overrides <option>-D</option> and
<envar>PGDATA</envar> for the location
of the data directory, but not for the location of the configuration
files.
</para>
<para>
If you wish, you can specify the configuration file names and locations
individually using the parameters <varname>config_file</>,
<varname>hba_file</> and/or <varname>ident_file</>.
<varname>config_file</> can only be specified on the
<command>postgres</command> command line, but the others can be
set within the main configuration file. If all three parameters plus
<varname>data_directory</> are explicitly set, then it is not necessary
to specify <option>-D</option> or <envar>PGDATA</envar>.
</para>
<para>
When setting any of these parameters, a relative path will be interpreted
with respect to the directory in which <command>postgres</command>
is started.
</para>
</sect1>
<sect1 id="runtime-config-connection">
<title>Connections and Authentication</title>
<sect2 id="runtime-config-connection-settings">
<title>Connection Settings</title>
<variablelist>
<varlistentry id="guc-listen-addresses" xreflabel="listen_addresses">
<term><varname>listen_addresses</varname> (<type>string</type>)
<indexterm>
<primary><varname>listen_addresses</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the TCP/IP address(es) on which the server is
to listen for connections from client applications.
The value takes the form of a comma-separated list of host names
and/or numeric IP addresses. The special entry <literal>*</>
corresponds to all available IP interfaces. The entry
<literal>0.0.0.0</> allows listening for all IPv4 addresses and
<literal>::</> allows listening for all IPv6 addresses.
If the list is empty, the server does not listen on any IP interface
at all, in which case only Unix-domain sockets can be used to connect
to it.
The default value is <systemitem class="systemname">localhost</>,
which allows only local TCP/IP <quote>loopback</> connections to be
made. While client authentication (<xref
linkend="client-authentication">) allows fine-grained control
over who can access the server, <varname>listen_addresses</varname>
controls which interfaces accept connection attempts, which
can help prevent repeated malicious connection requests on
insecure network interfaces. This parameter can only be set
at server start.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-port" xreflabel="port">
<term><varname>port</varname> (<type>integer</type>)
<indexterm>
<primary><varname>port</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
The TCP port the server listens on; 5432 by default. Note that the
same port number is used for all IP addresses the server listens on.
This parameter can only be set at server start.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-max-connections" xreflabel="max_connections">
<term><varname>max_connections</varname> (<type>integer</type>)
<indexterm>
<primary><varname>max_connections</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Determines the maximum number of concurrent connections to the
database server. The default is typically 100 connections, but
might be less if your kernel settings will not support it (as
determined during <application>initdb</>). This parameter can
only be set at server start.
</para>
<para>
When running a standby server, you must set this parameter to the
same or higher value than on the master server. Otherwise, queries
will not be allowed in the standby server.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-superuser-reserved-connections"
xreflabel="superuser_reserved_connections">
<term><varname>superuser_reserved_connections</varname>
(<type>integer</type>)
<indexterm>
<primary><varname>superuser_reserved_connections</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Determines the number of connection <quote>slots</quote> that
are reserved for connections by <productname>PostgreSQL</>
superusers. At most <xref linkend="guc-max-connections">
connections can ever be active simultaneously. Whenever the
number of active concurrent connections is at least
<varname>max_connections</> minus
<varname>superuser_reserved_connections</varname>, new
connections will be accepted only for superusers, and no
new replication connections will be accepted.
</para>
<para>
The default value is three connections. The value must be less
than the value of <varname>max_connections</varname>. This
parameter can only be set at server start.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-unix-socket-directories" xreflabel="unix_socket_directories">
<term><varname>unix_socket_directories</varname> (<type>string</type>)
<indexterm>
<primary><varname>unix_socket_directories</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the directory of the Unix-domain socket(s) on which the
server is to listen for connections from client applications.
Multiple sockets can be created by listing multiple directories
separated by commas. Whitespace between entries is
ignored; surround a directory name with double quotes if you need
to include whitespace or commas in the name.
An empty value
specifies not listening on any Unix-domain sockets, in which case
only TCP/IP sockets can be used to connect to the server.
The default value is normally
<filename>/tmp</filename>, but that can be changed at build time.
This parameter can only be set at server start.
</para>
<para>
In addition to the socket file itself, which is named
<literal>.s.PGSQL.<replaceable>nnnn</></literal> where
<replaceable>nnnn</> is the server's port number, an ordinary file
named <literal>.s.PGSQL.<replaceable>nnnn</>.lock</literal> will be
created in each of the <varname>unix_socket_directories</> directories.
Neither file should ever be removed manually.
</para>
<para>
This parameter is irrelevant on Windows, which does not have
Unix-domain sockets.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-unix-socket-group" xreflabel="unix_socket_group">
<term><varname>unix_socket_group</varname> (<type>string</type>)
<indexterm>
<primary><varname>unix_socket_group</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the owning group of the Unix-domain socket(s). (The owning
user of the sockets is always the user that starts the
server.) In combination with the parameter
<varname>unix_socket_permissions</varname> this can be used as
an additional access control mechanism for Unix-domain connections.
By default this is the empty string, which uses the default
group of the server user. This parameter can only be set at
server start.
</para>
<para>
This parameter is irrelevant on Windows, which does not have
Unix-domain sockets.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-unix-socket-permissions" xreflabel="unix_socket_permissions">
<term><varname>unix_socket_permissions</varname> (<type>integer</type>)
<indexterm>
<primary><varname>unix_socket_permissions</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the access permissions of the Unix-domain socket(s). Unix-domain
sockets use the usual Unix file system permission set.
The parameter value is expected to be a numeric mode
specified in the format accepted by the
<function>chmod</function> and <function>umask</function>
system calls. (To use the customary octal format the number
must start with a <literal>0</literal> (zero).)
</para>
<para>
The default permissions are <literal>0777</literal>, meaning
anyone can connect. Reasonable alternatives are
<literal>0770</literal> (only user and group, see also
<varname>unix_socket_group</varname>) and <literal>0700</literal>
(only user). (Note that for a Unix-domain socket, only write
permission matters, so there is no point in setting or revoking
read or execute permissions.)
</para>
<para>
This access control mechanism is independent of the one
described in <xref linkend="client-authentication">.
</para>
<para>
This parameter can only be set at server start.
</para>
<para>
This parameter is irrelevant on systems, notably Solaris as of Solaris
10, that ignore socket permissions entirely. There, one can achieve a
similar effect by pointing <varname>unix_socket_directories</> to a
directory having search permission limited to the desired audience.
This parameter is also irrelevant on Windows, which does not have
Unix-domain sockets.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-bonjour" xreflabel="bonjour">
<term><varname>bonjour</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>bonjour</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Enables advertising the server's existence via
<productname>Bonjour</productname>. The default is off.
This parameter can only be set at server start.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-bonjour-name" xreflabel="bonjour_name">
<term><varname>bonjour_name</varname> (<type>string</type>)
<indexterm>
<primary><varname>bonjour_name</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the <productname>Bonjour</productname> service
name. The computer name is used if this parameter is set to the
empty string <literal>''</> (which is the default). This parameter is
ignored if the server was not compiled with
<productname>Bonjour</productname> support.
This parameter can only be set at server start.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-tcp-keepalives-idle" xreflabel="tcp_keepalives_idle">
<term><varname>tcp_keepalives_idle</varname> (<type>integer</type>)
<indexterm>
<primary><varname>tcp_keepalives_idle</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the number of seconds of inactivity after which TCP
should send a keepalive message to the client. A value of 0 uses
the system default.
This parameter is supported only on systems that support the
<symbol>TCP_KEEPIDLE</> or <symbol>TCP_KEEPALIVE</> symbols, and on
Windows; on other systems, it must be zero.
In sessions connected via a Unix-domain socket, this parameter is
ignored and always reads as zero.
</para>
<note>
<para>
On Windows, a value of 0 will set this parameter to 2 hours,
since Windows does not provide a way to read the system default value.
</para>
</note>
</listitem>
</varlistentry>
<varlistentry id="guc-tcp-keepalives-interval" xreflabel="tcp_keepalives_interval">
<term><varname>tcp_keepalives_interval</varname> (<type>integer</type>)
<indexterm>
<primary><varname>tcp_keepalives_interval</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the number of seconds after which a TCP keepalive message
that is not acknowledged by the client should be retransmitted.
A value of 0 uses the system default.
This parameter is supported only on systems that support the
<symbol>TCP_KEEPINTVL</> symbol, and on Windows; on other systems, it
must be zero.
In sessions connected via a Unix-domain socket, this parameter is
ignored and always reads as zero.
</para>
<note>
<para>
On Windows, a value of 0 will set this parameter to 1 second,
since Windows does not provide a way to read the system default value.
</para>
</note>
</listitem>
</varlistentry>
<varlistentry id="guc-tcp-keepalives-count" xreflabel="tcp_keepalives_count">
<term><varname>tcp_keepalives_count</varname> (<type>integer</type>)
<indexterm>
<primary><varname>tcp_keepalives_count</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the number of TCP keepalives that can be lost before
the server's connection to the client is considered dead. A value of 0
uses the system default. This parameter is
supported only on systems that support the <symbol>TCP_KEEPCNT</>
symbol; on other systems, it must be zero.
In sessions connected via a Unix-domain socket, this parameter is
ignored and always reads as zero.
</para>
<note>
<para>
This parameter is not supported on Windows, and must be zero.
</para>
</note>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="runtime-config-connection-security">
<title>Security and Authentication</title>
<variablelist>
<varlistentry id="guc-authentication-timeout" xreflabel="authentication_timeout">
<term><varname>authentication_timeout</varname> (<type>integer</type>)
<indexterm><primary>timeout</><secondary>client authentication</></indexterm>
<indexterm><primary>client authentication</><secondary>timeout during</></indexterm>
<indexterm>
<primary><varname>authentication_timeout</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Maximum time to complete client authentication, in seconds. If a
would-be client has not completed the authentication protocol in
this much time, the server closes the connection. This prevents
hung clients from occupying a connection indefinitely.
The default is one minute (<literal>1m</>).
This parameter can only be set in the <filename>postgresql.conf</>
file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-ssl" xreflabel="ssl">
<term><varname>ssl</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>ssl</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Enables <acronym>SSL</> connections. Please read
<xref linkend="ssl-tcp"> before using this. The default
is <literal>off</>. This parameter can only be set at server
start. <acronym>SSL</> communication is only possible with
TCP/IP connections.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-ssl-ca-file" xreflabel="ssl_ca_file">
<term><varname>ssl_ca_file</varname> (<type>string</type>)
<indexterm>
<primary><varname>ssl_ca_file</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the name of the file containing the SSL server certificate
authority (CA). The default is empty, meaning no CA file is loaded,
and client certificate verification is not performed. (In previous
releases of PostgreSQL, the name of this file was hard-coded
as <filename>root.crt</filename>.) Relative paths are relative to the
data directory. This parameter can only be set at server start.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-ssl-cert-file" xreflabel="ssl_cert_file">
<term><varname>ssl_cert_file</varname> (<type>string</type>)
<indexterm>
<primary><varname>ssl_cert_file</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the name of the file containing the SSL server certificate.
The default is <filename>server.crt</filename>. Relative paths are
relative to the data directory. This parameter can only be set at
server start.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-ssl-crl-file" xreflabel="ssl_crl_file">
<term><varname>ssl_crl_file</varname> (<type>string</type>)
<indexterm>
<primary><varname>ssl_crl_file</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the name of the file containing the SSL server certificate
revocation list (CRL). The default is empty, meaning no CRL file is
loaded. (In previous releases of PostgreSQL, the name of this file was
hard-coded as <filename>root.crl</filename>.) Relative paths are
relative to the data directory. This parameter can only be set at
server start.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-ssl-key-file" xreflabel="ssl_key_file">
<term><varname>ssl_key_file</varname> (<type>string</type>)
<indexterm>
<primary><varname>ssl_key_file</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the name of the file containing the SSL server private key.
The default is <filename>server.key</filename>. Relative paths are
relative to the data directory. This parameter can only be set at
server start.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-ssl-renegotiation-limit" xreflabel="ssl_renegotiation_limit">
<term><varname>ssl_renegotiation_limit</varname> (<type>integer</type>)
<indexterm>
<primary><varname>ssl_renegotiation_limit</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies how much data can flow over an <acronym>SSL</>-encrypted
connection before renegotiation of the session keys will take
place. Renegotiation decreases an attacker's chances of doing
cryptanalysis when large amounts of traffic can be examined, but it
also carries a large performance penalty. The sum of sent and received
traffic is used to check the limit. If this parameter is set to 0,
renegotiation is disabled. The default is <literal>512MB</>.
</para>
<note>
<para>
SSL libraries from before November 2009 are insecure when using SSL
renegotiation, due to a vulnerability in the SSL protocol. As a
stop-gap fix for this vulnerability, some vendors shipped SSL
libraries incapable of doing renegotiation. If any such libraries
are in use on the client or server, SSL renegotiation should be
disabled.
</para>
</note>
</listitem>
</varlistentry>
<varlistentry id="guc-ssl-ciphers" xreflabel="ssl_ciphers">
<term><varname>ssl_ciphers</varname> (<type>string</type>)
<indexterm>
<primary><varname>ssl_ciphers</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies a list of <acronym>SSL</> cipher suites that are allowed to be
used on secure connections. See
the <citerefentry><refentrytitle>ciphers</></citerefentry> manual page
in the <application>OpenSSL</> package for the syntax of this setting
and a list of supported values. The default value is
<literal>HIGH:MEDIUM:+3DES:!aNULL</>. It is usually reasonable,
unless you have specific security requirements.
</para>
<para>
Explanation of the default value:
<variablelist>
<varlistentry>
<term><literal>HIGH</literal></term>
<listitem>
<para>
Cipher suites that use ciphers from <literal>HIGH</> group (e.g.,
AES, Camellia, 3DES)
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>MEDIUM</literal></term>
<listitem>
<para>
Cipher suites that use ciphers from <literal>MEDIUM</> group
(e.g., RC4, SEED)
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>+3DES</literal></term>
<listitem>
<para>
The OpenSSL default order for <literal>HIGH</> is problematic
because it orders 3DES higher than AES128. This is wrong because
3DES offers less security than AES128, and it is also much
slower. <literal>+3DES</> reorders it after all other
<literal>HIGH</> and <literal>MEDIUM</> ciphers.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>!aNULL</literal></term>
<listitem>
<para>
Disables anonymous cipher suites that do no authentication. Such
cipher suites are vulnerable to man-in-the-middle attacks and
therefore should not be used.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
<para>
Available cipher suite details will vary across OpenSSL versions. Use
the command
<literal>openssl ciphers -v 'HIGH:MEDIUM:+3DES:!aNULL'</literal> to
see actual details for the currently installed <application>OpenSSL</>
version. Note that this list is filtered at run time based on the
server key type.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-ssl-prefer-server-ciphers" xreflabel="ssl_prefer_server_ciphers">
<term><varname>ssl_prefer_server_ciphers</varname> (<type>bool</type>)
<indexterm>
<primary><varname>ssl_prefer_server_ciphers</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies whether to use the server's SSL cipher preferences, rather
than the client's. The default is true.
</para>
<para>
Older PostgreSQL versions do not have this setting and always use the
client's preferences. This setting is mainly for backward
compatibility with those versions. Using the server's preferences is
usually better because it is more likely that the server is appropriately
configured.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-ssl-ecdh-curve" xreflabel="ssl_ecdh_curve">
<term><varname>ssl_ecdh_curve</varname> (<type>string</type>)
<indexterm>
<primary><varname>ssl_ecdh_curve</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the name of the curve to use in <acronym>ECDH</> key
exchange. It needs to be supported by all clients that connect.
It does not need to be same curve as used by server's Elliptic
Curve key. The default is <literal>prime256v1</>.
</para>
<para>
OpenSSL names for most common curves:
<literal>prime256v1</> (NIST P-256),
<literal>secp384r1</> (NIST P-384),
<literal>secp521r1</> (NIST P-521).
</para>
<para>
The full list of available curves can be shown with the command
<command>openssl ecparam -list_curves</command>. Not all of them
are usable in <acronym>TLS</> though.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-password-encryption" xreflabel="password_encryption">
<term><varname>password_encryption</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>password_encryption</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
When a password is specified in <xref
linkend="sql-createuser"> or
<xref linkend="sql-alterrole">
without writing either <literal>ENCRYPTED</> or
<literal>UNENCRYPTED</>, this parameter determines whether the
password is to be encrypted. The default is <literal>on</>
(encrypt the password).
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-krb-server-keyfile" xreflabel="krb_server_keyfile">
<term><varname>krb_server_keyfile</varname> (<type>string</type>)
<indexterm>
<primary><varname>krb_server_keyfile</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the location of the Kerberos server key file. See
<xref linkend="gssapi-auth">
for details. This parameter can only be set in the
<filename>postgresql.conf</> file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-krb-caseins-users" xreflabel="krb_caseins_users">
<term><varname>krb_caseins_users</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>krb_caseins_users</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets whether GSSAPI user names should be treated
case-insensitively.
The default is <literal>off</> (case sensitive). This parameter can only be
set in the <filename>postgresql.conf</> file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-db-user-namespace" xreflabel="db_user_namespace">
<term><varname>db_user_namespace</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>db_user_namespace</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
This parameter enables per-database user names. It is off by default.
This parameter can only be set in the <filename>postgresql.conf</>
file or on the server command line.
</para>
<para>
If this is on, you should create users as <literal>username@dbname</>.
When <literal>username</> is passed by a connecting client,
<literal>@</> and the database name are appended to the user
name and that database-specific user name is looked up by the
server. Note that when you create users with names containing
<literal>@</> within the SQL environment, you will need to
quote the user name.
</para>
<para>
With this parameter enabled, you can still create ordinary global
users. Simply append <literal>@</> when specifying the user
name in the client, e.g. <literal>joe@</>. The <literal>@</>
will be stripped off before the user name is looked up by the
server.
</para>
<para>
<varname>db_user_namespace</> causes the client's and
server's user name representation to differ.
Authentication checks are always done with the server's user name
so authentication methods must be configured for the
server's user name, not the client's. Because
<literal>md5</> uses the user name as salt on both the
client and server, <literal>md5</> cannot be used with
<varname>db_user_namespace</>.
</para>
<note>
<para>
This feature is intended as a temporary measure until a
complete solution is found. At that time, this option will
be removed.
</para>
</note>
</listitem>
</varlistentry>
</variablelist>
</sect2>
</sect1>
<sect1 id="runtime-config-resource">
<title>Resource Consumption</title>
<sect2 id="runtime-config-resource-memory">
<title>Memory</title>
<variablelist>
<varlistentry id="guc-shared-buffers" xreflabel="shared_buffers">
<term><varname>shared_buffers</varname> (<type>integer</type>)
<indexterm>
<primary><varname>shared_buffers</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the amount of memory the database server uses for shared
memory buffers. The default is typically 128 megabytes
(<literal>128MB</>), but might be less if your kernel settings will
not support it (as determined during <application>initdb</>).
This setting must be at least 128 kilobytes. (Non-default
values of <symbol>BLCKSZ</symbol> change the minimum.) However,
settings significantly higher than the minimum are usually needed
for good performance. This parameter can only be set at server start.
</para>
<para>
If you have a dedicated database server with 1GB or more of RAM, a
reasonable starting value for <varname>shared_buffers</varname> is 25%
of the memory in your system. There are some workloads where even
large settings for <varname>shared_buffers</varname> are effective, but
because <productname>PostgreSQL</productname> also relies on the
operating system cache, it is unlikely that an allocation of more than
40% of RAM to <varname>shared_buffers</varname> will work better than a
smaller amount. Larger settings for <varname>shared_buffers</varname>
usually require a corresponding increase in
<varname>checkpoint_segments</varname>, in order to spread out the
process of writing large quantities of new or changed data over a
longer period of time.
</para>
<para>
On systems with less than 1GB of RAM, a smaller percentage of RAM is
appropriate, so as to leave adequate space for the operating system.
Also, on Windows, large values for <varname>shared_buffers</varname>
aren't as effective. You may find better results keeping the setting
relatively low and using the operating system cache more instead. The
useful range for <varname>shared_buffers</varname> on Windows systems
is generally from 64MB to 512MB.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-huge-pages" xreflabel="huge_pages">
<term><varname>huge_pages</varname> (<type>enum</type>)
<indexterm>
<primary><varname>huge_pages</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Enables/disables the use of huge memory pages. Valid values are
<literal>try</literal> (the default), <literal>on</literal>,
and <literal>off</literal>.
</para>
<para>
At present, this feature is supported only on Linux. The setting is
ignored on other systems when set to <literal>try</literal>.
</para>
<para>
The use of huge pages results in smaller page tables and less CPU time
spent on memory management, increasing performance. For more details,
see <xref linkend="linux-huge-pages">.
</para>
<para>
With <varname>huge_pages</varname> set to <literal>try</literal>,
the server will try to use huge pages, but fall back to using
normal allocation if that fails. With <literal>on</literal>, failure
to use huge pages will prevent the server from starting up. With
<literal>off</literal>, huge pages will not be used.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-temp-buffers" xreflabel="temp_buffers">
<term><varname>temp_buffers</varname> (<type>integer</type>)
<indexterm>
<primary><varname>temp_buffers</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the maximum number of temporary buffers used by each database
session. These are session-local buffers used only for access to
temporary tables. The default is eight megabytes
(<literal>8MB</>). The setting can be changed within individual
sessions, but only before the first use of temporary tables
within the session; subsequent attempts to change the value will
have no effect on that session.
</para>
<para>
A session will allocate temporary buffers as needed up to the limit
given by <varname>temp_buffers</>. The cost of setting a large
value in sessions that do not actually need many temporary
buffers is only a buffer descriptor, or about 64 bytes, per
increment in <varname>temp_buffers</>. However if a buffer is
actually used an additional 8192 bytes will be consumed for it
(or in general, <symbol>BLCKSZ</symbol> bytes).
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-max-prepared-transactions" xreflabel="max_prepared_transactions">
<term><varname>max_prepared_transactions</varname> (<type>integer</type>)
<indexterm>
<primary><varname>max_prepared_transactions</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the maximum number of transactions that can be in the
<quote>prepared</> state simultaneously (see <xref
linkend="sql-prepare-transaction">).
Setting this parameter to zero (which is the default)
disables the prepared-transaction feature.
This parameter can only be set at server start.
</para>
<para>
If you are not planning to use prepared transactions, this parameter
should be set to zero to prevent accidental creation of prepared
transactions. If you are using prepared transactions, you will
probably want <varname>max_prepared_transactions</varname> to be at
least as large as <xref linkend="guc-max-connections">, so that every
session can have a prepared transaction pending.
</para>
<para>
When running a standby server, you must set this parameter to the
same or higher value than on the master server. Otherwise, queries
will not be allowed in the standby server.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-work-mem" xreflabel="work_mem">
<term><varname>work_mem</varname> (<type>integer</type>)
<indexterm>
<primary><varname>work_mem</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the amount of memory to be used by internal sort operations
and hash tables before writing to temporary disk files. The value
defaults to four megabytes (<literal>4MB</>).
Note that for a complex query, several sort or hash operations might be
running in parallel; each operation will be allowed to use as much memory
as this value specifies before it starts to write data into temporary
files. Also, several running sessions could be doing such operations
concurrently. Therefore, the total memory used could be many
times the value of <varname>work_mem</varname>; it is necessary to
keep this fact in mind when choosing the value. Sort operations are
used for <literal>ORDER BY</>, <literal>DISTINCT</>, and
merge joins.
Hash tables are used in hash joins, hash-based aggregation, and
hash-based processing of <literal>IN</> subqueries.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-maintenance-work-mem" xreflabel="maintenance_work_mem">
<term><varname>maintenance_work_mem</varname> (<type>integer</type>)
<indexterm>
<primary><varname>maintenance_work_mem</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the maximum amount of memory to be used by maintenance
operations, such as <command>VACUUM</command>, <command>CREATE
INDEX</>, and <command>ALTER TABLE ADD FOREIGN KEY</>. It defaults
to 64 megabytes (<literal>64MB</>). Since only one of these
operations can be executed at a time by a database session, and
an installation normally doesn't have many of them running
concurrently, it's safe to set this value significantly larger
than <varname>work_mem</varname>. Larger settings might improve
performance for vacuuming and for restoring database dumps.
</para>
<para>
Note that when autovacuum runs, up to
<xref linkend="guc-autovacuum-max-workers"> times this memory
may be allocated, so be careful not to set the default value
too high. It may be useful to control for this by separately
setting <xref linkend="guc-autovacuum-work-mem">.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-autovacuum-work-mem" xreflabel="autovacuum_work_mem">
<term><varname>autovacuum_work_mem</varname> (<type>integer</type>)
<indexterm>
<primary><varname>autovacuum_work_mem</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the maximum amount of memory to be used by each
autovacuum worker process. It defaults to -1, indicating that
the value of <xref linkend="guc-maintenance-work-mem"> should
be used instead. The setting has no effect on the behavior of
<command>VACUUM</command> when run in other contexts.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-max-stack-depth" xreflabel="max_stack_depth">
<term><varname>max_stack_depth</varname> (<type>integer</type>)
<indexterm>
<primary><varname>max_stack_depth</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the maximum safe depth of the server's execution stack.
The ideal setting for this parameter is the actual stack size limit
enforced by the kernel (as set by <literal>ulimit -s</> or local
equivalent), less a safety margin of a megabyte or so. The safety
margin is needed because the stack depth is not checked in every
routine in the server, but only in key potentially-recursive routines
such as expression evaluation. The default setting is two
megabytes (<literal>2MB</>), which is conservatively small and
unlikely to risk crashes. However, it might be too small to allow
execution of complex functions. Only superusers can change this
setting.
</para>
<para>
Setting <varname>max_stack_depth</> higher than
the actual kernel limit will mean that a runaway recursive function
can crash an individual backend process. On platforms where
<productname>PostgreSQL</productname> can determine the kernel limit,
the server will not allow this variable to be set to an unsafe
value. However, not all platforms provide the information,
so caution is recommended in selecting a value.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-dynamic-shared-memory-type" xreflabel="dynamic_shared_memory_type">
<term><varname>dynamic_shared_memory_type</varname> (<type>enum</type>)
<indexterm>
<primary><varname>dynamic_shared_memory_type</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the dynamic shared memory implementation that the server
should use. Possible values are <literal>posix</> (for POSIX shared
memory allocated using <literal>shm_open</>), <literal>sysv</literal>
(for System V shared memory allocated via <literal>shmget</>),
<literal>windows</> (for Windows shared memory), <literal>mmap</>
(to simulate shared memory using memory-mapped files stored in the
data directory), and <literal>none</> (to disable this feature).
Not all values are supported on all platforms; the first supported
option is the default for that platform. The use of the
<literal>mmap</> option, which is not the default on any platform,
is generally discouraged because the operating system may write
modified pages back to disk repeatedly, increasing system I/O load;
however, it may be useful for debugging, when the
<literal>pg_dynshmem</> directory is stored on a RAM disk, or when
other shared memory facilities are not available.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="runtime-config-resource-disk">
<title>Disk</title>
<variablelist>
<varlistentry id="guc-temp-file-limit" xreflabel="temp_file_limit">
<term><varname>temp_file_limit</varname> (<type>integer</type>)
<indexterm>
<primary><varname>temp_file_limit</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the maximum amount of disk space that a session can use
for temporary files, such as sort and hash temporary files, or the
storage file for a held cursor. A transaction attempting to exceed
this limit will be canceled.
The value is specified in kilobytes, and <literal>-1</> (the
default) means no limit.
Only superusers can change this setting.
</para>
<para>
This setting constrains the total space used at any instant by all
temporary files used by a given <productname>PostgreSQL</> session.
It should be noted that disk space used for explicit temporary
tables, as opposed to temporary files used behind-the-scenes in query
execution, does <emphasis>not</emphasis> count against this limit.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="runtime-config-resource-kernel">
<title>Kernel Resource Usage</title>
<variablelist>
<varlistentry id="guc-max-files-per-process" xreflabel="max_files_per_process">
<term><varname>max_files_per_process</varname> (<type>integer</type>)
<indexterm>
<primary><varname>max_files_per_process</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the maximum number of simultaneously open files allowed to each
server subprocess. The default is one thousand files. If the kernel is enforcing
a safe per-process limit, you don't need to worry about this setting.
But on some platforms (notably, most BSD systems), the kernel will
allow individual processes to open many more files than the system
can actually support if many processes all try to open
that many files. If you find yourself seeing <quote>Too many open
files</> failures, try reducing this setting.
This parameter can only be set at server start.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="runtime-config-resource-vacuum-cost">
<title>Cost-based Vacuum Delay</title>
<para>
During the execution of <xref linkend="sql-vacuum">
and <xref linkend="sql-analyze">
commands, the system maintains an
internal counter that keeps track of the estimated cost of the
various I/O operations that are performed. When the accumulated
cost reaches a limit (specified by
<varname>vacuum_cost_limit</varname>), the process performing
the operation will sleep for a short period of time, as specified by
<varname>vacuum_cost_delay</varname>. Then it will reset the
counter and continue execution.
</para>
<para>
The intent of this feature is to allow administrators to reduce
the I/O impact of these commands on concurrent database
activity. There are many situations where it is not
important that maintenance commands like
<command>VACUUM</command> and <command>ANALYZE</command> finish
quickly; however, it is usually very important that these
commands do not significantly interfere with the ability of the
system to perform other database operations. Cost-based vacuum
delay provides a way for administrators to achieve this.
</para>
<para>
This feature is disabled by default for manually issued
<command>VACUUM</command> commands. To enable it, set the
<varname>vacuum_cost_delay</varname> variable to a nonzero
value.
</para>
<variablelist>
<varlistentry id="guc-vacuum-cost-delay" xreflabel="vacuum_cost_delay">
<term><varname>vacuum_cost_delay</varname> (<type>integer</type>)
<indexterm>
<primary><varname>vacuum_cost_delay</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
The length of time, in milliseconds, that the process will sleep
when the cost limit has been exceeded.
The default value is zero, which disables the cost-based vacuum
delay feature. Positive values enable cost-based vacuuming.
Note that on many systems, the effective resolution
of sleep delays is 10 milliseconds; setting
<varname>vacuum_cost_delay</varname> to a value that is
not a multiple of 10 might have the same results as setting it
to the next higher multiple of 10.
</para>
<para>
When using cost-based vacuuming, appropriate values for
<varname>vacuum_cost_delay</> are usually quite small, perhaps
10 or 20 milliseconds. Adjusting vacuum's resource consumption
is best done by changing the other vacuum cost parameters.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-vacuum-cost-page-hit" xreflabel="vacuum_cost_page_hit">
<term><varname>vacuum_cost_page_hit</varname> (<type>integer</type>)
<indexterm>
<primary><varname>vacuum_cost_page_hit</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
The estimated cost for vacuuming a buffer found in the shared buffer
cache. It represents the cost to lock the buffer pool, lookup
the shared hash table and scan the content of the page. The
default value is one.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-vacuum-cost-page-miss" xreflabel="vacuum_cost_page_miss">
<term><varname>vacuum_cost_page_miss</varname> (<type>integer</type>)
<indexterm>
<primary><varname>vacuum_cost_page_miss</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
The estimated cost for vacuuming a buffer that has to be read from
disk. This represents the effort to lock the buffer pool,
lookup the shared hash table, read the desired block in from
the disk and scan its content. The default value is 10.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-vacuum-cost-page-dirty" xreflabel="vacuum_cost_page_dirty">
<term><varname>vacuum_cost_page_dirty</varname> (<type>integer</type>)
<indexterm>
<primary><varname>vacuum_cost_page_dirty</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
The estimated cost charged when vacuum modifies a block that was
previously clean. It represents the extra I/O required to
flush the dirty block out to disk again. The default value is
20.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-vacuum-cost-limit" xreflabel="vacuum_cost_limit">
<term><varname>vacuum_cost_limit</varname> (<type>integer</type>)
<indexterm>
<primary><varname>vacuum_cost_limit</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
The accumulated cost that will cause the vacuuming process to sleep.
The default value is 200.
</para>
</listitem>
</varlistentry>
</variablelist>
<note>
<para>
There are certain operations that hold critical locks and should
therefore complete as quickly as possible. Cost-based vacuum
delays do not occur during such operations. Therefore it is
possible that the cost accumulates far higher than the specified
limit. To avoid uselessly long delays in such cases, the actual
delay is calculated as <varname>vacuum_cost_delay</varname> *
<varname>accumulated_balance</varname> /
<varname>vacuum_cost_limit</varname> with a maximum of
<varname>vacuum_cost_delay</varname> * 4.
</para>
</note>
</sect2>
<sect2 id="runtime-config-resource-background-writer">
<title>Background Writer</title>
<para>
There is a separate server
process called the <firstterm>background writer</>, whose function
is to issue writes of <quote>dirty</> (new or modified) shared
buffers. It writes shared buffers so server processes handling
user queries seldom or never need to wait for a write to occur.
However, the background writer does cause a net overall
increase in I/O load, because while a repeatedly-dirtied page might
otherwise be written only once per checkpoint interval, the
background writer might write it several times as it is dirtied
in the same interval. The parameters discussed in this subsection
can be used to tune the behavior for local needs.
</para>
<variablelist>
<varlistentry id="guc-bgwriter-delay" xreflabel="bgwriter_delay">
<term><varname>bgwriter_delay</varname> (<type>integer</type>)
<indexterm>
<primary><varname>bgwriter_delay</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the delay between activity rounds for the
background writer. In each round the writer issues writes
for some number of dirty buffers (controllable by the
following parameters). It then sleeps for <varname>bgwriter_delay</>
milliseconds, and repeats. When there are no dirty buffers in the
buffer pool, though, it goes into a longer sleep regardless of
<varname>bgwriter_delay</>. The default value is 200
milliseconds (<literal>200ms</>). Note that on many systems, the
effective resolution of sleep delays is 10 milliseconds; setting
<varname>bgwriter_delay</> to a value that is not a multiple of 10
might have the same results as setting it to the next higher multiple
of 10. This parameter can only be set in the
<filename>postgresql.conf</> file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-bgwriter-lru-maxpages" xreflabel="bgwriter_lru_maxpages">
<term><varname>bgwriter_lru_maxpages</varname> (<type>integer</type>)
<indexterm>
<primary><varname>bgwriter_lru_maxpages</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
In each round, no more than this many buffers will be written
by the background writer. Setting this to zero disables
background writing. (Note that checkpoints, which are managed by
a separate, dedicated auxiliary process, are unaffected.)
The default value is 100 buffers.
This parameter can only be set in the <filename>postgresql.conf</>
file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-bgwriter-lru-multiplier" xreflabel="bgwriter_lru_multiplier">
<term><varname>bgwriter_lru_multiplier</varname> (<type>floating point</type>)
<indexterm>
<primary><varname>bgwriter_lru_multiplier</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
The number of dirty buffers written in each round is based on the
number of new buffers that have been needed by server processes
during recent rounds. The average recent need is multiplied by
<varname>bgwriter_lru_multiplier</> to arrive at an estimate of the
number of buffers that will be needed during the next round. Dirty
buffers are written until there are that many clean, reusable buffers
available. (However, no more than <varname>bgwriter_lru_maxpages</>
buffers will be written per round.)
Thus, a setting of 1.0 represents a <quote>just in time</> policy
of writing exactly the number of buffers predicted to be needed.
Larger values provide some cushion against spikes in demand,
while smaller values intentionally leave writes to be done by
server processes.
The default is 2.0.
This parameter can only be set in the <filename>postgresql.conf</>
file or on the server command line.
</para>
</listitem>
</varlistentry>
</variablelist>
<para>
Smaller values of <varname>bgwriter_lru_maxpages</varname> and
<varname>bgwriter_lru_multiplier</varname> reduce the extra I/O load
caused by the background writer, but make it more likely that server
processes will have to issue writes for themselves, delaying interactive
queries.
</para>
</sect2>
<sect2 id="runtime-config-resource-async-behavior">
<title>Asynchronous Behavior</title>
<variablelist>
<varlistentry id="guc-effective-io-concurrency" xreflabel="effective_io_concurrency">
<term><varname>effective_io_concurrency</varname> (<type>integer</type>)
<indexterm>
<primary><varname>effective_io_concurrency</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the number of concurrent disk I/O operations that
<productname>PostgreSQL</> expects can be executed
simultaneously. Raising this value will increase the number of I/O
operations that any individual <productname>PostgreSQL</> session
attempts to initiate in parallel. The allowed range is 1 to 1000,
or zero to disable issuance of asynchronous I/O requests. Currently,
this setting only affects bitmap heap scans.
</para>
<para>
A good starting point for this setting is the number of separate
drives comprising a RAID 0 stripe or RAID 1 mirror being used for the
database. (For RAID 5 the parity drive should not be counted.)
However, if the database is often busy with multiple queries issued in
concurrent sessions, lower values may be sufficient to keep the disk
array busy. A value higher than needed to keep the disks busy will
only result in extra CPU overhead.
</para>
<para>
For more exotic systems, such as memory-based storage or a RAID array
that is limited by bus bandwidth, the correct value might be the
number of I/O paths available. Some experimentation may be needed
to find the best value.
</para>
<para>
Asynchronous I/O depends on an effective <function>posix_fadvise</>
function, which some operating systems lack. If the function is not
present then setting this parameter to anything but zero will result
in an error. On some operating systems (e.g., Solaris), the function
is present but does not actually do anything.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-max-worker-processes" xreflabel="max_worker_processes">
<term><varname>max_worker_processes</varname> (<type>integer</type>)
<indexterm>
<primary><varname>max_worker_processes</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the maximum number of background processes that the system
can support. This parameter can only be set at server start.
</para>
<para>
When running a standby server, you must set this parameter to the
same or higher value than on the master server. Otherwise, queries
will not be allowed in the standby server.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
</sect1>
<sect1 id="runtime-config-wal">
<title>Write Ahead Log</title>
<para>
For additional information on tuning these settings,
see <xref linkend="wal-configuration">.
</para>
<sect2 id="runtime-config-wal-settings">
<title>Settings</title>
<variablelist>
<varlistentry id="guc-wal-level" xreflabel="wal_level">
<term><varname>wal_level</varname> (<type>enum</type>)
<indexterm>
<primary><varname>wal_level</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
<varname>wal_level</> determines how much information is written
to the WAL. The default value is <literal>minimal</>, which writes
only the information needed to recover from a crash or immediate
shutdown. <literal>archive</> adds logging required for WAL archiving;
<literal>hot_standby</> further adds information required to run
read-only queries on a standby server; and, finally
<literal>logical</> adds information necessary to support logical
decoding. Each level includes the information logged at all lower
levels. This parameter can only be set at server start.
</para>
<para>
In <literal>minimal</> level, WAL-logging of some bulk
operations can be safely skipped, which can make those
operations much faster (see <xref linkend="populate-pitr">).
Operations in which this optimization can be applied include:
<simplelist>
<member><command>CREATE TABLE AS</></member>
<member><command>CREATE INDEX</></member>
<member><command>CLUSTER</></member>
<member><command>COPY</> into tables that were created or truncated in the same
transaction</member>
</simplelist>
But minimal WAL does not contain enough information to reconstruct the
data from a base backup and the WAL logs, so <literal>archive</> or
higher must be used to enable WAL archiving
(<xref linkend="guc-archive-mode">) and streaming replication.
</para>
<para>
In <literal>hot_standby</> level, the same information is logged as
with <literal>archive</>, plus information needed to reconstruct
the status of running transactions from the WAL. To enable read-only
queries on a standby server, <varname>wal_level</> must be set to
<literal>hot_standby</> or higher on the primary, and
<xref linkend="guc-hot-standby"> must be enabled in the standby. It is
thought that there is little measurable difference in performance
between using <literal>hot_standby</> and <literal>archive</> levels,
so feedback is welcome if any production impacts are noticeable.
</para>
<para>
In <literal>logical</> level, the same information is logged as
with <literal>hot_standby</>, plus information needed to allow
extracting logical changesets from the WAL. Using a level of
<literal>logical</> will increase the WAL volume, particularly if many
tables are configured for <literal>REPLICA IDENTITY FULL</literal> and
many <command>UPDATE</> and <command>DELETE</> statements are
executed.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-fsync" xreflabel="fsync">
<term><varname>fsync</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>fsync</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
If this parameter is on, the <productname>PostgreSQL</> server
will try to make sure that updates are physically written to
disk, by issuing <function>fsync()</> system calls or various
equivalent methods (see <xref linkend="guc-wal-sync-method">).
This ensures that the database cluster can recover to a
consistent state after an operating system or hardware crash.
</para>
<para>
While turning off <varname>fsync</varname> is often a performance
benefit, this can result in unrecoverable data corruption in
the event of a power failure or system crash. Thus it
is only advisable to turn off <varname>fsync</varname> if
you can easily recreate your entire database from external
data.
</para>
<para>
Examples of safe circumstances for turning off
<varname>fsync</varname> include the initial loading of a new
database cluster from a backup file, using a database cluster
for processing a batch of data after which the database
will be thrown away and recreated,
or for a read-only database clone which
gets recreated frequently and is not used for failover. High
quality hardware alone is not a sufficient justification for
turning off <varname>fsync</varname>.
</para>
<para>
For reliable recovery when changing <varname>fsync</varname>
off to on, it is necessary to force all modified buffers in the
kernel to durable storage. This can be done while the cluster
is shutdown or while fsync is on by running <command>initdb
--sync-only</command>, running <command>sync</>, unmounting the
file system, or rebooting the server.
</para>
<para>
In many situations, turning off <xref linkend="guc-synchronous-commit">
for noncritical transactions can provide much of the potential
performance benefit of turning off <varname>fsync</varname>, without
the attendant risks of data corruption.
</para>
<para>
<varname>fsync</varname> can only be set in the <filename>postgresql.conf</>
file or on the server command line.
If you turn this parameter off, also consider turning off
<xref linkend="guc-full-page-writes">.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-synchronous-commit" xreflabel="synchronous_commit">
<term><varname>synchronous_commit</varname> (<type>enum</type>)
<indexterm>
<primary><varname>synchronous_commit</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies whether transaction commit will wait for WAL records
to be written to disk before the command returns a <quote>success</>
indication to the client. Valid values are <literal>on</>,
<literal>remote_write</>, <literal>local</>, and <literal>off</>.
The default, and safe, setting
is <literal>on</>. When <literal>off</>, there can be a delay between
when success is reported to the client and when the transaction is
really guaranteed to be safe against a server crash. (The maximum
delay is three times <xref linkend="guc-wal-writer-delay">.) Unlike
<xref linkend="guc-fsync">, setting this parameter to <literal>off</>
does not create any risk of database inconsistency: an operating
system or database crash might
result in some recent allegedly-committed transactions being lost, but
the database state will be just the same as if those transactions had
been aborted cleanly. So, turning <varname>synchronous_commit</> off
can be a useful alternative when performance is more important than
exact certainty about the durability of a transaction. For more
discussion see <xref linkend="wal-async-commit">.
</para>
<para>
If <xref linkend="guc-synchronous-standby-names"> is set, this
parameter also controls whether or not transaction commits will wait
for the transaction's WAL records to be replicated to the standby
server.
When set to <literal>on</>, commits will wait until a reply
from the current synchronous standby indicates it has received
the commit record of the transaction and flushed it to disk. This
ensures the transaction will not be lost unless both primary and
standby suffer corruption of their database storage.
When set to <literal>remote_write</>, commits will wait
until a reply from the current synchronous standby indicates it has
received the commit record of the transaction and written it out to
the standby's operating system, but the data has not necessarily
reached stable storage on the standby. This setting is sufficient to
ensure data preservation even if the standby instance of
<productname>PostgreSQL</> were to crash, but not if the standby
suffers an operating-system-level crash.
</para>
<para>
When synchronous
replication is in use, it will normally be sensible either to wait
for both local flush to disk and replication of WAL records, or
to allow the transaction to commit asynchronously. However, the
setting <literal>local</> is available for transactions that
wish to wait for local flush to disk, but not synchronous replication.
If <varname>synchronous_standby_names</> is not set, the settings
<literal>on</>, <literal>remote_write</> and <literal>local</> all
provide the same synchronization level: transaction commits only wait
for local flush to disk.
</para>
<para>
This parameter can be changed at any time; the behavior for any
one transaction is determined by the setting in effect when it
commits. It is therefore possible, and useful, to have some
transactions commit synchronously and others asynchronously.
For example, to make a single multistatement transaction commit
asynchronously when the default is the opposite, issue <command>SET
LOCAL synchronous_commit TO OFF</> within the transaction.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-wal-sync-method" xreflabel="wal_sync_method">
<term><varname>wal_sync_method</varname> (<type>enum</type>)
<indexterm>
<primary><varname>wal_sync_method</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Method used for forcing WAL updates out to disk.
If <varname>fsync</varname> is off then this setting is irrelevant,
since WAL file updates will not be forced out at all.
Possible values are:
</para>
<itemizedlist>
<listitem>
<para>
<literal>open_datasync</> (write WAL files with <function>open()</> option <symbol>O_DSYNC</>)
</para>
</listitem>
<listitem>
<para>
<literal>fdatasync</> (call <function>fdatasync()</> at each commit)
</para>
</listitem>
<listitem>
<para>
<literal>fsync</> (call <function>fsync()</> at each commit)
</para>
</listitem>
<listitem>
<para>
<literal>fsync_writethrough</> (call <function>fsync()</> at each commit, forcing write-through of any disk write cache)
</para>
</listitem>
<listitem>
<para>
<literal>open_sync</> (write WAL files with <function>open()</> option <symbol>O_SYNC</>)
</para>
</listitem>
</itemizedlist>
<para>
The <literal>open_</>* options also use <literal>O_DIRECT</> if available.
Not all of these choices are available on all platforms.
The default is the first method in the above list that is supported
by the platform, except that <literal>fdatasync</> is the default on
Linux. The default is not necessarily ideal; it might be
necessary to change this setting or other aspects of your system
configuration in order to create a crash-safe configuration or
achieve optimal performance.
These aspects are discussed in <xref linkend="wal-reliability">.
This parameter can only be set in the <filename>postgresql.conf</>
file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-full-page-writes" xreflabel="full_page_writes">
<term><varname>full_page_writes</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>full_page_writes</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
When this parameter is on, the <productname>PostgreSQL</> server
writes the entire content of each disk page to WAL during the
first modification of that page after a checkpoint.
This is needed because
a page write that is in process during an operating system crash might
be only partially completed, leading to an on-disk page
that contains a mix of old and new data. The row-level change data
normally stored in WAL will not be enough to completely restore
such a page during post-crash recovery. Storing the full page image
guarantees that the page can be correctly restored, but at the price
of increasing the amount of data that must be written to WAL.
(Because WAL replay always starts from a checkpoint, it is sufficient
to do this during the first change of each page after a checkpoint.
Therefore, one way to reduce the cost of full-page writes is to
increase the checkpoint interval parameters.)
</para>
<para>
Turning this parameter off speeds normal operation, but
might lead to either unrecoverable data corruption, or silent
data corruption, after a system failure. The risks are similar to turning off
<varname>fsync</varname>, though smaller, and it should be turned off
only based on the same circumstances recommended for that parameter.
</para>
<para>
Turning off this parameter does not affect use of
WAL archiving for point-in-time recovery (PITR)
(see <xref linkend="continuous-archiving">).
</para>
<para>
This parameter can only be set in the <filename>postgresql.conf</>
file or on the server command line.
The default is <literal>on</>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-wal-log-hints" xreflabel="wal_log_hints">
<term><varname>wal_log_hints</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>wal_log_hints</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
When this parameter is <literal>on</>, the <productname>PostgreSQL</>
server writes the entire content of each disk page to WAL during the
first modification of that page after a checkpoint, even for
non-critical modifications of so-called hint bits.
</para>
<para>
If data checksums are enabled, hint bit updates are always WAL-logged
and this setting is ignored. You can use this setting to test how much
extra WAL-logging would occur if your database had data checksums
enabled.
</para>
<para>
This parameter can only be set at server start. The default value is <literal>off</>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-wal-buffers" xreflabel="wal_buffers">
<term><varname>wal_buffers</varname> (<type>integer</type>)
<indexterm>
<primary><varname>wal_buffers</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
The amount of shared memory used for WAL data that has not yet been
written to disk. The default setting of -1 selects a size equal to
1/32nd (about 3%) of <xref linkend="guc-shared-buffers">, but not less
than <literal>64kB</literal> nor more than the size of one WAL
segment, typically <literal>16MB</literal>. This value can be set
manually if the automatic choice is too large or too small,
but any positive value less than <literal>32kB</literal> will be
treated as <literal>32kB</literal>.
This parameter can only be set at server start.
</para>
<para>
The contents of the WAL buffers are written out to disk at every
transaction commit, so extremely large values are unlikely to
provide a significant benefit. However, setting this value to at
least a few megabytes can improve write performance on a busy
server where many clients are committing at once. The auto-tuning
selected by the default setting of -1 should give reasonable
results in most cases.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-wal-writer-delay" xreflabel="wal_writer_delay">
<term><varname>wal_writer_delay</varname> (<type>integer</type>)
<indexterm>
<primary><varname>wal_writer_delay</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the delay between activity rounds for the WAL writer.
In each round the writer will flush WAL to disk. It then sleeps for
<varname>wal_writer_delay</> milliseconds, and repeats. The default
value is 200 milliseconds (<literal>200ms</>). Note that on many
systems, the effective resolution of sleep delays is 10 milliseconds;
setting <varname>wal_writer_delay</> to a value that is not a multiple
of 10 might have the same results as setting it to the next higher
multiple of 10. This parameter can only be set in the
<filename>postgresql.conf</> file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-commit-delay" xreflabel="commit_delay">
<term><varname>commit_delay</varname> (<type>integer</type>)
<indexterm>
<primary><varname>commit_delay</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
<varname>commit_delay</varname> adds a time delay, measured in
microseconds, before a WAL flush is initiated. This can improve
group commit throughput by allowing a larger number of transactions
to commit via a single WAL flush, if system load is high enough
that additional transactions become ready to commit within the
given interval. However, it also increases latency by up to
<varname>commit_delay</varname> microseconds for each WAL
flush. Because the delay is just wasted if no other transactions
become ready to commit, a delay is only performed if at least
<varname>commit_siblings</varname> other transactions are active
when a flush is about to be initiated. Also, no delays are
performed if <varname>fsync</varname> is disabled.
The default <varname>commit_delay</> is zero (no delay).
Only superusers can change this setting.
</para>
<para>
In <productname>PostgreSQL</> releases prior to 9.3,
<varname>commit_delay</varname> behaved differently and was much
less effective: it affected only commits, rather than all WAL flushes,
and waited for the entire configured delay even if the WAL flush
was completed sooner. Beginning in <productname>PostgreSQL</> 9.3,
the first process that becomes ready to flush waits for the configured
interval, while subsequent processes wait only until the leader
completes the flush operation.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-commit-siblings" xreflabel="commit_siblings">
<term><varname>commit_siblings</varname> (<type>integer</type>)
<indexterm>
<primary><varname>commit_siblings</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Minimum number of concurrent open transactions to require
before performing the <varname>commit_delay</> delay. A larger
value makes it more probable that at least one other
transaction will become ready to commit during the delay
interval. The default is five transactions.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="runtime-config-wal-checkpoints">
<title>Checkpoints</title>
<variablelist>
<varlistentry id="guc-checkpoint-segments" xreflabel="checkpoint_segments">
<term><varname>checkpoint_segments</varname> (<type>integer</type>)
<indexterm>
<primary><varname>checkpoint_segments</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Maximum number of log file segments between automatic WAL
checkpoints (each segment is normally 16 megabytes). The default
is three segments. Increasing this parameter can increase the
amount of time needed for crash recovery.
This parameter can only be set in the <filename>postgresql.conf</>
file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-checkpoint-timeout" xreflabel="checkpoint_timeout">
<term><varname>checkpoint_timeout</varname> (<type>integer</type>)
<indexterm>
<primary><varname>checkpoint_timeout</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Maximum time between automatic WAL checkpoints, in
seconds. The default is five minutes (<literal>5min</>).
Increasing this parameter can increase the amount of time needed
for crash recovery.
This parameter can only be set in the <filename>postgresql.conf</>
file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-checkpoint-completion-target" xreflabel="checkpoint_completion_target">
<term><varname>checkpoint_completion_target</varname> (<type>floating point</type>)
<indexterm>
<primary><varname>checkpoint_completion_target</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the target of checkpoint completion, as a fraction of
total time between checkpoints. The default is 0.5.
This parameter can only be set in the <filename>postgresql.conf</>
file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-checkpoint-warning" xreflabel="checkpoint_warning">
<term><varname>checkpoint_warning</varname> (<type>integer</type>)
<indexterm>
<primary><varname>checkpoint_warning</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Write a message to the server log if checkpoints caused by
the filling of checkpoint segment files happen closer together
than this many seconds (which suggests that
<varname>checkpoint_segments</> ought to be raised). The default is
30 seconds (<literal>30s</>). Zero disables the warning.
No warnings will be generated if <varname>checkpoint_timeout</varname>
is less than <varname>checkpoint_warning</varname>.
This parameter can only be set in the <filename>postgresql.conf</>
file or on the server command line.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="runtime-config-wal-archiving">
<title>Archiving</title>
<variablelist>
<varlistentry id="guc-archive-mode" xreflabel="archive_mode">
<term><varname>archive_mode</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>archive_mode</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
When <varname>archive_mode</> is enabled, completed WAL segments
are sent to archive storage by setting
<xref linkend="guc-archive-command">.
<varname>archive_mode</> and <varname>archive_command</> are
separate variables so that <varname>archive_command</> can be
changed without leaving archiving mode.
This parameter can only be set at server start.
<varname>archive_mode</> cannot be enabled when
<varname>wal_level</> is set to <literal>minimal</>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-archive-command" xreflabel="archive_command">
<term><varname>archive_command</varname> (<type>string</type>)
<indexterm>
<primary><varname>archive_command</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
The local shell command to execute to archive a completed WAL file
segment. Any <literal>%p</> in the string is
replaced by the path name of the file to archive, and any
<literal>%f</> is replaced by only the file name.
(The path name is relative to the working directory of the server,
i.e., the cluster's data directory.)
Use <literal>%%</> to embed an actual <literal>%</> character in the
command. It is important for the command to return a zero
exit status only if it succeeds. For more information see
<xref linkend="backup-archiving-wal">.
</para>
<para>
This parameter can only be set in the <filename>postgresql.conf</>
file or on the server command line. It is ignored unless
<varname>archive_mode</> was enabled at server start.
If <varname>archive_command</> is an empty string (the default) while
<varname>archive_mode</> is enabled, WAL archiving is temporarily
disabled, but the server continues to accumulate WAL segment files in
the expectation that a command will soon be provided. Setting
<varname>archive_command</> to a command that does nothing but
return true, e.g. <literal>/bin/true</> (<literal>REM</> on
Windows), effectively disables
archiving, but also breaks the chain of WAL files needed for
archive recovery, so it should only be used in unusual circumstances.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-archive-timeout" xreflabel="archive_timeout">
<term><varname>archive_timeout</varname> (<type>integer</type>)
<indexterm>
<primary><varname>archive_timeout</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
The <xref linkend="guc-archive-command"> is only invoked for
completed WAL segments. Hence, if your server generates little WAL
traffic (or has slack periods where it does so), there could be a
long delay between the completion of a transaction and its safe
recording in archive storage. To limit how old unarchived
data can be, you can set <varname>archive_timeout</> to force the
server to switch to a new WAL segment file periodically. When this
parameter is greater than zero, the server will switch to a new
segment file whenever this many seconds have elapsed since the last
segment file switch, and there has been any database activity,
including a single checkpoint. (Increasing
<varname>checkpoint_timeout</> will reduce unnecessary
checkpoints on an idle system.)
Note that archived files that are closed early
due to a forced switch are still the same length as completely full
files. Therefore, it is unwise to use a very short
<varname>archive_timeout</> &mdash; it will bloat your archive
storage. <varname>archive_timeout</> settings of a minute or so are
usually reasonable. You should consider using streaming replication,
instead of archiving, if you want data to be copied off the master
server more quickly than that.
This parameter can only be set in the
<filename>postgresql.conf</> file or on the server command line.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
</sect1>
<sect1 id="runtime-config-replication">
<title>Replication</title>
<para>
These settings control the behavior of the built-in
<firstterm>streaming replication</> feature (see
<xref linkend="streaming-replication">). Servers will be either a
Master or a Standby server. Masters can send data, while Standby(s)
are always receivers of replicated data. When cascading replication
(see <xref linkend="cascading-replication">) is used, Standby server(s)
can also be senders, as well as receivers.
Parameters are mainly for Sending and Standby servers, though some
parameters have meaning only on the Master server. Settings may vary
across the cluster without problems if that is required.
</para>
<sect2 id="runtime-config-replication-sender">
<title>Sending Server(s)</title>
<para>
These parameters can be set on any server that is
to send replication data to one or more standby servers.
The master is always a sending server, so these parameters must
always be set on the master.
The role and meaning of these parameters does not change after a
standby becomes the master.
</para>
<variablelist>
<varlistentry id="guc-max-wal-senders" xreflabel="max_wal_senders">
<term><varname>max_wal_senders</varname> (<type>integer</type>)
<indexterm>
<primary><varname>max_wal_senders</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the maximum number of concurrent connections from
standby servers or streaming base backup clients (i.e., the
maximum number of simultaneously running WAL sender
processes). The default is zero, meaning replication is
disabled. WAL sender processes count towards the total number
of connections, so the parameter cannot be set higher than
<xref linkend="guc-max-connections">. Abrupt streaming client
disconnection might cause an orphaned connection slot until
a timeout is reached, so this parameter should be set slightly
higher than the maximum number of expected clients so disconnected
clients can immediately reconnect. This parameter can only
be set at server start. <varname>wal_level</> must be set to
<literal>archive</> or higher to allow connections from standby
servers.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-max-replication-slots" xreflabel="max_replication_slots">
<term><varname>max_replication_slots</varname> (<type>integer</type>)
<indexterm>
<primary><varname>max_replication_slots</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the maximum number of replication slots
(see <xref linkend="streaming-replication-slots">) that the server
can support. The default is zero. This parameter can only be set at
server start.
<varname>wal_level</varname> must be set
to <literal>archive</literal> or higher to allow replication slots to
be used. Setting it to a lower value than the number of currently
existing replication slots will prevent the server from starting.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-wal-keep-segments" xreflabel="wal_keep_segments">
<term><varname>wal_keep_segments</varname> (<type>integer</type>)
<indexterm>
<primary><varname>wal_keep_segments</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the minimum number of past log file segments kept in the
<filename>pg_xlog</>
directory, in case a standby server needs to fetch them for streaming
replication. Each segment is normally 16 megabytes. If a standby
server connected to the sending server falls behind by more than
<varname>wal_keep_segments</> segments, the sending server might remove
a WAL segment still needed by the standby, in which case the
replication connection will be terminated. Downstream connections
will also eventually fail as a result. (However, the standby
server can recover by fetching the segment from archive, if WAL
archiving is in use.)
</para>
<para>
This sets only the minimum number of segments retained in
<filename>pg_xlog</>; the system might need to retain more segments
for WAL archival or to recover from a checkpoint. If
<varname>wal_keep_segments</> is zero (the default), the system
doesn't keep any extra segments for standby purposes, so the number
of old WAL segments available to standby servers is a function of
the location of the previous checkpoint and status of WAL
archiving.
This parameter can only be set in the
<filename>postgresql.conf</> file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-wal-sender-timeout" xreflabel="wal_sender_timeout">
<term><varname>wal_sender_timeout</varname> (<type>integer</type>)
<indexterm>
<primary><varname>wal_sender_timeout</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Terminate replication connections that are inactive longer
than the specified number of milliseconds. This is useful for
the sending server to detect a standby crash or network outage.
A value of zero disables the timeout mechanism. This parameter
can only be set in
the <filename>postgresql.conf</> file or on the server command line.
The default value is 60 seconds.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="runtime-config-replication-master">
<title>Master Server</title>
<para>
These parameters can be set on the master/primary server that is
to send replication data to one or more standby servers.
Note that in addition to these parameters,
<xref linkend="guc-wal-level"> must be set appropriately on the master
server, and optionally WAL archiving can be enabled as
well (see <xref linkend="runtime-config-wal-archiving">).
The values of these parameters on standby servers are irrelevant,
although you may wish to set them there in preparation for the
possibility of a standby becoming the master.
</para>
<variablelist>
<varlistentry id="guc-synchronous-standby-names" xreflabel="synchronous_standby_names">
<term><varname>synchronous_standby_names</varname> (<type>string</type>)
<indexterm>
<primary><varname>synchronous_standby_names</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies a comma-separated list of standby names that can support
<firstterm>synchronous replication</>, as described in
<xref linkend="synchronous-replication">.
At any one time there will be at most one active synchronous standby;
transactions waiting for commit will be allowed to proceed after
this standby server confirms receipt of their data.
The synchronous standby will be the first standby named in this list
that is both currently connected and streaming data in real-time
(as shown by a state of <literal>streaming</literal> in the
<link linkend="monitoring-stats-views-table">
<literal>pg_stat_replication</></link> view).
Other standby servers appearing later in this list represent potential
synchronous standbys.
If the current synchronous standby disconnects for whatever reason,
it will be replaced immediately with the next-highest-priority standby.
Specifying more than one standby name can allow very high availability.
</para>
<para>
The name of a standby server for this purpose is the
<varname>application_name</> setting of the standby, as set in the
<varname>primary_conninfo</> of the standby's WAL receiver. There is
no mechanism to enforce uniqueness. In case of duplicates one of the
matching standbys will be chosen to be the synchronous standby, though
exactly which one is indeterminate.
The special entry <literal>*</> matches any
<varname>application_name</>, including the default application name
of <literal>walreceiver</>.
</para>
<para>
If no synchronous standby names are specified here, then synchronous
replication is not enabled and transaction commits will not wait for
replication. This is the default configuration. Even when
synchronous replication is enabled, individual transactions can be
configured not to wait for replication by setting the
<xref linkend="guc-synchronous-commit"> parameter to
<literal>local</> or <literal>off</>.
</para>
<para>
This parameter can only be set in the <filename>postgresql.conf</>
file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-vacuum-defer-cleanup-age" xreflabel="vacuum_defer_cleanup_age">
<term><varname>vacuum_defer_cleanup_age</varname> (<type>integer</type>)
<indexterm>
<primary><varname>vacuum_defer_cleanup_age</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the number of transactions by which <command>VACUUM</> and
<acronym>HOT</> updates will defer cleanup of dead row versions. The
default is zero transactions, meaning that dead row versions can be
removed as soon as possible, that is, as soon as they are no longer
visible to any open transaction. You may wish to set this to a
non-zero value on a primary server that is supporting hot standby
servers, as described in <xref linkend="hot-standby">. This allows
more time for queries on the standby to complete without incurring
conflicts due to early cleanup of rows. However, since the value
is measured in terms of number of write transactions occurring on the
primary server, it is difficult to predict just how much additional
grace time will be made available to standby queries.
This parameter can only be set in the <filename>postgresql.conf</>
file or on the server command line.
</para>
<para>
You should also consider setting <varname>hot_standby_feedback</>
on standby server(s) as an alternative to using this parameter.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="runtime-config-replication-standby">
<title>Standby Servers</title>
<para>
These settings control the behavior of a standby server that is
to receive replication data. Their values on the master server
are irrelevant.
</para>
<variablelist>
<varlistentry id="guc-hot-standby" xreflabel="hot_standby">
<term><varname>hot_standby</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>hot_standby</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies whether or not you can connect and run queries during
recovery, as described in <xref linkend="hot-standby">.
The default value is <literal>off</literal>.
This parameter can only be set at server start. It only has effect
during archive recovery or in standby mode.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-max-standby-archive-delay" xreflabel="max_standby_archive_delay">
<term><varname>max_standby_archive_delay</varname> (<type>integer</type>)
<indexterm>
<primary><varname>max_standby_archive_delay</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
When Hot Standby is active, this parameter determines how long the
standby server should wait before canceling standby queries that
conflict with about-to-be-applied WAL entries, as described in
<xref linkend="hot-standby-conflict">.
<varname>max_standby_archive_delay</> applies when WAL data is
being read from WAL archive (and is therefore not current).
The default is 30 seconds. Units are milliseconds if not specified.
A value of -1 allows the standby to wait forever for conflicting
queries to complete.
This parameter can only be set in the <filename>postgresql.conf</>
file or on the server command line.
</para>
<para>
Note that <varname>max_standby_archive_delay</> is not the same as the
maximum length of time a query can run before cancellation; rather it
is the maximum total time allowed to apply any one WAL segment's data.
Thus, if one query has resulted in significant delay earlier in the
WAL segment, subsequent conflicting queries will have much less grace
time.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-max-standby-streaming-delay" xreflabel="max_standby_streaming_delay">
<term><varname>max_standby_streaming_delay</varname> (<type>integer</type>)
<indexterm>
<primary><varname>max_standby_streaming_delay</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
When Hot Standby is active, this parameter determines how long the
standby server should wait before canceling standby queries that
conflict with about-to-be-applied WAL entries, as described in
<xref linkend="hot-standby-conflict">.
<varname>max_standby_streaming_delay</> applies when WAL data is
being received via streaming replication.
The default is 30 seconds. Units are milliseconds if not specified.
A value of -1 allows the standby to wait forever for conflicting
queries to complete.
This parameter can only be set in the <filename>postgresql.conf</>
file or on the server command line.
</para>
<para>
Note that <varname>max_standby_streaming_delay</> is not the same as
the maximum length of time a query can run before cancellation; rather
it is the maximum total time allowed to apply WAL data once it has
been received from the primary server. Thus, if one query has
resulted in significant delay, subsequent conflicting queries will
have much less grace time until the standby server has caught up
again.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-wal-receiver-status-interval" xreflabel="wal_receiver_status_interval">
<term><varname>wal_receiver_status_interval</varname> (<type>integer</type>)
<indexterm>
<primary><varname>wal_receiver_status_interval</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the minimum frequency for the WAL receiver
process on the standby to send information about replication progress
to the primary or upstream standby, where it can be seen using the
<link linkend="monitoring-stats-views-table">
<literal>pg_stat_replication</></link> view. The standby will report
the last transaction log position it has written, the last position it
has flushed to disk, and the last position it has applied.
This parameter's
value is the maximum interval, in seconds, between reports. Updates are
sent each time the write or flush positions change, or at least as
often as specified by this parameter. Thus, the apply position may
lag slightly behind the true position. Setting this parameter to zero
disables status updates completely. This parameter can only be set in
the <filename>postgresql.conf</> file or on the server command line.
The default value is 10 seconds.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-hot-standby-feedback" xreflabel="hot_standby_feedback">
<term><varname>hot_standby_feedback</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>hot_standby_feedback</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies whether or not a hot standby will send feedback to the primary
or upstream standby
about queries currently executing on the standby. This parameter can
be used to eliminate query cancels caused by cleanup records, but
can cause database bloat on the primary for some workloads.
Feedback messages will not be sent more frequently than once per
<varname>wal_receiver_status_interval</>. The default value is
<literal>off</literal>. This parameter can only be set in the
<filename>postgresql.conf</> file or on the server command line.
</para>
<para>
If cascaded replication is in use the feedback is passed upstream
until it eventually reaches the primary. Standbys make no other use
of feedback they receive other than to pass upstream.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-wal-receiver-timeout" xreflabel="wal_receiver_timeout">
<term><varname>wal_receiver_timeout</varname> (<type>integer</type>)
<indexterm>
<primary><varname>wal_receiver_timeout</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Terminate replication connections that are inactive longer
than the specified number of milliseconds. This is useful for
the receiving standby server to detect a primary node crash or network
outage.
A value of zero disables the timeout mechanism. This parameter
can only be set in
the <filename>postgresql.conf</> file or on the server command line.
The default value is 60 seconds.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
</sect1>
<sect1 id="runtime-config-query">
<title>Query Planning</title>
<sect2 id="runtime-config-query-enable">
<title>Planner Method Configuration</title>
<para>
These configuration parameters provide a crude method of
influencing the query plans chosen by the query optimizer. If
the default plan chosen by the optimizer for a particular query
is not optimal, a <emphasis>temporary</> solution is to use one
of these configuration parameters to force the optimizer to
choose a different plan.
Better ways to improve the quality of the
plans chosen by the optimizer include adjusting the planer cost
constants (see <xref linkend="runtime-config-query-constants">),
running <xref linkend="sql-analyze"> manually, increasing
the value of the <xref
linkend="guc-default-statistics-target"> configuration parameter,
and increasing the amount of statistics collected for
specific columns using <command>ALTER TABLE SET
STATISTICS</command>.
</para>
<variablelist>
<varlistentry id="guc-enable-bitmapscan" xreflabel="enable_bitmapscan">
<term><varname>enable_bitmapscan</varname> (<type>boolean</type>)
<indexterm>
<primary>bitmap scan</primary>
</indexterm>
<indexterm>
<primary><varname>enable_bitmapscan</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Enables or disables the query planner's use of bitmap-scan plan
types. The default is <literal>on</>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-enable-hashagg" xreflabel="enable_hashagg">
<term><varname>enable_hashagg</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>enable_hashagg</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Enables or disables the query planner's use of hashed
aggregation plan types. The default is <literal>on</>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-enable-hashjoin" xreflabel="enable_hashjoin">
<term><varname>enable_hashjoin</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>enable_hashjoin</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Enables or disables the query planner's use of hash-join plan
types. The default is <literal>on</>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-enable-indexscan" xreflabel="enable_indexscan">
<term><varname>enable_indexscan</varname> (<type>boolean</type>)
<indexterm>
<primary>index scan</primary>
</indexterm>
<indexterm>
<primary><varname>enable_indexscan</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Enables or disables the query planner's use of index-scan plan
types. The default is <literal>on</>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-enable-indexonlyscan" xreflabel="enable_indexonlyscan">
<term><varname>enable_indexonlyscan</varname> (<type>boolean</type>)
<indexterm>
<primary>index-only scan</primary>
</indexterm>
<indexterm>
<primary><varname>enable_indexonlyscan</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Enables or disables the query planner's use of index-only-scan plan
types. The default is <literal>on</>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-enable-material" xreflabel="enable_material">
<term><varname>enable_material</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>enable_material</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Enables or disables the query planner's use of materialization.
It is impossible to suppress materialization entirely,
but turning this variable off prevents the planner from inserting
materialize nodes except in cases where it is required for correctness.
The default is <literal>on</>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-enable-mergejoin" xreflabel="enable_mergejoin">
<term><varname>enable_mergejoin</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>enable_mergejoin</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Enables or disables the query planner's use of merge-join plan
types. The default is <literal>on</>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-enable-nestloop" xreflabel="enable_nestloop">
<term><varname>enable_nestloop</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>enable_nestloop</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Enables or disables the query planner's use of nested-loop join
plans. It is impossible to suppress nested-loop joins entirely,
but turning this variable off discourages the planner from using
one if there are other methods available. The default is
<literal>on</>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-enable-seqscan" xreflabel="enable_seqscan">
<term><varname>enable_seqscan</varname> (<type>boolean</type>)
<indexterm>
<primary>sequential scan</primary>
</indexterm>
<indexterm>
<primary><varname>enable_seqscan</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Enables or disables the query planner's use of sequential scan
plan types. It is impossible to suppress sequential scans
entirely, but turning this variable off discourages the planner
from using one if there are other methods available. The
default is <literal>on</>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-enable-sort" xreflabel="enable_sort">
<term><varname>enable_sort</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>enable_sort</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Enables or disables the query planner's use of explicit sort
steps. It is impossible to suppress explicit sorts entirely,
but turning this variable off discourages the planner from
using one if there are other methods available. The default
is <literal>on</>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-enable-tidscan" xreflabel="enable_tidscan">
<term><varname>enable_tidscan</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>enable_tidscan</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Enables or disables the query planner's use of <acronym>TID</>
scan plan types. The default is <literal>on</>.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="runtime-config-query-constants">
<title>Planner Cost Constants</title>
<para>
The <firstterm>cost</> variables described in this section are measured
on an arbitrary scale. Only their relative values matter, hence
scaling them all up or down by the same factor will result in no change
in the planner's choices. By default, these cost variables are based on
the cost of sequential page fetches; that is,
<varname>seq_page_cost</> is conventionally set to <literal>1.0</>
and the other cost variables are set with reference to that. But
you can use a different scale if you prefer, such as actual execution
times in milliseconds on a particular machine.
</para>
<note>
<para>
Unfortunately, there is no well-defined method for determining ideal
values for the cost variables. They are best treated as averages over
the entire mix of queries that a particular installation will receive. This
means that changing them on the basis of just a few experiments is very
risky.
</para>
</note>
<variablelist>
<varlistentry id="guc-seq-page-cost" xreflabel="seq_page_cost">
<term><varname>seq_page_cost</varname> (<type>floating point</type>)
<indexterm>
<primary><varname>seq_page_cost</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the planner's estimate of the cost of a disk page fetch
that is part of a series of sequential fetches. The default is 1.0.
This value can be overridden for tables and indexes in a particular
tablespace by setting the tablespace parameter of the same name
(see <xref linkend="sql-altertablespace">).
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-random-page-cost" xreflabel="random_page_cost">
<term><varname>random_page_cost</varname> (<type>floating point</type>)
<indexterm>
<primary><varname>random_page_cost</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the planner's estimate of the cost of a
non-sequentially-fetched disk page. The default is 4.0.
This value can be overridden for tables and indexes in a particular
tablespace by setting the tablespace parameter of the same name
(see <xref linkend="sql-altertablespace">).
</para>
<para>
Reducing this value relative to <varname>seq_page_cost</>
will cause the system to prefer index scans; raising it will
make index scans look relatively more expensive. You can raise
or lower both values together to change the importance of disk I/O
costs relative to CPU costs, which are described by the following
parameters.
</para>
<para>
Random access to mechanical disk storage is normally much more expensive
than four times sequential access. However, a lower default is used
(4.0) because the majority of random accesses to disk, such as indexed
reads, are assumed to be in cache. The default value can be thought of
as modeling random access as 40 times slower than sequential, while
expecting 90% of random reads to be cached.
</para>
<para>
If you believe a 90% cache rate is an incorrect assumption
for your workload, you can increase random_page_cost to better
reflect the true cost of random storage reads. Correspondingly,
if your data is likely to be completely in cache, such as when
the database is smaller than the total server memory, decreasing
random_page_cost can be appropriate. Storage that has a low random
read cost relative to sequential, e.g. solid-state drives, might
also be better modeled with a lower value for random_page_cost.
</para>
<tip>
<para>
Although the system will let you set <varname>random_page_cost</> to
less than <varname>seq_page_cost</>, it is not physically sensible
to do so. However, setting them equal makes sense if the database
is entirely cached in RAM, since in that case there is no penalty
for touching pages out of sequence. Also, in a heavily-cached
database you should lower both values relative to the CPU parameters,
since the cost of fetching a page already in RAM is much smaller
than it would normally be.
</para>
</tip>
</listitem>
</varlistentry>
<varlistentry id="guc-cpu-tuple-cost" xreflabel="cpu_tuple_cost">
<term><varname>cpu_tuple_cost</varname> (<type>floating point</type>)
<indexterm>
<primary><varname>cpu_tuple_cost</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the planner's estimate of the cost of processing
each row during a query.
The default is 0.01.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-cpu-index-tuple-cost" xreflabel="cpu_index_tuple_cost">
<term><varname>cpu_index_tuple_cost</varname> (<type>floating point</type>)
<indexterm>
<primary><varname>cpu_index_tuple_cost</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the planner's estimate of the cost of processing
each index entry during an index scan.
The default is 0.005.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-cpu-operator-cost" xreflabel="cpu_operator_cost">
<term><varname>cpu_operator_cost</varname> (<type>floating point</type>)
<indexterm>
<primary><varname>cpu_operator_cost</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the planner's estimate of the cost of processing each
operator or function executed during a query.
The default is 0.0025.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-effective-cache-size" xreflabel="effective_cache_size">
<term><varname>effective_cache_size</varname> (<type>integer</type>)
<indexterm>
<primary><varname>effective_cache_size</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the planner's assumption about the effective size of the
disk cache that is available to a single query. This is
factored into estimates of the cost of using an index; a
higher value makes it more likely index scans will be used, a
lower value makes it more likely sequential scans will be
used. When setting this parameter you should consider both
<productname>PostgreSQL</productname>'s shared buffers and the
portion of the kernel's disk cache that will be used for
<productname>PostgreSQL</productname> data files. Also, take
into account the expected number of concurrent queries on different
tables, since they will have to share the available
space. This parameter has no effect on the size of shared
memory allocated by <productname>PostgreSQL</productname>, nor
does it reserve kernel disk cache; it is used only for estimation
purposes. The system also does not assume data remains in
the disk cache between queries. The default is 4 gigabytes
(<literal>4GB</>).
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="runtime-config-query-geqo">
<title>Genetic Query Optimizer</title>
<para>
The genetic query optimizer (GEQO) is an algorithm that does query
planning using heuristic searching. This reduces planning time for
complex queries (those joining many relations), at the cost of producing
plans that are sometimes inferior to those found by the normal
exhaustive-search algorithm.
For more information see <xref linkend="geqo">.
</para>
<variablelist>
<varlistentry id="guc-geqo" xreflabel="geqo">
<term><varname>geqo</varname> (<type>boolean</type>)
<indexterm>
<primary>genetic query optimization</primary>
</indexterm>
<indexterm>
<primary>GEQO</primary>
<see>genetic query optimization</see>
</indexterm>
<indexterm>
<primary><varname>geqo</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Enables or disables genetic query optimization.
This is on by default. It is usually best not to turn it off in
production; the <varname>geqo_threshold</varname> variable provides
more granular control of GEQO.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-geqo-threshold" xreflabel="geqo_threshold">
<term><varname>geqo_threshold</varname> (<type>integer</type>)
<indexterm>
<primary><varname>geqo_threshold</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Use genetic query optimization to plan queries with at least
this many <literal>FROM</> items involved. (Note that a
<literal>FULL OUTER JOIN</> construct counts as only one <literal>FROM</>
item.) The default is 12. For simpler queries it is usually best
to use the regular, exhaustive-search planner, but for queries with
many tables the exhaustive search takes too long, often
longer than the penalty of executing a suboptimal plan. Thus,
a threshold on the size of the query is a convenient way to manage
use of GEQO.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-geqo-effort" xreflabel="geqo_effort">
<term><varname>geqo_effort</varname> (<type>integer</type>)
<indexterm>
<primary><varname>geqo_effort</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Controls the trade-off between planning time and query plan
quality in GEQO. This variable must be an integer in the
range from 1 to 10. The default value is five. Larger values
increase the time spent doing query planning, but also
increase the likelihood that an efficient query plan will be
chosen.
</para>
<para>
<varname>geqo_effort</varname> doesn't actually do anything
directly; it is only used to compute the default values for
the other variables that influence GEQO behavior (described
below). If you prefer, you can set the other parameters by
hand instead.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-geqo-pool-size" xreflabel="geqo_pool_size">
<term><varname>geqo_pool_size</varname> (<type>integer</type>)
<indexterm>
<primary><varname>geqo_pool_size</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Controls the pool size used by GEQO, that is the
number of individuals in the genetic population. It must be
at least two, and useful values are typically 100 to 1000. If
it is set to zero (the default setting) then a suitable
value is chosen based on <varname>geqo_effort</varname> and
the number of tables in the query.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-geqo-generations" xreflabel="geqo_generations">
<term><varname>geqo_generations</varname> (<type>integer</type>)
<indexterm>
<primary><varname>geqo_generations</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Controls the number of generations used by GEQO, that is
the number of iterations of the algorithm. It must
be at least one, and useful values are in the same range as
the pool size. If it is set to zero (the default setting)
then a suitable value is chosen based on
<varname>geqo_pool_size</varname>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-geqo-selection-bias" xreflabel="geqo_selection_bias">
<term><varname>geqo_selection_bias</varname> (<type>floating point</type>)
<indexterm>
<primary><varname>geqo_selection_bias</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Controls the selection bias used by GEQO. The selection bias
is the selective pressure within the population. Values can be
from 1.50 to 2.00; the latter is the default.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-geqo-seed" xreflabel="geqo_seed">
<term><varname>geqo_seed</varname> (<type>floating point</type>)
<indexterm>
<primary><varname>geqo_seed</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Controls the initial value of the random number generator used
by GEQO to select random paths through the join order search space.
The value can range from zero (the default) to one. Varying the
value changes the set of join paths explored, and may result in a
better or worse best path being found.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="runtime-config-query-other">
<title>Other Planner Options</title>
<variablelist>
<varlistentry id="guc-default-statistics-target" xreflabel="default_statistics_target">
<term><varname>default_statistics_target</varname> (<type>integer</type>)
<indexterm>
<primary><varname>default_statistics_target</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the default statistics target for table columns without
a column-specific target set via <command>ALTER TABLE
SET STATISTICS</>. Larger values increase the time needed to
do <command>ANALYZE</>, but might improve the quality of the
planner's estimates. The default is 100. For more information
on the use of statistics by the <productname>PostgreSQL</>
query planner, refer to <xref linkend="planner-stats">.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-constraint-exclusion" xreflabel="constraint_exclusion">
<term><varname>constraint_exclusion</varname> (<type>enum</type>)
<indexterm>
<primary>constraint exclusion</primary>
</indexterm>
<indexterm>
<primary><varname>constraint_exclusion</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Controls the query planner's use of table constraints to
optimize queries.
The allowed values of <varname>constraint_exclusion</> are
<literal>on</> (examine constraints for all tables),
<literal>off</> (never examine constraints), and
<literal>partition</> (examine constraints only for inheritance child
tables and <literal>UNION ALL</> subqueries).
<literal>partition</> is the default setting.
It is often used with inheritance and partitioned tables to
improve performance.
</para>
<para>
When this parameter allows it for a particular table, the planner
compares query conditions with the table's <literal>CHECK</>
constraints, and omits scanning tables for which the conditions
contradict the constraints. For example:
<programlisting>
CREATE TABLE parent(key integer, ...);
CREATE TABLE child1000(check (key between 1000 and 1999)) INHERITS(parent);
CREATE TABLE child2000(check (key between 2000 and 2999)) INHERITS(parent);
...
SELECT * FROM parent WHERE key = 2400;
</programlisting>
With constraint exclusion enabled, this <command>SELECT</>
will not scan <structname>child1000</> at all, improving performance.
</para>
<para>
Currently, constraint exclusion is enabled by default
only for cases that are often used to implement table partitioning.
Turning it on for all tables imposes extra planning overhead that is
quite noticeable on simple queries, and most often will yield no
benefit for simple queries. If you have no partitioned tables
you might prefer to turn it off entirely.
</para>
<para>
Refer to <xref linkend="ddl-partitioning-constraint-exclusion"> for
more information on using constraint exclusion and partitioning.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-cursor-tuple-fraction" xreflabel="cursor_tuple_fraction">
<term><varname>cursor_tuple_fraction</varname> (<type>floating point</type>)
<indexterm>
<primary><varname>cursor_tuple_fraction</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the planner's estimate of the fraction of a cursor's rows that
will be retrieved. The default is 0.1. Smaller values of this
setting bias the planner towards using <quote>fast start</> plans
for cursors, which will retrieve the first few rows quickly while
perhaps taking a long time to fetch all rows. Larger values
put more emphasis on the total estimated time. At the maximum
setting of 1.0, cursors are planned exactly like regular queries,
considering only the total estimated time and not how soon the
first rows might be delivered.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-from-collapse-limit" xreflabel="from_collapse_limit">
<term><varname>from_collapse_limit</varname> (<type>integer</type>)
<indexterm>
<primary><varname>from_collapse_limit</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
The planner will merge sub-queries into upper queries if the
resulting <literal>FROM</literal> list would have no more than
this many items. Smaller values reduce planning time but might
yield inferior query plans. The default is eight.
For more information see <xref linkend="explicit-joins">.
</para>
<para>
Setting this value to <xref linkend="guc-geqo-threshold"> or more
may trigger use of the GEQO planner, resulting in non-optimal
plans. See <xref linkend="runtime-config-query-geqo">.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-join-collapse-limit" xreflabel="join_collapse_limit">
<term><varname>join_collapse_limit</varname> (<type>integer</type>)
<indexterm>
<primary><varname>join_collapse_limit</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
The planner will rewrite explicit <literal>JOIN</>
constructs (except <literal>FULL JOIN</>s) into lists of
<literal>FROM</> items whenever a list of no more than this many items
would result. Smaller values reduce planning time but might
yield inferior query plans.
</para>
<para>
By default, this variable is set the same as
<varname>from_collapse_limit</varname>, which is appropriate
for most uses. Setting it to 1 prevents any reordering of
explicit <literal>JOIN</>s. Thus, the explicit join order
specified in the query will be the actual order in which the
relations are joined. Because the query planner does not always choose
the optimal join order, advanced users can elect to
temporarily set this variable to 1, and then specify the join
order they desire explicitly.
For more information see <xref linkend="explicit-joins">.
</para>
<para>
Setting this value to <xref linkend="guc-geqo-threshold"> or more
may trigger use of the GEQO planner, resulting in non-optimal
plans. See <xref linkend="runtime-config-query-geqo">.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
</sect1>
<sect1 id="runtime-config-logging">
<title>Error Reporting and Logging</title>
<indexterm zone="runtime-config-logging">
<primary>server log</primary>
</indexterm>
<sect2 id="runtime-config-logging-where">
<title>Where To Log</title>
<indexterm zone="runtime-config-logging-where">
<primary>where to log</primary>
</indexterm>
<variablelist>
<varlistentry id="guc-log-destination" xreflabel="log_destination">
<term><varname>log_destination</varname> (<type>string</type>)
<indexterm>
<primary><varname>log_destination</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
<productname>PostgreSQL</productname> supports several methods
for logging server messages, including
<systemitem>stderr</systemitem>, <systemitem>csvlog</systemitem> and
<systemitem>syslog</systemitem>. On Windows,
<systemitem>eventlog</systemitem> is also supported. Set this
parameter to a list of desired log destinations separated by
commas. The default is to log to <systemitem>stderr</systemitem>
only.
This parameter can only be set in the <filename>postgresql.conf</>
file or on the server command line.
</para>
<para>
If <systemitem>csvlog</> is included in <varname>log_destination</>,
log entries are output in <quote>comma separated
value</> (<acronym>CSV</>) format, which is convenient for
loading logs into programs.
See <xref linkend="runtime-config-logging-csvlog"> for details.
<xref linkend="guc-logging-collector"> must be enabled to generate
CSV-format log output.
</para>
<note>
<para>
On most Unix systems, you will need to alter the configuration of
your system's <application>syslog</application> daemon in order
to make use of the <systemitem>syslog</systemitem> option for
<varname>log_destination</>. <productname>PostgreSQL</productname>
can log to <application>syslog</application> facilities
<literal>LOCAL0</> through <literal>LOCAL7</> (see <xref
linkend="guc-syslog-facility">), but the default
<application>syslog</application> configuration on most platforms
will discard all such messages. You will need to add something like:
<programlisting>
local0.* /var/log/postgresql
</programlisting>
to the <application>syslog</application> daemon's configuration file
to make it work.
</para>
<para>
On Windows, when you use the <literal>eventlog</literal>
option for <varname>log_destination</>, you should
register an event source and its library with the operating
system so that the Windows Event Viewer can display event
log messages cleanly.
See <xref linkend="event-log-registration"> for details.
</para>
</note>
</listitem>
</varlistentry>
<varlistentry id="guc-logging-collector" xreflabel="logging_collector">
<term><varname>logging_collector</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>logging_collector</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
This parameter enables the <firstterm>logging collector</>, which
is a background process that captures log messages
sent to <systemitem>stderr</> and redirects them into log files.
This approach is often more useful than
logging to <application>syslog</>, since some types of messages
might not appear in <application>syslog</> output. (One common
example is dynamic-linker failure messages; another is error messages
produced by scripts such as <varname>archive_command</>.)
This parameter can only be set at server start.
</para>
<note>
<para>
It is possible to log to <systemitem>stderr</> without using the
logging collector; the log messages will just go to wherever the
server's <systemitem>stderr</> is directed. However, that method is
only suitable for low log volumes, since it provides no convenient
way to rotate log files. Also, on some platforms not using the
logging collector can result in lost or garbled log output, because
multiple processes writing concurrently to the same log file can
overwrite each other's output.
</para>
</note>
<note>
<para>
The logging collector is designed to never lose messages. This means
that in case of extremely high load, server processes could be
blocked while trying to send additional log messages when the
collector has fallen behind. In contrast, <application>syslog</>
prefers to drop messages if it cannot write them, which means it
may fail to log some messages in such cases but it will not block
the rest of the system.
</para>
</note>
</listitem>
</varlistentry>
<varlistentry id="guc-log-directory" xreflabel="log_directory">
<term><varname>log_directory</varname> (<type>string</type>)
<indexterm>
<primary><varname>log_directory</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
When <varname>logging_collector</> is enabled,
this parameter determines the directory in which log files will be created.
It can be specified as an absolute path, or relative to the
cluster data directory.
This parameter can only be set in the <filename>postgresql.conf</>
file or on the server command line.
The default is <literal>pg_log</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-log-filename" xreflabel="log_filename">
<term><varname>log_filename</varname> (<type>string</type>)
<indexterm>
<primary><varname>log_filename</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
When <varname>logging_collector</varname> is enabled,
this parameter sets the file names of the created log files. The value
is treated as a <function>strftime</function> pattern,
so <literal>%</literal>-escapes can be used to specify time-varying
file names. (Note that if there are
any time-zone-dependent <literal>%</literal>-escapes, the computation
is done in the zone specified
by <xref linkend="guc-log-timezone">.)
The supported <literal>%</literal>-escapes are similar to those
listed in the Open Group's <ulink
url="http://pubs.opengroup.org/onlinepubs/009695399/functions/strftime.html">strftime
</ulink> specification.
Note that the system's <systemitem>strftime</systemitem> is not used
directly, so platform-specific (nonstandard) extensions do not work.
The default is <literal>postgresql-%Y-%m-%d_%H%M%S.log</literal>.
</para>
<para>
If you specify a file name without escapes, you should plan to
use a log rotation utility to avoid eventually filling the
entire disk. In releases prior to 8.4, if
no <literal>%</literal> escapes were
present, <productname>PostgreSQL</productname> would append
the epoch of the new log file's creation time, but this is no
longer the case.
</para>
<para>
If CSV-format output is enabled in <varname>log_destination</>,
<literal>.csv</> will be appended to the timestamped
log file name to create the file name for CSV-format output.
(If <varname>log_filename</> ends in <literal>.log</>, the suffix is
replaced instead.)
</para>
<para>
This parameter can only be set in the <filename>postgresql.conf</>
file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-log-file-mode" xreflabel="log_file_mode">
<term><varname>log_file_mode</varname> (<type>integer</type>)
<indexterm>
<primary><varname>log_file_mode</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
On Unix systems this parameter sets the permissions for log files
when <varname>logging_collector</varname> is enabled. (On Microsoft
Windows this parameter is ignored.)
The parameter value is expected to be a numeric mode
specified in the format accepted by the
<function>chmod</function> and <function>umask</function>
system calls. (To use the customary octal format the number
must start with a <literal>0</literal> (zero).)
</para>
<para>
The default permissions are <literal>0600</>, meaning only the
server owner can read or write the log files. The other commonly
useful setting is <literal>0640</>, allowing members of the owner's
group to read the files. Note however that to make use of such a
setting, you'll need to alter <xref linkend="guc-log-directory"> to
store the files somewhere outside the cluster data directory. In
any case, it's unwise to make the log files world-readable, since
they might contain sensitive data.
</para>
<para>
This parameter can only be set in the <filename>postgresql.conf</>
file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-log-rotation-age" xreflabel="log_rotation_age">
<term><varname>log_rotation_age</varname> (<type>integer</type>)
<indexterm>
<primary><varname>log_rotation_age</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
When <varname>logging_collector</varname> is enabled,
this parameter determines the maximum lifetime of an individual log file.
After this many minutes have elapsed, a new log file will
be created. Set to zero to disable time-based creation of
new log files.
This parameter can only be set in the <filename>postgresql.conf</>
file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-log-rotation-size" xreflabel="log_rotation_size">
<term><varname>log_rotation_size</varname> (<type>integer</type>)
<indexterm>
<primary><varname>log_rotation_size</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
When <varname>logging_collector</varname> is enabled,
this parameter determines the maximum size of an individual log file.
After this many kilobytes have been emitted into a log file,
a new log file will be created. Set to zero to disable size-based
creation of new log files.
This parameter can only be set in the <filename>postgresql.conf</>
file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-log-truncate-on-rotation" xreflabel="log_truncate_on_rotation">
<term><varname>log_truncate_on_rotation</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>log_truncate_on_rotation</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
When <varname>logging_collector</varname> is enabled,
this parameter will cause <productname>PostgreSQL</productname> to truncate (overwrite),
rather than append to, any existing log file of the same name.
However, truncation will occur only when a new file is being opened
due to time-based rotation, not during server startup or size-based
rotation. When off, pre-existing files will be appended to in
all cases. For example, using this setting in combination with
a <varname>log_filename</varname> like <literal>postgresql-%H.log</literal>
would result in generating twenty-four hourly log files and then
cyclically overwriting them.
This parameter can only be set in the <filename>postgresql.conf</>
file or on the server command line.
</para>
<para>
Example: To keep 7 days of logs, one log file per day named
<literal>server_log.Mon</literal>, <literal>server_log.Tue</literal>,
etc, and automatically overwrite last week's log with this week's log,
set <varname>log_filename</varname> to <literal>server_log.%a</literal>,
<varname>log_truncate_on_rotation</varname> to <literal>on</literal>, and
<varname>log_rotation_age</varname> to <literal>1440</literal>.
</para>
<para>
Example: To keep 24 hours of logs, one log file per hour, but
also rotate sooner if the log file size exceeds 1GB, set
<varname>log_filename</varname> to <literal>server_log.%H%M</literal>,
<varname>log_truncate_on_rotation</varname> to <literal>on</literal>,
<varname>log_rotation_age</varname> to <literal>60</literal>, and
<varname>log_rotation_size</varname> to <literal>1000000</literal>.
Including <literal>%M</> in <varname>log_filename</varname> allows
any size-driven rotations that might occur to select a file name
different from the hour's initial file name.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-syslog-facility" xreflabel="syslog_facility">
<term><varname>syslog_facility</varname> (<type>enum</type>)
<indexterm>
<primary><varname>syslog_facility</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
When logging to <application>syslog</> is enabled, this parameter
determines the <application>syslog</application>
<quote>facility</quote> to be used. You can choose
from <literal>LOCAL0</>, <literal>LOCAL1</>,
<literal>LOCAL2</>, <literal>LOCAL3</>, <literal>LOCAL4</>,
<literal>LOCAL5</>, <literal>LOCAL6</>, <literal>LOCAL7</>;
the default is <literal>LOCAL0</>. See also the
documentation of your system's
<application>syslog</application> daemon.
This parameter can only be set in the <filename>postgresql.conf</>
file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-syslog-ident" xreflabel="syslog_ident">
<term><varname>syslog_ident</varname> (<type>string</type>)
<indexterm>
<primary><varname>syslog_ident</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
When logging to <application>syslog</> is enabled, this parameter
determines the program name used to identify
<productname>PostgreSQL</productname> messages in
<application>syslog</application> logs. The default is
<literal>postgres</literal>.
This parameter can only be set in the <filename>postgresql.conf</>
file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-event-source" xreflabel="event_source">
<term><varname>event_source</varname> (<type>string</type>)
<indexterm>
<primary><varname>event_source</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
When logging to <application>event log</> is enabled, this parameter
determines the program name used to identify
<productname>PostgreSQL</productname> messages in
the log. The default is <literal>PostgreSQL</literal>.
This parameter can only be set in the <filename>postgresql.conf</>
file or on the server command line.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="runtime-config-logging-when">
<title>When To Log</title>
<variablelist>
<varlistentry id="guc-client-min-messages" xreflabel="client_min_messages">
<term><varname>client_min_messages</varname> (<type>enum</type>)
<indexterm>
<primary><varname>client_min_messages</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Controls which message levels are sent to the client.
Valid values are <literal>DEBUG5</>,
<literal>DEBUG4</>, <literal>DEBUG3</>, <literal>DEBUG2</>,
<literal>DEBUG1</>, <literal>LOG</>, <literal>NOTICE</>,
<literal>WARNING</>, <literal>ERROR</>, <literal>FATAL</>,
and <literal>PANIC</>. Each level
includes all the levels that follow it. The later the level,
the fewer messages are sent. The default is
<literal>NOTICE</>. Note that <literal>LOG</> has a different
rank here than in <varname>log_min_messages</>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-log-min-messages" xreflabel="log_min_messages">
<term><varname>log_min_messages</varname> (<type>enum</type>)
<indexterm>
<primary><varname>log_min_messages</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Controls which message levels are written to the server log.
Valid values are <literal>DEBUG5</>, <literal>DEBUG4</>,
<literal>DEBUG3</>, <literal>DEBUG2</>, <literal>DEBUG1</>,
<literal>INFO</>, <literal>NOTICE</>, <literal>WARNING</>,
<literal>ERROR</>, <literal>LOG</>, <literal>FATAL</>, and
<literal>PANIC</>. Each level includes all the levels that
follow it. The later the level, the fewer messages are sent
to the log. The default is <literal>WARNING</>. Note that
<literal>LOG</> has a different rank here than in
<varname>client_min_messages</>.
Only superusers can change this setting.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-log-min-error-statement" xreflabel="log_min_error_statement">
<term><varname>log_min_error_statement</varname> (<type>enum</type>)
<indexterm>
<primary><varname>log_min_error_statement</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Controls which SQL statements that cause an error
condition are recorded in the server log. The current
SQL statement is included in the log entry for any message of
the specified severity or higher.
Valid values are <literal>DEBUG5</literal>,
<literal>DEBUG4</literal>, <literal>DEBUG3</literal>,
<literal>DEBUG2</literal>, <literal>DEBUG1</literal>,
<literal>INFO</literal>, <literal>NOTICE</literal>,
<literal>WARNING</literal>, <literal>ERROR</literal>,
<literal>LOG</literal>,
<literal>FATAL</literal>, and <literal>PANIC</literal>.
The default is <literal>ERROR</literal>, which means statements
causing errors, log messages, fatal errors, or panics will be logged.
To effectively turn off logging of failing statements,
set this parameter to <literal>PANIC</literal>.
Only superusers can change this setting.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-log-min-duration-statement" xreflabel="log_min_duration_statement">
<term><varname>log_min_duration_statement</varname> (<type>integer</type>)
<indexterm>
<primary><varname>log_min_duration_statement</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Causes the duration of each completed statement to be logged
if the statement ran for at least the specified number of
milliseconds. Setting this to zero prints all statement durations.
Minus-one (the default) disables logging statement durations.
For example, if you set it to <literal>250ms</literal>
then all SQL statements that run 250ms or longer will be
logged. Enabling this parameter can be helpful in tracking down
unoptimized queries in your applications.
Only superusers can change this setting.
</para>
<para>
For clients using extended query protocol, durations of the Parse,
Bind, and Execute steps are logged independently.
</para>
<note>
<para>
When using this option together with
<xref linkend="guc-log-statement">,
the text of statements that are logged because of
<varname>log_statement</> will not be repeated in the
duration log message.
If you are not using <application>syslog</>, it is recommended
that you log the PID or session ID using
<xref linkend="guc-log-line-prefix">
so that you can link the statement message to the later
duration message using the process ID or session ID.
</para>
</note>
</listitem>
</varlistentry>
</variablelist>
<para>
<xref linkend="runtime-config-severity-levels"> explains the message
severity levels used by <productname>PostgreSQL</>. If logging output
is sent to <systemitem>syslog</systemitem> or Windows'
<systemitem>eventlog</systemitem>, the severity levels are translated
as shown in the table.
</para>
<table id="runtime-config-severity-levels">
<title>Message Severity Levels</title>
<tgroup cols="4">
<thead>
<row>
<entry>Severity</entry>
<entry>Usage</entry>
<entry><systemitem>syslog</></entry>
<entry><systemitem>eventlog</></entry>
</row>
</thead>
<tbody>
<row>
<entry><literal>DEBUG1..DEBUG5</></entry>
<entry>Provides successively-more-detailed information for use by
developers.</entry>
<entry><literal>DEBUG</></entry>
<entry><literal>INFORMATION</></entry>
</row>
<row>
<entry><literal>INFO</></entry>
<entry>Provides information implicitly requested by the user,
e.g., output from <command>VACUUM VERBOSE</>.</entry>
<entry><literal>INFO</></entry>
<entry><literal>INFORMATION</></entry>
</row>
<row>
<entry><literal>NOTICE</></entry>
<entry>Provides information that might be helpful to users, e.g.,
notice of truncation of long identifiers.</entry>
<entry><literal>NOTICE</></entry>
<entry><literal>INFORMATION</></entry>
</row>
<row>
<entry><literal>WARNING</></entry>
<entry>Provides warnings of likely problems, e.g., <command>COMMIT</>
outside a transaction block.</entry>
<entry><literal>NOTICE</></entry>
<entry><literal>WARNING</></entry>
</row>
<row>
<entry><literal>ERROR</></entry>
<entry>Reports an error that caused the current command to
abort.</entry>
<entry><literal>WARNING</></entry>
<entry><literal>ERROR</></entry>
</row>
<row>
<entry><literal>LOG</></entry>
<entry>Reports information of interest to administrators, e.g.,
checkpoint activity.</entry>
<entry><literal>INFO</></entry>
<entry><literal>INFORMATION</></entry>
</row>
<row>
<entry><literal>FATAL</></entry>
<entry>Reports an error that caused the current session to
abort.</entry>
<entry><literal>ERR</></entry>
<entry><literal>ERROR</></entry>
</row>
<row>
<entry><literal>PANIC</></entry>
<entry>Reports an error that caused all database sessions to abort.</entry>
<entry><literal>CRIT</></entry>
<entry><literal>ERROR</></entry>
</row>
</tbody>
</tgroup>
</table>
</sect2>
<sect2 id="runtime-config-logging-what">
<title>What To Log</title>
<variablelist>
<varlistentry id="guc-application-name" xreflabel="application_name">
<term><varname>application_name</varname> (<type>string</type>)
<indexterm>
<primary><varname>application_name</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
The <varname>application_name</varname> can be any string of less than
<symbol>NAMEDATALEN</> characters (64 characters in a standard build).
It is typically set by an application upon connection to the server.
The name will be displayed in the <structname>pg_stat_activity</> view
and included in CSV log entries. It can also be included in regular
log entries via the <xref linkend="guc-log-line-prefix"> parameter.
Only printable ASCII characters may be used in the
<varname>application_name</varname> value. Other characters will be
replaced with question marks (<literal>?</literal>).
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-cluster-name" xreflabel="cluster_name">
<term><varname>cluster_name</varname> (<type>string</type>)</term>
<indexterm>
<primary><varname>cluster_name</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Sets the cluster name that appears in the process title for all
processes in this cluster. The name can be any string of less than
<symbol>NAMEDATALEN</> characters (64 characters in a standard
build). Only printable ASCII characters may be used in the
<varname>cluster_name</varname> value. Other characters will be
replaced with question marks (<literal>?</literal>). No name is shown
if this parameter is set to the empty string <literal>''</> (which is
the default). This parameter can only be set at server start.
</para>
<para>
The process title is typically viewed using programs like
<application>ps</> or, on Windows, <application>Process Explorer</>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>debug_print_parse</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>debug_print_parse</> configuration parameter</primary>
</indexterm>
</term>
<term><varname>debug_print_rewritten</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>debug_print_rewritten</> configuration parameter</primary>
</indexterm>
</term>
<term><varname>debug_print_plan</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>debug_print_plan</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
These parameters enable various debugging output to be emitted.
When set, they print the resulting parse tree, the query rewriter
output, or the execution plan for each executed query.
These messages are emitted at <literal>LOG</> message level, so by
default they will appear in the server log but will not be sent to the
client. You can change that by adjusting
<xref linkend="guc-client-min-messages"> and/or
<xref linkend="guc-log-min-messages">.
These parameters are off by default.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>debug_pretty_print</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>debug_pretty_print</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
When set, <varname>debug_pretty_print</varname> indents the messages
produced by <varname>debug_print_parse</varname>,
<varname>debug_print_rewritten</varname>, or
<varname>debug_print_plan</varname>. This results in more readable
but much longer output than the <quote>compact</> format used when
it is off. It is on by default.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-log-checkpoints" xreflabel="log_checkpoints">
<term><varname>log_checkpoints</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>log_checkpoints</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Causes checkpoints and restartpoints to be logged in the server log.
Some statistics are included in the log messages, including the number
of buffers written and the time spent writing them.
This parameter can only be set in the <filename>postgresql.conf</>
file or on the server command line. The default is off.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-log-connections" xreflabel="log_connections">
<term><varname>log_connections</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>log_connections</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Causes each attempted connection to the server to be logged,
as well as successful completion of client authentication.
This parameter cannot be changed after session start.
The default is off.
</para>
<note>
<para>
Some client programs, like <application>psql</>, attempt
to connect twice while determining if a password is required, so
duplicate <quote>connection received</> messages do not
necessarily indicate a problem.
</para>
</note>
</listitem>
</varlistentry>
<varlistentry id="guc-log-disconnections" xreflabel="log_disconnections">
<term><varname>log_disconnections</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>log_disconnections</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
This outputs a line in the server log similar to
<varname>log_connections</varname> but at session termination,
and includes the duration of the session. This is off by
default.
This parameter cannot be changed after session start.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-log-duration" xreflabel="log_duration">
<term><varname>log_duration</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>log_duration</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Causes the duration of every completed statement to be logged.
The default is <literal>off</>.
Only superusers can change this setting.
</para>
<para>
For clients using extended query protocol, durations of the Parse,
Bind, and Execute steps are logged independently.
</para>
<note>
<para>
The difference between setting this option and setting
<xref linkend="guc-log-min-duration-statement"> to zero is that
exceeding <varname>log_min_duration_statement</> forces the text of
the query to be logged, but this option doesn't. Thus, if
<varname>log_duration</> is <literal>on</> and
<varname>log_min_duration_statement</> has a positive value, all
durations are logged but the query text is included only for
statements exceeding the threshold. This behavior can be useful for
gathering statistics in high-load installations.
</para>
</note>
</listitem>
</varlistentry>
<varlistentry id="guc-log-error-verbosity" xreflabel="log_error_verbosity">
<term><varname>log_error_verbosity</varname> (<type>enum</type>)
<indexterm>
<primary><varname>log_error_verbosity</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Controls the amount of detail written in the server log for each
message that is logged. Valid values are <literal>TERSE</>,
<literal>DEFAULT</>, and <literal>VERBOSE</>, each adding more
fields to displayed messages. <literal>TERSE</> excludes
the logging of <literal>DETAIL</>, <literal>HINT</>,
<literal>QUERY</>, and <literal>CONTEXT</> error information.
<literal>VERBOSE</> output includes the <symbol>SQLSTATE</> error
code (see also <xref linkend="errcodes-appendix">) and the source code file name, function name,
and line number that generated the error.
Only superusers can change this setting.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-log-hostname" xreflabel="log_hostname">
<term><varname>log_hostname</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>log_hostname</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
By default, connection log messages only show the IP address of the
connecting host. Turning this parameter on causes logging of the
host name as well. Note that depending on your host name resolution
setup this might impose a non-negligible performance penalty.
This parameter can only be set in the <filename>postgresql.conf</>
file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-log-line-prefix" xreflabel="log_line_prefix">
<term><varname>log_line_prefix</varname> (<type>string</type>)
<indexterm>
<primary><varname>log_line_prefix</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
This is a <function>printf</>-style string that is output at the
beginning of each log line.
<literal>%</> characters begin <quote>escape sequences</>
that are replaced with status information as outlined below.
Unrecognized escapes are ignored. Other
characters are copied straight to the log line. Some escapes are
only recognized by session processes, and will be treated as empty by
background processes such as the main server process. Status
information may be aligned either left or right by specifying a
numeric literal after the % and before the option. A negative
value will cause the status information to be padded on the
right with spaces to give it a minimum width, whereas a positive
value will pad on the left. Padding can be useful to aid human
readability in log files.
This parameter can only be set in the <filename>postgresql.conf</>
file or on the server command line. The default is an empty string.
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Escape</entry>
<entry>Effect</entry>
<entry>Session only</entry>
</row>
</thead>
<tbody>
<row>
<entry><literal>%a</literal></entry>
<entry>Application name</entry>
<entry>yes</entry>
</row>
<row>
<entry><literal>%u</literal></entry>
<entry>User name</entry>
<entry>yes</entry>
</row>
<row>
<entry><literal>%d</literal></entry>
<entry>Database name</entry>
<entry>yes</entry>
</row>
<row>
<entry><literal>%r</literal></entry>
<entry>Remote host name or IP address, and remote port</entry>
<entry>yes</entry>
</row>
<row>
<entry><literal>%h</literal></entry>
<entry>Remote host name or IP address</entry>
<entry>yes</entry>
</row>
<row>
<entry><literal>%p</literal></entry>
<entry>Process ID</entry>
<entry>no</entry>
</row>
<row>
<entry><literal>%t</literal></entry>
<entry>Time stamp without milliseconds</entry>
<entry>no</entry>
</row>
<row>
<entry><literal>%m</literal></entry>
<entry>Time stamp with milliseconds</entry>
<entry>no</entry>
</row>
<row>
<entry><literal>%i</literal></entry>
<entry>Command tag: type of session's current command</entry>
<entry>yes</entry>
</row>
<row>
<entry><literal>%e</literal></entry>
<entry>SQLSTATE error code</entry>
<entry>no</entry>
</row>
<row>
<entry><literal>%c</literal></entry>
<entry>Session ID: see below</entry>
<entry>no</entry>
</row>
<row>
<entry><literal>%l</literal></entry>
<entry>Number of the log line for each session or process, starting at 1</entry>
<entry>no</entry>
</row>
<row>
<entry><literal>%s</literal></entry>
<entry>Process start time stamp</entry>
<entry>no</entry>
</row>
<row>
<entry><literal>%v</literal></entry>
<entry>Virtual transaction ID (backendID/localXID)</entry>
<entry>no</entry>
</row>
<row>
<entry><literal>%x</literal></entry>
<entry>Transaction ID (0 if none is assigned)</entry>
<entry>no</entry>
</row>
<row>
<entry><literal>%q</literal></entry>
<entry>Produces no output, but tells non-session
processes to stop at this point in the string; ignored by
session processes</entry>
<entry>no</entry>
</row>
<row>
<entry><literal>%%</literal></entry>
<entry>Literal <literal>%</></entry>
<entry>no</entry>
</row>
</tbody>
</tgroup>
</informaltable>
The <literal>%c</> escape prints a quasi-unique session identifier,
consisting of two 4-byte hexadecimal numbers (without leading zeros)
separated by a dot. The numbers are the process start time and the
process ID, so <literal>%c</> can also be used as a space saving way
of printing those items. For example, to generate the session
identifier from <literal>pg_stat_activity</>, use this query:
<programlisting>
SELECT to_hex(EXTRACT(EPOCH FROM backend_start)::integer) || '.' ||
to_hex(pid)
FROM pg_stat_activity;
</programlisting>
</para>
<tip>
<para>
If you set a nonempty value for <varname>log_line_prefix</>,
you should usually make its last character be a space, to provide
visual separation from the rest of the log line. A punctuation
character can be used too.
</para>
</tip>
<tip>
<para>
<application>Syslog</> produces its own
time stamp and process ID information, so you probably do not want to
include those escapes if you are logging to <application>syslog</>.
</para>
</tip>
</listitem>
</varlistentry>
<varlistentry id="guc-log-lock-waits" xreflabel="log_lock_waits">
<term><varname>log_lock_waits</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>log_lock_waits</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Controls whether a log message is produced when a session waits
longer than <xref linkend="guc-deadlock-timeout"> to acquire a
lock. This is useful in determining if lock waits are causing
poor performance. The default is <literal>off</>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-log-statement" xreflabel="log_statement">
<term><varname>log_statement</varname> (<type>enum</type>)
<indexterm>
<primary><varname>log_statement</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Controls which SQL statements are logged. Valid values are
<literal>none</> (off), <literal>ddl</>, <literal>mod</>, and
<literal>all</> (all statements). <literal>ddl</> logs all data definition
statements, such as <command>CREATE</>, <command>ALTER</>, and
<command>DROP</> statements. <literal>mod</> logs all
<literal>ddl</> statements, plus data-modifying statements
such as <command>INSERT</>,
<command>UPDATE</>, <command>DELETE</>, <command>TRUNCATE</>,
and <command>COPY FROM</>.
<command>PREPARE</>, <command>EXECUTE</>, and
<command>EXPLAIN ANALYZE</> statements are also logged if their
contained command is of an appropriate type. For clients using
extended query protocol, logging occurs when an Execute message
is received, and values of the Bind parameters are included
(with any embedded single-quote marks doubled).
</para>
<para>
The default is <literal>none</>. Only superusers can change this
setting.
</para>
<note>
<para>
Statements that contain simple syntax errors are not logged
even by the <varname>log_statement</> = <literal>all</> setting,
because the log message is emitted only after basic parsing has
been done to determine the statement type. In the case of extended
query protocol, this setting likewise does not log statements that
fail before the Execute phase (i.e., during parse analysis or
planning). Set <varname>log_min_error_statement</> to
<literal>ERROR</> (or lower) to log such statements.
</para>
</note>
</listitem>
</varlistentry>
<varlistentry id="guc-log-temp-files" xreflabel="log_temp_files">
<term><varname>log_temp_files</varname> (<type>integer</type>)
<indexterm>
<primary><varname>log_temp_files</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Controls logging of temporary file names and sizes.
Temporary files can be
created for sorts, hashes, and temporary query results.
A log entry is made for each temporary file when it is deleted.
A value of zero logs all temporary file information, while positive
values log only files whose size is greater than or equal to
the specified number of kilobytes. The
default setting is -1, which disables such logging.
Only superusers can change this setting.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-log-timezone" xreflabel="log_timezone">
<term><varname>log_timezone</varname> (<type>string</type>)
<indexterm>
<primary><varname>log_timezone</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the time zone used for timestamps written in the server log.
Unlike <xref linkend="guc-timezone">, this value is cluster-wide,
so that all sessions will report timestamps consistently.
The built-in default is <literal>GMT</>, but that is typically
overridden in <filename>postgresql.conf</>; <application>initdb</>
will install a setting there corresponding to its system environment.
See <xref linkend="datatype-timezones"> for more information.
This parameter can only be set in the <filename>postgresql.conf</>
file or on the server command line.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="runtime-config-logging-csvlog">
<title>Using CSV-Format Log Output</title>
<para>
Including <literal>csvlog</> in the <varname>log_destination</> list
provides a convenient way to import log files into a database table.
This option emits log lines in comma-separated-values
(<acronym>CSV</>) format,
with these columns:
time stamp with milliseconds,
user name,
database name,
process ID,
client host:port number,
session ID,
per-session line number,
command tag,
session start time,
virtual transaction ID,
regular transaction ID,
error severity,
SQLSTATE code,
error message,
error message detail,
hint,
internal query that led to the error (if any),
character count of the error position therein,
error context,
user query that led to the error (if any and enabled by
<varname>log_min_error_statement</>),
character count of the error position therein,
location of the error in the PostgreSQL source code
(if <varname>log_error_verbosity</> is set to <literal>verbose</>),
and application name.
Here is a sample table definition for storing CSV-format log output:
<programlisting>
CREATE TABLE postgres_log
(
log_time timestamp(3) with time zone,
user_name text,
database_name text,
process_id integer,
connection_from text,
session_id text,
session_line_num bigint,
command_tag text,
session_start_time timestamp with time zone,
virtual_transaction_id text,
transaction_id bigint,
error_severity text,
sql_state_code text,
message text,
detail text,
hint text,
internal_query text,
internal_query_pos integer,
context text,
query text,
query_pos integer,
location text,
application_name text,
PRIMARY KEY (session_id, session_line_num)
);
</programlisting>
</para>
<para>
To import a log file into this table, use the <command>COPY FROM</>
command:
<programlisting>
COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
</programlisting>
</para>
<para>
There are a few things you need to do to simplify importing CSV log
files:
<orderedlist>
<listitem>
<para>
Set <varname>log_filename</varname> and
<varname>log_rotation_age</> to provide a consistent,
predictable naming scheme for your log files. This lets you
predict what the file name will be and know when an individual log
file is complete and therefore ready to be imported.
</para>
</listitem>
<listitem>
<para>
Set <varname>log_rotation_size</varname> to 0 to disable
size-based log rotation, as it makes the log file name difficult
to predict.
</para>
</listitem>
<listitem>
<para>
Set <varname>log_truncate_on_rotation</varname> to <literal>on</> so
that old log data isn't mixed with the new in the same file.
</para>
</listitem>
<listitem>
<para>
The table definition above includes a primary key specification.
This is useful to protect against accidentally importing the same
information twice. The <command>COPY</> command commits all of the
data it imports at one time, so any error will cause the entire
import to fail. If you import a partial log file and later import
the file again when it is complete, the primary key violation will
cause the import to fail. Wait until the log is complete and
closed before importing. This procedure will also protect against
accidentally importing a partial line that hasn't been completely
written, which would also cause <command>COPY</> to fail.
</para>
</listitem>
</orderedlist>
</para>
</sect2>
</sect1>
<sect1 id="runtime-config-statistics">
<title>Run-time Statistics</title>
<sect2 id="runtime-config-statistics-collector">
<title>Query and Index Statistics Collector</title>
<para>
These parameters control server-wide statistics collection features.
When statistics collection is enabled, the data that is produced can be
accessed via the <structname>pg_stat</structname> and
<structname>pg_statio</structname> family of system views.
Refer to <xref linkend="monitoring"> for more information.
</para>
<variablelist>
<varlistentry id="guc-track-activities" xreflabel="track_activities">
<term><varname>track_activities</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>track_activities</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Enables the collection of information on the currently
executing command of each session, along with the time when
that command began execution. This parameter is on by
default. Note that even when enabled, this information is not
visible to all users, only to superusers and the user owning
the session being reported on, so it should not represent a
security risk.
Only superusers can change this setting.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-track-activity-query-size" xreflabel="track_activity_query_size">
<term><varname>track_activity_query_size</varname> (<type>integer</type>)
<indexterm>
<primary><varname>track_activity_query_size</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the number of bytes reserved to track the currently
executing command for each active session, for the
<structname>pg_stat_activity</>.<structfield>query</> field.
The default value is 1024. This parameter can only be set at server
start.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-track-counts" xreflabel="track_counts">
<term><varname>track_counts</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>track_counts</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Enables collection of statistics on database activity.
This parameter is on by default, because the autovacuum
daemon needs the collected information.
Only superusers can change this setting.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-track-io-timing" xreflabel="track_io_timing">
<term><varname>track_io_timing</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>track_io_timing</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Enables timing of database I/O calls. This parameter is off by
default, because it will repeatedly query the operating system for
the current time, which may cause significant overhead on some
platforms. You can use the <xref linkend="pgtesttiming"> tool to
measure the overhead of timing on your system.
I/O timing information is
displayed in <xref linkend="pg-stat-database-view">, in the output of
<xref linkend="sql-explain"> when the <literal>BUFFERS</> option is
used, and by <xref linkend="pgstatstatements">. Only superusers can
change this setting.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-track-functions" xreflabel="track_functions">
<term><varname>track_functions</varname> (<type>enum</type>)
<indexterm>
<primary><varname>track_functions</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Enables tracking of function call counts and time used. Specify
<literal>pl</literal> to track only procedural-language functions,
<literal>all</literal> to also track SQL and C language functions.
The default is <literal>none</literal>, which disables function
statistics tracking. Only superusers can change this setting.
</para>
<note>
<para>
SQL-language functions that are simple enough to be <quote>inlined</>
into the calling query will not be tracked, regardless of this
setting.
</para>
</note>
</listitem>
</varlistentry>
<varlistentry id="guc-update-process-title" xreflabel="update_process_title">
<term><varname>update_process_title</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>update_process_title</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Enables updating of the process title every time a new SQL command
is received by the server. The process title is typically viewed
by the <command>ps</> command,
or in Windows by using the <application>Process Explorer</>.
Only superusers can change this setting.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-stats-temp-directory" xreflabel="stats_temp_directory">
<term><varname>stats_temp_directory</varname> (<type>string</type>)
<indexterm>
<primary><varname>stats_temp_directory</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the directory to store temporary statistics data in. This can be
a path relative to the data directory or an absolute path. The default
is <filename>pg_stat_tmp</filename>. Pointing this at a RAM-based
file system will decrease physical I/O requirements and can lead to
improved performance.
This parameter can only be set in the <filename>postgresql.conf</>
file or on the server command line.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="runtime-config-statistics-monitor">
<title>Statistics Monitoring</title>
<variablelist>
<varlistentry>
<term><varname>log_statement_stats</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>log_statement_stats</> configuration parameter</primary>
</indexterm>
</term>
<term><varname>log_parser_stats</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>log_parser_stats</> configuration parameter</primary>
</indexterm>
</term>
<term><varname>log_planner_stats</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>log_planner_stats</> configuration parameter</primary>
</indexterm>
</term>
<term><varname>log_executor_stats</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>log_executor_stats</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
For each query, output performance statistics of the respective
module to the server log. This is a crude profiling
instrument, similar to the Unix <function>getrusage()</> operating
system facility. <varname>log_statement_stats</varname> reports total
statement statistics, while the others report per-module statistics.
<varname>log_statement_stats</varname> cannot be enabled together with
any of the per-module options. All of these options are disabled by
default. Only superusers can change these settings.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
</sect1>
<sect1 id="runtime-config-autovacuum">
<title>Automatic Vacuuming</title>
<indexterm>
<primary>autovacuum</primary>
<secondary>configuration parameters</secondary>
</indexterm>
<para>
These settings control the behavior of the <firstterm>autovacuum</>
feature. Refer to <xref linkend="autovacuum"> for
more information.
</para>
<variablelist>
<varlistentry id="guc-autovacuum" xreflabel="autovacuum">
<term><varname>autovacuum</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>autovacuum</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Controls whether the server should run the
autovacuum launcher daemon. This is on by default; however,
<xref linkend="guc-track-counts"> must also be enabled for
autovacuum to work.
This parameter can only be set in the <filename>postgresql.conf</>
file or on the server command line.
</para>
<para>
Note that even when this parameter is disabled, the system
will launch autovacuum processes if necessary to
prevent transaction ID wraparound. See <xref
linkend="vacuum-for-wraparound"> for more information.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-log-autovacuum-min-duration" xreflabel="log_autovacuum_min_duration">
<term><varname>log_autovacuum_min_duration</varname> (<type>integer</type>)
<indexterm>
<primary><varname>log_autovacuum_min_duration</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Causes each action executed by autovacuum to be logged if it ran for at
least the specified number of milliseconds. Setting this to zero logs
all autovacuum actions. Minus-one (the default) disables logging
autovacuum actions. For example, if you set this to
<literal>250ms</literal> then all automatic vacuums and analyzes that run
250ms or longer will be logged. In addition, when this parameter is
set to any value other than <literal>-1</literal>, a message will be
logged if an autovacuum action is skipped due to the existence of a
conflicting lock. Enabling this parameter can be helpful
in tracking autovacuum activity. This setting can only be set in
the <filename>postgresql.conf</> file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-autovacuum-max-workers" xreflabel="autovacuum_max_workers">
<term><varname>autovacuum_max_workers</varname> (<type>integer</type>)
<indexterm>
<primary><varname>autovacuum_max_workers</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the maximum number of autovacuum processes (other than the
autovacuum launcher) which may be running at any one time. The default
is three. This parameter can only be set at server start.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-autovacuum-naptime" xreflabel="autovacuum_naptime">
<term><varname>autovacuum_naptime</varname> (<type>integer</type>)
<indexterm>
<primary><varname>autovacuum_naptime</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the minimum delay between autovacuum runs on any given
database. In each round the daemon examines the
database and issues <command>VACUUM</> and <command>ANALYZE</> commands
as needed for tables in that database. The delay is measured
in seconds, and the default is one minute (<literal>1min</>).
This parameter can only be set in the <filename>postgresql.conf</>
file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-autovacuum-vacuum-threshold" xreflabel="autovacuum_vacuum_threshold">
<term><varname>autovacuum_vacuum_threshold</varname> (<type>integer</type>)
<indexterm>
<primary><varname>autovacuum_vacuum_threshold</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the minimum number of updated or deleted tuples needed
to trigger a <command>VACUUM</> in any one table.
The default is 50 tuples.
This parameter can only be set in the <filename>postgresql.conf</>
file or on the server command line.
This setting can be overridden for individual tables by
changing storage parameters.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-autovacuum-analyze-threshold" xreflabel="autovacuum_analyze_threshold">
<term><varname>autovacuum_analyze_threshold</varname> (<type>integer</type>)
<indexterm>
<primary><varname>autovacuum_analyze_threshold</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the minimum number of inserted, updated or deleted tuples
needed to trigger an <command>ANALYZE</> in any one table.
The default is 50 tuples.
This parameter can only be set in the <filename>postgresql.conf</>
file or on the server command line.
This setting can be overridden for individual tables by
changing storage parameters.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-autovacuum-vacuum-scale-factor" xreflabel="autovacuum_vacuum_scale_factor">
<term><varname>autovacuum_vacuum_scale_factor</varname> (<type>floating point</type>)
<indexterm>
<primary><varname>autovacuum_vacuum_scale_factor</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies a fraction of the table size to add to
<varname>autovacuum_vacuum_threshold</varname>
when deciding whether to trigger a <command>VACUUM</>.
The default is 0.2 (20% of table size).
This parameter can only be set in the <filename>postgresql.conf</>
file or on the server command line.
This setting can be overridden for individual tables by
changing storage parameters.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-autovacuum-analyze-scale-factor" xreflabel="autovacuum_analyze_scale_factor">
<term><varname>autovacuum_analyze_scale_factor</varname> (<type>floating point</type>)
<indexterm>
<primary><varname>autovacuum_analyze_scale_factor</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies a fraction of the table size to add to
<varname>autovacuum_analyze_threshold</varname>
when deciding whether to trigger an <command>ANALYZE</>.
The default is 0.1 (10% of table size).
This parameter can only be set in the <filename>postgresql.conf</>
file or on the server command line.
This setting can be overridden for individual tables by
changing storage parameters.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-autovacuum-freeze-max-age" xreflabel="autovacuum_freeze_max_age">
<term><varname>autovacuum_freeze_max_age</varname> (<type>integer</type>)
<indexterm>
<primary><varname>autovacuum_freeze_max_age</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the maximum age (in transactions) that a table's
<structname>pg_class</>.<structfield>relfrozenxid</> field can
attain before a <command>VACUUM</> operation is forced
to prevent transaction ID wraparound within the table.
Note that the system will launch autovacuum processes to
prevent wraparound even when autovacuum is otherwise disabled.
</para>
<para>
Vacuum also allows removal of old files from the
<filename>pg_clog</> subdirectory, which is why the default
is a relatively low 200 million transactions.
This parameter can only be set at server start, but the setting
can be reduced for individual tables by
changing storage parameters.
For more information see <xref linkend="vacuum-for-wraparound">.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-autovacuum-multixact-freeze-max-age" xreflabel="autovacuum_multixact_freeze_max_age">
<term><varname>autovacuum_multixact_freeze_max_age</varname> (<type>integer</type>)
<indexterm>
<primary><varname>autovacuum_multixact_freeze_max_age</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the maximum age (in multixacts) that a table's
<structname>pg_class</>.<structfield>relminmxid</> field can
attain before a <command>VACUUM</> operation is forced to
prevent multixact ID wraparound within the table.
Note that the system will launch autovacuum processes to
prevent wraparound even when autovacuum is otherwise disabled.
</para>
<para>
Vacuuming multixacts also allows removal of old files from the
<filename>pg_multixact/members</> and <filename>pg_multixact/offsets</>
subdirectories, which is why the default is a relatively low
400 million multixacts.
This parameter can only be set at server start, but the setting
can be reduced for individual tables by changing storage parameters.
For more information see <xref linkend="vacuum-for-multixact-wraparound">.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-autovacuum-vacuum-cost-delay" xreflabel="autovacuum_vacuum_cost_delay">
<term><varname>autovacuum_vacuum_cost_delay</varname> (<type>integer</type>)
<indexterm>
<primary><varname>autovacuum_vacuum_cost_delay</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the cost delay value that will be used in automatic
<command>VACUUM</> operations. If -1 is specified, the regular
<xref linkend="guc-vacuum-cost-delay"> value will be used.
The default value is 20 milliseconds.
This parameter can only be set in the <filename>postgresql.conf</>
file or on the server command line.
This setting can be overridden for individual tables by
changing storage parameters.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-autovacuum-vacuum-cost-limit" xreflabel="autovacuum_vacuum_cost_limit">
<term><varname>autovacuum_vacuum_cost_limit</varname> (<type>integer</type>)
<indexterm>
<primary><varname>autovacuum_vacuum_cost_limit</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the cost limit value that will be used in automatic
<command>VACUUM</> operations. If -1 is specified (which is the
default), the regular
<xref linkend="guc-vacuum-cost-limit"> value will be used. Note that
the value is distributed proportionally among the running autovacuum
workers, if there is more than one, so that the sum of the limits of
each worker never exceeds the limit on this variable.
This parameter can only be set in the <filename>postgresql.conf</>
file or on the server command line.
This setting can be overridden for individual tables by
changing storage parameters.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect1>
<sect1 id="runtime-config-client">
<title>Client Connection Defaults</title>
<sect2 id="runtime-config-client-statement">
<title>Statement Behavior</title>
<variablelist>
<varlistentry id="guc-search-path" xreflabel="search_path">
<term><varname>search_path</varname> (<type>string</type>)
<indexterm>
<primary><varname>search_path</> configuration parameter</primary>
</indexterm>
<indexterm><primary>path</><secondary>for schemas</></>
</term>
<listitem>
<para>
This variable specifies the order in which schemas are searched
when an object (table, data type, function, etc.) is referenced by a
simple name with no schema specified. When there are objects of
identical names in different schemas, the one found first
in the search path is used. An object that is not in any of the
schemas in the search path can only be referenced by specifying
its containing schema with a qualified (dotted) name.
</para>
<para>
The value for <varname>search_path</varname> must be a comma-separated
list of schema names. Any name that is not an existing schema, or is
a schema for which the user does not have <literal>USAGE</>
permission, is silently ignored.
</para>
<para>
If one of the list items is the special name
<literal>$user</literal>, then the schema having the name returned by
<function>SESSION_USER</> is substituted, if there is such a schema
and the user has <literal>USAGE</> permission for it.
(If not, <literal>$user</literal> is ignored.)
</para>
<para>
The system catalog schema, <literal>pg_catalog</>, is always
searched, whether it is mentioned in the path or not. If it is
mentioned in the path then it will be searched in the specified
order. If <literal>pg_catalog</> is not in the path then it will
be searched <emphasis>before</> searching any of the path items.
</para>
<para>
Likewise, the current session's temporary-table schema,
<literal>pg_temp_<replaceable>nnn</></>, is always searched if it
exists. It can be explicitly listed in the path by using the
alias <literal>pg_temp</><indexterm><primary>pg_temp</></>. If it is not listed in the path then
it is searched first (even before <literal>pg_catalog</>). However,
the temporary schema is only searched for relation (table, view,
sequence, etc) and data type names. It is never searched for
function or operator names.
</para>
<para>
When objects are created without specifying a particular target
schema, they will be placed in the first valid schema named in
<varname>search_path</varname>. An error is reported if the search
path is empty.
</para>
<para>
The default value for this parameter is
<literal>"$user", public</literal>.
This setting supports shared use of a database (where no users
have private schemas, and all share use of <literal>public</>),
private per-user schemas, and combinations of these. Other
effects can be obtained by altering the default search path
setting, either globally or per-user.
</para>
<para>
The current effective value of the search path can be examined
via the <acronym>SQL</acronym> function
<function>current_schemas</>
(see <xref linkend="functions-info">).
This is not quite the same as
examining the value of <varname>search_path</varname>, since
<function>current_schemas</> shows how the items
appearing in <varname>search_path</varname> were resolved.
</para>
<para>
For more information on schema handling, see <xref linkend="ddl-schemas">.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-default-tablespace" xreflabel="default_tablespace">
<term><varname>default_tablespace</varname> (<type>string</type>)
<indexterm>
<primary><varname>default_tablespace</> configuration parameter</primary>
</indexterm>
<indexterm><primary>tablespace</><secondary>default</></>
</term>
<listitem>
<para>
This variable specifies the default tablespace in which to create
objects (tables and indexes) when a <command>CREATE</> command does
not explicitly specify a tablespace.
</para>
<para>
The value is either the name of a tablespace, or an empty string
to specify using the default tablespace of the current database.
If the value does not match the name of any existing tablespace,
<productname>PostgreSQL</> will automatically use the default
tablespace of the current database. If a nondefault tablespace
is specified, the user must have <literal>CREATE</> privilege
for it, or creation attempts will fail.
</para>
<para>
This variable is not used for temporary tables; for them,
<xref linkend="guc-temp-tablespaces"> is consulted instead.
</para>
<para>
This variable is also not used when creating databases.
By default, a new database inherits its tablespace setting from
the template database it is copied from.
</para>
<para>
For more information on tablespaces,
see <xref linkend="manage-ag-tablespaces">.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-temp-tablespaces" xreflabel="temp_tablespaces">
<term><varname>temp_tablespaces</varname> (<type>string</type>)
<indexterm>
<primary><varname>temp_tablespaces</> configuration parameter</primary>
</indexterm>
<indexterm><primary>tablespace</><secondary>temporary</></>
</term>
<listitem>
<para>
This variable specifies tablespaces in which to create temporary
objects (temp tables and indexes on temp tables) when a
<command>CREATE</> command does not explicitly specify a tablespace.
Temporary files for purposes such as sorting large data sets
are also created in these tablespaces.
</para>
<para>
The value is a list of names of tablespaces. When there is more than
one name in the list, <productname>PostgreSQL</> chooses a random
member of the list each time a temporary object is to be created;
except that within a transaction, successively created temporary
objects are placed in successive tablespaces from the list.
If the selected element of the list is an empty string,
<productname>PostgreSQL</> will automatically use the default
tablespace of the current database instead.
</para>
<para>
When <varname>temp_tablespaces</> is set interactively, specifying a
nonexistent tablespace is an error, as is specifying a tablespace for
which the user does not have <literal>CREATE</> privilege. However,
when using a previously set value, nonexistent tablespaces are
ignored, as are tablespaces for which the user lacks
<literal>CREATE</> privilege. In particular, this rule applies when
using a value set in <filename>postgresql.conf</>.
</para>
<para>
The default value is an empty string, which results in all temporary
objects being created in the default tablespace of the current
database.
</para>
<para>
See also <xref linkend="guc-default-tablespace">.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-check-function-bodies" xreflabel="check_function_bodies">
<term><varname>check_function_bodies</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>check_function_bodies</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
This parameter is normally on. When set to <literal>off</>, it
disables validation of the function body string during <xref
linkend="sql-createfunction">. Disabling validation avoids side
effects of the validation process and avoids false positives due
to problems such as forward references. Set this parameter
to <literal>off</> before loading functions on behalf of other
users; <application>pg_dump</> does so automatically.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-default-transaction-isolation" xreflabel="default_transaction_isolation">
<term><varname>default_transaction_isolation</varname> (<type>enum</type>)
<indexterm>
<primary>transaction isolation level</primary>
<secondary>setting default</secondary>
</indexterm>
<indexterm>
<primary><varname>default_transaction_isolation</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Each SQL transaction has an isolation level, which can be
either <quote>read uncommitted</quote>, <quote>read
committed</quote>, <quote>repeatable read</quote>, or
<quote>serializable</quote>. This parameter controls the
default isolation level of each new transaction. The default
is <quote>read committed</quote>.
</para>
<para>
Consult <xref linkend="mvcc"> and <xref
linkend="sql-set-transaction"> for more information.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-default-transaction-read-only" xreflabel="default_transaction_read_only">
<term><varname>default_transaction_read_only</varname> (<type>boolean</type>)
<indexterm>
<primary>read-only transaction</primary>
<secondary>setting default</secondary>
</indexterm>
<indexterm>
<primary><varname>default_transaction_read_only</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
A read-only SQL transaction cannot alter non-temporary tables.
This parameter controls the default read-only status of each new
transaction. The default is <literal>off</> (read/write).
</para>
<para>
Consult <xref linkend="sql-set-transaction"> for more information.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-default-transaction-deferrable" xreflabel="default_transaction_deferrable">
<term><varname>default_transaction_deferrable</varname> (<type>boolean</type>)
<indexterm>
<primary>deferrable transaction</primary>
<secondary>setting default</secondary>
</indexterm>
<indexterm>
<primary><varname>default_transaction_deferrable</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
When running at the <literal>serializable</> isolation level,
a deferrable read-only SQL transaction may be delayed before
it is allowed to proceed. However, once it begins executing
it does not incur any of the overhead required to ensure
serializability; so serialization code will have no reason to
force it to abort because of concurrent updates, making this
option suitable for long-running read-only transactions.
</para>
<para>
This parameter controls the default deferrable status of each
new transaction. It currently has no effect on read-write
transactions or those operating at isolation levels lower
than <literal>serializable</>. The default is <literal>off</>.
</para>
<para>
Consult <xref linkend="sql-set-transaction"> for more information.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-session-replication-role" xreflabel="session_replication_role">
<term><varname>session_replication_role</varname> (<type>enum</type>)
<indexterm>
<primary><varname>session_replication_role</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Controls firing of replication-related triggers and rules for the
current session. Setting this variable requires
superuser privilege and results in discarding any previously cached
query plans. Possible values are <literal>origin</> (the default),
<literal>replica</> and <literal>local</>.
See <xref linkend="sql-altertable"> for
more information.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-statement-timeout" xreflabel="statement_timeout">
<term><varname>statement_timeout</varname> (<type>integer</type>)
<indexterm>
<primary><varname>statement_timeout</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Abort any statement that takes more than the specified number of
milliseconds, starting from the time the command arrives at the server
from the client. If <varname>log_min_error_statement</> is set to
<literal>ERROR</> or lower, the statement that timed out will also be
logged. A value of zero (the default) turns this off.
</para>
<para>
Setting <varname>statement_timeout</> in
<filename>postgresql.conf</> is not recommended because it would
affect all sessions.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-lock-timeout" xreflabel="lock_timeout">
<term><varname>lock_timeout</varname> (<type>integer</type>)
<indexterm>
<primary><varname>lock_timeout</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Abort any statement that waits longer than the specified number of
milliseconds while attempting to acquire a lock on a table, index,
row, or other database object. The time limit applies separately to
each lock acquisition attempt. The limit applies both to explicit
locking requests (such as <command>LOCK TABLE</>, or <command>SELECT
FOR UPDATE</> without <literal>NOWAIT</>) and to implicitly-acquired
locks. If <varname>log_min_error_statement</> is set to
<literal>ERROR</> or lower, the statement that timed out will be
logged. A value of zero (the default) turns this off.
</para>
<para>
Unlike <varname>statement_timeout</>, this timeout can only occur
while waiting for locks. Note that if <varname>statement_timeout</>
is nonzero, it is rather pointless to set <varname>lock_timeout</> to
the same or larger value, since the statement timeout would always
trigger first.
</para>
<para>
Setting <varname>lock_timeout</> in
<filename>postgresql.conf</> is not recommended because it would
affect all sessions.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-vacuum-freeze-table-age" xreflabel="vacuum_freeze_table_age">
<term><varname>vacuum_freeze_table_age</varname> (<type>integer</type>)
<indexterm>
<primary><varname>vacuum_freeze_table_age</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
<command>VACUUM</> performs a whole-table scan if the table's
<structname>pg_class</>.<structfield>relfrozenxid</> field has reached
the age specified by this setting. The default is 150 million
transactions. Although users can set this value anywhere from zero to
two billions, <command>VACUUM</> will silently limit the effective value
to 95% of <xref linkend="guc-autovacuum-freeze-max-age">, so that a
periodical manual <command>VACUUM</> has a chance to run before an
anti-wraparound autovacuum is launched for the table. For more
information see
<xref linkend="vacuum-for-wraparound">.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-vacuum-freeze-min-age" xreflabel="vacuum_freeze_min_age">
<term><varname>vacuum_freeze_min_age</varname> (<type>integer</type>)
<indexterm>
<primary><varname>vacuum_freeze_min_age</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the cutoff age (in transactions) that <command>VACUUM</>
should use to decide whether to freeze row versions
while scanning a table.
The default is 50 million transactions. Although
users can set this value anywhere from zero to one billion,
<command>VACUUM</> will silently limit the effective value to half
the value of <xref linkend="guc-autovacuum-freeze-max-age">, so
that there is not an unreasonably short time between forced
autovacuums. For more information see <xref
linkend="vacuum-for-wraparound">.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-vacuum-multixact-freeze-table-age" xreflabel="vacuum_multixact_freeze_table_age">
<term><varname>vacuum_multixact_freeze_table_age</varname> (<type>integer</type>)
<indexterm>
<primary><varname>vacuum_multixact_freeze_table_age</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
<command>VACUUM</> performs a whole-table scan if the table's
<structname>pg_class</>.<structfield>relminmxid</> field has reached
the age specified by this setting. The default is 150 million multixacts.
Although users can set this value anywhere from zero to two billions,
<command>VACUUM</> will silently limit the effective value to 95% of
<xref linkend="guc-autovacuum-multixact-freeze-max-age">, so that a
periodical manual <command>VACUUM</> has a chance to run before an
anti-wraparound is launched for the table.
For more information see <xref linkend="vacuum-for-multixact-wraparound">.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-vacuum-multixact-freeze-min-age" xreflabel="vacuum_multixact_freeze_min_age">
<term><varname>vacuum_multixact_freeze_min_age</varname> (<type>integer</type>)
<indexterm>
<primary><varname>vacuum_multixact_freeze_min_age</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the cutoff age (in multixacts) that <command>VACUUM</>
should use to decide whether to replace multixact IDs with a newer
transaction ID or multixact ID while scanning a table. The default
is 5 million multixacts.
Although users can set this value anywhere from zero to one billion,
<command>VACUUM</> will silently limit the effective value to half
the value of <xref linkend="guc-autovacuum-multixact-freeze-max-age">,
so that there is not an unreasonably short time between forced
autovacuums.
For more information see <xref linkend="vacuum-for-multixact-wraparound">.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-bytea-output" xreflabel="bytea_output">
<term><varname>bytea_output</varname> (<type>enum</type>)
<indexterm>
<primary><varname>bytea_output</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the output format for values of type <type>bytea</type>.
Valid values are <literal>hex</literal> (the default)
and <literal>escape</literal> (the traditional PostgreSQL
format). See <xref linkend="datatype-binary"> for more
information. The <type>bytea</type> type always
accepts both formats on input, regardless of this setting.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-xmlbinary" xreflabel="xmlbinary">
<term><varname>xmlbinary</varname> (<type>enum</type>)
<indexterm>
<primary><varname>xmlbinary</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets how binary values are to be encoded in XML. This applies
for example when <type>bytea</type> values are converted to
XML by the functions <function>xmlelement</function> or
<function>xmlforest</function>. Possible values are
<literal>base64</literal> and <literal>hex</literal>, which
are both defined in the XML Schema standard. The default is
<literal>base64</literal>. For further information about
XML-related functions, see <xref linkend="functions-xml">.
</para>
<para>
The actual choice here is mostly a matter of taste,
constrained only by possible restrictions in client
applications. Both methods support all possible values,
although the hex encoding will be somewhat larger than the
base64 encoding.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-xmloption" xreflabel="xmloption">
<term><varname>xmloption</varname> (<type>enum</type>)
<indexterm>
<primary><varname>xmloption</> configuration parameter</primary>
</indexterm>
<indexterm>
<primary><varname>SET XML OPTION</></primary>
</indexterm>
<indexterm>
<primary>XML option</primary>
</indexterm>
</term>
<listitem>
<para>
Sets whether <literal>DOCUMENT</literal> or
<literal>CONTENT</literal> is implicit when converting between
XML and character string values. See <xref
linkend="datatype-xml"> for a description of this. Valid
values are <literal>DOCUMENT</literal> and
<literal>CONTENT</literal>. The default is
<literal>CONTENT</literal>.
</para>
<para>
According to the SQL standard, the command to set this option is
<synopsis>
SET XML OPTION { DOCUMENT | CONTENT };
</synopsis>
This syntax is also available in PostgreSQL.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="runtime-config-client-format">
<title>Locale and Formatting</title>
<variablelist>
<varlistentry id="guc-datestyle" xreflabel="DateStyle">
<term><varname>DateStyle</varname> (<type>string</type>)
<indexterm>
<primary><varname>DateStyle</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the display format for date and time values, as well as the
rules for interpreting ambiguous date input values. For
historical reasons, this variable contains two independent
components: the output format specification (<literal>ISO</>,
<literal>Postgres</>, <literal>SQL</>, or <literal>German</>)
and the input/output specification for year/month/day ordering
(<literal>DMY</>, <literal>MDY</>, or <literal>YMD</>). These
can be set separately or together. The keywords <literal>Euro</>
and <literal>European</> are synonyms for <literal>DMY</>; the
keywords <literal>US</>, <literal>NonEuro</>, and
<literal>NonEuropean</> are synonyms for <literal>MDY</>. See
<xref linkend="datatype-datetime"> for more information. The
built-in default is <literal>ISO, MDY</>, but
<application>initdb</application> will initialize the
configuration file with a setting that corresponds to the
behavior of the chosen <varname>lc_time</varname> locale.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-intervalstyle" xreflabel="IntervalStyle">
<term><varname>IntervalStyle</varname> (<type>enum</type>)
<indexterm>
<primary><varname>IntervalStyle</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the display format for interval values.
The value <literal>sql_standard</> will produce
output matching <acronym>SQL</acronym> standard interval literals.
The value <literal>postgres</> (which is the default) will produce
output matching <productname>PostgreSQL</> releases prior to 8.4
when the <xref linkend="guc-datestyle">
parameter was set to <literal>ISO</>.
The value <literal>postgres_verbose</> will produce output
matching <productname>PostgreSQL</> releases prior to 8.4
when the <varname>DateStyle</>
parameter was set to non-<literal>ISO</> output.
The value <literal>iso_8601</> will produce output matching the time
interval <quote>format with designators</> defined in section
4.4.3.2 of ISO 8601.
</para>
<para>
The <varname>IntervalStyle</> parameter also affects the
interpretation of ambiguous interval input. See
<xref linkend="datatype-interval-input"> for more information.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-timezone" xreflabel="TimeZone">
<term><varname>TimeZone</varname> (<type>string</type>)
<indexterm>
<primary><varname>TimeZone</> configuration parameter</primary>
</indexterm>
<indexterm><primary>time zone</></>
</term>
<listitem>
<para>
Sets the time zone for displaying and interpreting time stamps.
The built-in default is <literal>GMT</>, but that is typically
overridden in <filename>postgresql.conf</>; <application>initdb</>
will install a setting there corresponding to its system environment.
See <xref linkend="datatype-timezones"> for more information.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-timezone-abbreviations" xreflabel="timezone_abbreviations">
<term><varname>timezone_abbreviations</varname> (<type>string</type>)
<indexterm>
<primary><varname>timezone_abbreviations</> configuration parameter</primary>
</indexterm>
<indexterm><primary>time zone names</></>
</term>
<listitem>
<para>
Sets the collection of time zone abbreviations that will be accepted
by the server for datetime input. The default is <literal>'Default'</>,
which is a collection that works in most of the world; there are
also <literal>'Australia'</literal> and <literal>'India'</literal>, and other collections can be defined
for a particular installation. See <xref
linkend="datetime-appendix"> for more information.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-extra-float-digits" xreflabel="extra_float_digits">
<term><varname>extra_float_digits</varname> (<type>integer</type>)
<indexterm>
<primary>significant digits</primary>
</indexterm>
<indexterm>
<primary>floating-point</primary>
<secondary>display</secondary>
</indexterm>
<indexterm>
<primary><varname>extra_float_digits</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
This parameter adjusts the number of digits displayed for
floating-point values, including <type>float4</>, <type>float8</>,
and geometric data types. The parameter value is added to the
standard number of digits (<literal>FLT_DIG</> or <literal>DBL_DIG</>
as appropriate). The value can be set as high as 3, to include
partially-significant digits; this is especially useful for dumping
float data that needs to be restored exactly. Or it can be set
negative to suppress unwanted digits.
See also <xref linkend="datatype-float">.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-client-encoding" xreflabel="client_encoding">
<term><varname>client_encoding</varname> (<type>string</type>)
<indexterm>
<primary><varname>client_encoding</> configuration parameter</primary>
</indexterm>
<indexterm><primary>character set</></>
</term>
<listitem>
<para>
Sets the client-side encoding (character set).
The default is to use the database encoding.
The character sets supported by the <productname>PostgreSQL</productname>
server are described in <xref linkend="multibyte-charset-supported">.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-lc-messages" xreflabel="lc_messages">
<term><varname>lc_messages</varname> (<type>string</type>)
<indexterm>
<primary><varname>lc_messages</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the language in which messages are displayed. Acceptable
values are system-dependent; see <xref linkend="locale"> for
more information. If this variable is set to the empty string
(which is the default) then the value is inherited from the
execution environment of the server in a system-dependent way.
</para>
<para>
On some systems, this locale category does not exist. Setting
this variable will still work, but there will be no effect.
Also, there is a chance that no translated messages for the
desired language exist. In that case you will continue to see
the English messages.
</para>
<para>
Only superusers can change this setting, because it affects the
messages sent to the server log as well as to the client, and
an improper value might obscure the readability of the server
logs.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-lc-monetary" xreflabel="lc_monetary">
<term><varname>lc_monetary</varname> (<type>string</type>)
<indexterm>
<primary><varname>lc_monetary</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the locale to use for formatting monetary amounts, for
example with the <function>to_char</function> family of
functions. Acceptable values are system-dependent; see <xref
linkend="locale"> for more information. If this variable is
set to the empty string (which is the default) then the value
is inherited from the execution environment of the server in a
system-dependent way.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-lc-numeric" xreflabel="lc_numeric">
<term><varname>lc_numeric</varname> (<type>string</type>)
<indexterm>
<primary><varname>lc_numeric</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the locale to use for formatting numbers, for example
with the <function>to_char</function> family of
functions. Acceptable values are system-dependent; see <xref
linkend="locale"> for more information. If this variable is
set to the empty string (which is the default) then the value
is inherited from the execution environment of the server in a
system-dependent way.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-lc-time" xreflabel="lc_time">
<term><varname>lc_time</varname> (<type>string</type>)
<indexterm>
<primary><varname>lc_time</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Sets the locale to use for formatting dates and times, for example
with the <function>to_char</function> family of
functions. Acceptable values are system-dependent; see <xref
linkend="locale"> for more information. If this variable is
set to the empty string (which is the default) then the value
is inherited from the execution environment of the server in a
system-dependent way.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-default-text-search-config" xreflabel="default_text_search_config">
<term><varname>default_text_search_config</varname> (<type>string</type>)
<indexterm>
<primary><varname>default_text_search_config</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Selects the text search configuration that is used by those variants
of the text search functions that do not have an explicit argument
specifying the configuration.
See <xref linkend="textsearch"> for further information.
The built-in default is <literal>pg_catalog.simple</>, but
<application>initdb</application> will initialize the
configuration file with a setting that corresponds to the
chosen <varname>lc_ctype</varname> locale, if a configuration
matching that locale can be identified.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="runtime-config-client-preload">
<title>Shared Library Preloading</title>
<para>
Several settings are available for preloading shared libraries into the
server, in order to load additional functionality or achieve performance
benefits. For example, a setting of
<literal>'$libdir/mylib'</literal> would cause
<literal>mylib.so</> (or on some platforms,
<literal>mylib.sl</>) to be preloaded from the installation's standard
library directory. The differences between the settings are when they
take effect and what privileges are required to change them.
</para>
<para>
<productname>PostgreSQL</productname> procedural language libraries can
be preloaded in this way, typically by using the
syntax <literal>'$libdir/plXXX'</literal> where
<literal>XXX</literal> is <literal>pgsql</>, <literal>perl</>,
<literal>tcl</>, or <literal>python</>.
</para>
<para>
For each parameter, if more than one library is to be loaded, separate
their names with commas. All library names are converted to lower case
unless double-quoted.
</para>
<para>
Only shared libraries specifically intended to be used with PostgreSQL
can be loaded this way. Every PostgreSQL-supported library has
a <quote>magic block</> that is checked to guarantee compatibility. For
this reason, non-PostgreSQL libraries cannot be loaded in this way. You
might be able to use operating-system facilities such
as <envar>LD_PRELOAD</envar> for that.
</para>
<para>
In general, refer to the documentation of a specific module for the
recommended way to load that module.
</para>
<variablelist>
<varlistentry id="guc-local-preload-libraries" xreflabel="local_preload_libraries">
<term><varname>local_preload_libraries</varname> (<type>string</type>)
<indexterm>
<primary><varname>local_preload_libraries</> configuration parameter</primary>
</indexterm>
<indexterm>
<primary><filename>$libdir/plugins</></primary>
</indexterm>
</term>
<listitem>
<para>
This variable specifies one or more shared libraries that are to be
preloaded at connection start. This parameter cannot be changed after
the start of a particular session. If a specified library is not
found, the connection attempt will fail.
</para>
<para>
This option can be set by any user. Because of that, the libraries
that can be loaded are restricted to those appearing in the
<filename>plugins</> subdirectory of the installation's
standard library directory. (It is the database administrator's
responsibility to ensure that only <quote>safe</> libraries
are installed there.) Entries in <varname>local_preload_libraries</>
can specify this directory explicitly, for example
<literal>$libdir/plugins/mylib</literal>, or just specify
the library name &mdash; <literal>mylib</literal> would have
the same effect as <literal>$libdir/plugins/mylib</literal>.
</para>
<para>
Unless a module is specifically designed to be used in this way by
non-superusers, this is usually not the right setting to use. Look
at <xref linkend="guc-session-preload-libraries"> instead.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-session-preload-libraries" xreflabel="session_preload_libraries">
<term><varname>session_preload_libraries</varname> (<type>string</type>)
<indexterm>
<primary><varname>session_preload_libraries</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
This variable specifies one or more shared libraries that are to be
preloaded at connection start. Only superusers can change this setting.
The parameter value only takes effect at the start of the connection.
Subsequent changes have no effect. If a specified library is not
found, the connection attempt will fail.
</para>
<para>
The intent of this feature is to allow debugging or
performance-measurement libraries to be loaded into specific sessions
without an explicit
<command>LOAD</> command being given. For
example, <xref linkend="auto-explain"> could be enabled for all
sessions under a given user name by setting this parameter
with <command>ALTER ROLE SET</>. Also, this parameter can be changed
without restarting the server (but changes only take effect when a new
session is started), so it is easier to add new modules this way, even
if they should apply to all sessions.
</para>
<para>
Unlike <xref linkend="guc-shared-preload-libraries">, there is no large
performance advantage to loading a library at session start rather than
when it is first used. There is some advantage, however, when
connection pooling is used.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-shared-preload-libraries" xreflabel="shared_preload_libraries">
<term><varname>shared_preload_libraries</varname> (<type>string</type>)
<indexterm>
<primary><varname>shared_preload_libraries</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
This variable specifies one or more shared libraries to be preloaded at
server start. with commas. This parameter can only be set at server
start. If a specified library is not found, the server will fail to
start.
</para>
<para>
Some libraries need to perform certain operations that can only take
place at postmaster start, such as allocating shared memory, reserving
light-weight locks, or starting background workers. Those libraries
must be loaded at server start through this parameter. See the
documentation of each library for details.
</para>
<para>
Other libraries can also be preloaded. By preloading a shared library,
the library startup time is avoided when the library is first used.
However, the time to start each new server process might increase
slightly, even if that process never uses the library. So this
parameter is recommended only for libraries that will be used in most
sessions. Also, changing this parameter requires a server restart, so
this is not the right setting to use for short-term debugging tasks,
say. Use <xref linkend="guc-session-preload-libraries"> for that
instead.
</para>
<note>
<para>
On Windows hosts, preloading a library at server start will not reduce
the time required to start each new server process; each server process
will re-load all preload libraries. However, <varname>shared_preload_libraries
</varname> is still useful on Windows hosts for libraries that need to
perform operations at postmaster start time.
</para>
</note>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="runtime-config-client-other">
<title>Other Defaults</title>
<variablelist>
<varlistentry id="guc-dynamic-library-path" xreflabel="dynamic_library_path">
<term><varname>dynamic_library_path</varname> (<type>string</type>)
<indexterm>
<primary><varname>dynamic_library_path</> configuration parameter</primary>
</indexterm>
<indexterm><primary>dynamic loading</></>
</term>
<listitem>
<para>
If a dynamically loadable module needs to be opened and the
file name specified in the <command>CREATE FUNCTION</command> or
<command>LOAD</command> command
does not have a directory component (i.e., the
name does not contain a slash), the system will search this
path for the required file.
</para>
<para>
The value for <varname>dynamic_library_path</varname> must be a
list of absolute directory paths separated by colons (or semi-colons
on Windows). If a list element starts
with the special string <literal>$libdir</literal>, the
compiled-in <productname>PostgreSQL</productname> package
library directory is substituted for <literal>$libdir</literal>; this
is where the modules provided by the standard
<productname>PostgreSQL</productname> distribution are installed.
(Use <literal>pg_config --pkglibdir</literal> to find out the name of
this directory.) For example:
<programlisting>
dynamic_library_path = '/usr/local/lib/postgresql:/home/my_project/lib:$libdir'
</programlisting>
or, in a Windows environment:
<programlisting>
dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
</programlisting>
</para>
<para>
The default value for this parameter is
<literal>'$libdir'</literal>. If the value is set to an empty
string, the automatic path search is turned off.
</para>
<para>
This parameter can be changed at run time by superusers, but a
setting done that way will only persist until the end of the
client connection, so this method should be reserved for
development purposes. The recommended way to set this parameter
is in the <filename>postgresql.conf</filename> configuration
file.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-gin-fuzzy-search-limit" xreflabel="gin_fuzzy_search_limit">
<term><varname>gin_fuzzy_search_limit</varname> (<type>integer</type>)
<indexterm>
<primary><varname>gin_fuzzy_search_limit</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Soft upper limit of the size of the set returned by GIN index scans. For more
information see <xref linkend="gin-tips">.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
</sect1>
<sect1 id="runtime-config-locks">
<title>Lock Management</title>
<variablelist>
<varlistentry id="guc-deadlock-timeout" xreflabel="deadlock_timeout">
<term><varname>deadlock_timeout</varname> (<type>integer</type>)
<indexterm>
<primary>deadlock</primary>
<secondary>timeout during</secondary>
</indexterm>
<indexterm>
<primary>timeout</primary>
<secondary>deadlock</secondary>
</indexterm>
<indexterm>
<primary><varname>deadlock_timeout</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
This is the amount of time, in milliseconds, to wait on a lock
before checking to see if there is a deadlock condition. The
check for deadlock is relatively expensive, so the server doesn't run
it every time it waits for a lock. We optimistically assume
that deadlocks are not common in production applications and
just wait on the lock for a while before checking for a
deadlock. Increasing this value reduces the amount of time
wasted in needless deadlock checks, but slows down reporting of
real deadlock errors. The default is one second (<literal>1s</>),
which is probably about the smallest value you would want in
practice. On a heavily loaded server you might want to raise it.
Ideally the setting should exceed your typical transaction time,
so as to improve the odds that a lock will be released before
the waiter decides to check for deadlock. Only superusers can change
this setting.
</para>
<para>
When <xref linkend="guc-log-lock-waits"> is set,
this parameter also determines the length of time to wait before
a log message is issued about the lock wait. If you are trying
to investigate locking delays you might want to set a shorter than
normal <varname>deadlock_timeout</varname>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-max-locks-per-transaction" xreflabel="max_locks_per_transaction">
<term><varname>max_locks_per_transaction</varname> (<type>integer</type>)
<indexterm>
<primary><varname>max_locks_per_transaction</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
The shared lock table tracks locks on
<varname>max_locks_per_transaction</varname> * (<xref
linkend="guc-max-connections"> + <xref
linkend="guc-max-prepared-transactions">) objects (e.g., tables);
hence, no more than this many distinct objects can be locked at
any one time. This parameter controls the average number of object
locks allocated for each transaction; individual transactions
can lock more objects as long as the locks of all transactions
fit in the lock table. This is <emphasis>not</> the number of
rows that can be locked; that value is unlimited. The default,
64, has historically proven sufficient, but you might need to
raise this value if you have queries that touch many different
tables in a single transaction, e.g. query of a parent table with
many children. This parameter can only be set at server start.
</para>
<para>
When running a standby server, you must set this parameter to the
same or higher value than on the master server. Otherwise, queries
will not be allowed in the standby server.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-max-pred-locks-per-transaction" xreflabel="max_pred_locks_per_transaction">
<term><varname>max_pred_locks_per_transaction</varname> (<type>integer</type>)
<indexterm>
<primary><varname>max_pred_locks_per_transaction</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
The shared predicate lock table tracks locks on
<varname>max_pred_locks_per_transaction</varname> * (<xref
linkend="guc-max-connections"> + <xref
linkend="guc-max-prepared-transactions">) objects (e.g., tables);
hence, no more than this many distinct objects can be locked at
any one time. This parameter controls the average number of object
locks allocated for each transaction; individual transactions
can lock more objects as long as the locks of all transactions
fit in the lock table. This is <emphasis>not</> the number of
rows that can be locked; that value is unlimited. The default,
64, has generally been sufficient in testing, but you might need to
raise this value if you have clients that touch many different
tables in a single serializable transaction. This parameter can
only be set at server start.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect1>
<sect1 id="runtime-config-compatible">
<title>Version and Platform Compatibility</title>
<sect2 id="runtime-config-compatible-version">
<title>Previous PostgreSQL Versions</title>
<variablelist>
<varlistentry id="guc-array-nulls" xreflabel="array_nulls">
<term><varname>array_nulls</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>array_nulls</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
This controls whether the array input parser recognizes
unquoted <literal>NULL</> as specifying a null array element.
By default, this is <literal>on</>, allowing array values containing
null values to be entered. However, <productname>PostgreSQL</> versions
before 8.2 did not support null values in arrays, and therefore would
treat <literal>NULL</> as specifying a normal array element with
the string value <quote>NULL</>. For backward compatibility with
applications that require the old behavior, this variable can be
turned <literal>off</>.
</para>
<para>
Note that it is possible to create array values containing null values
even when this variable is <literal>off</>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-backslash-quote" xreflabel="backslash_quote">
<term><varname>backslash_quote</varname> (<type>enum</type>)
<indexterm><primary>strings</><secondary>backslash quotes</></>
<indexterm>
<primary><varname>backslash_quote</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
This controls whether a quote mark can be represented by
<literal>\'</> in a string literal. The preferred, SQL-standard way
to represent a quote mark is by doubling it (<literal>''</>) but
<productname>PostgreSQL</> has historically also accepted
<literal>\'</>. However, use of <literal>\'</> creates security risks
because in some client character set encodings, there are multibyte
characters in which the last byte is numerically equivalent to ASCII
<literal>\</>. If client-side code does escaping incorrectly then a
SQL-injection attack is possible. This risk can be prevented by
making the server reject queries in which a quote mark appears to be
escaped by a backslash.
The allowed values of <varname>backslash_quote</> are
<literal>on</> (allow <literal>\'</> always),
<literal>off</> (reject always), and
<literal>safe_encoding</> (allow only if client encoding does not
allow ASCII <literal>\</> within a multibyte character).
<literal>safe_encoding</> is the default setting.
</para>
<para>
Note that in a standard-conforming string literal, <literal>\</> just
means <literal>\</> anyway. This parameter only affects the handling of
non-standard-conforming literals, including
escape string syntax (<literal>E'...'</>).
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-default-with-oids" xreflabel="default_with_oids">
<term><varname>default_with_oids</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>default_with_oids</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
This controls whether <command>CREATE TABLE</command> and
<command>CREATE TABLE AS</command> include an OID column in
newly-created tables, if neither <literal>WITH OIDS</literal>
nor <literal>WITHOUT OIDS</literal> is specified. It also
determines whether OIDs will be included in tables created by
<command>SELECT INTO</command>. The parameter is <literal>off</>
by default; in <productname>PostgreSQL</> 8.0 and earlier, it
was on by default.
</para>
<para>
The use of OIDs in user tables is considered deprecated, so
most installations should leave this variable disabled.
Applications that require OIDs for a particular table should
specify <literal>WITH OIDS</literal> when creating the
table. This variable can be enabled for compatibility with old
applications that do not follow this behavior.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-escape-string-warning" xreflabel="escape_string_warning">
<term><varname>escape_string_warning</varname> (<type>boolean</type>)
<indexterm><primary>strings</><secondary>escape warning</></>
<indexterm>
<primary><varname>escape_string_warning</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
When on, a warning is issued if a backslash (<literal>\</>)
appears in an ordinary string literal (<literal>'...'</>
syntax) and <varname>standard_conforming_strings</varname> is off.
The default is <literal>on</>.
</para>
<para>
Applications that wish to use backslash as escape should be
modified to use escape string syntax (<literal>E'...'</>),
because the default behavior of ordinary strings is now to treat
backslash as an ordinary character, per SQL standard. This variable
can be enabled to help locate code that needs to be changed.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-lo-compat-privileges" xreflabel="lo_compat_privileges">
<term><varname>lo_compat_privileges</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>lo_compat_privileges</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
In <productname>PostgreSQL</> releases prior to 9.0, large objects
did not have access privileges and were, therefore, always readable
and writable by all users. Setting this variable to <literal>on</>
disables the new privilege checks, for compatibility with prior
releases. The default is <literal>off</>.
Only superusers can change this setting.
</para>
<para>
Setting this variable does not disable all security checks related to
large objects &mdash; only those for which the default behavior has
changed in <productname>PostgreSQL</> 9.0.
For example, <literal>lo_import()</literal> and
<literal>lo_export()</literal> need superuser privileges regardless
of this setting.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-quote-all-identifiers" xreflabel="quote-all-identifiers">
<term><varname>quote_all_identifiers</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>quote_all_identifiers</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
When the database generates SQL, force all identifiers to be quoted,
even if they are not (currently) keywords. This will affect the
output of <command>EXPLAIN</> as well as the results of functions
like <function>pg_get_viewdef</>. See also the
<option>--quote-all-identifiers</option> option of
<xref linkend="app-pgdump"> and <xref linkend="app-pg-dumpall">.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-sql-inheritance" xreflabel="sql_inheritance">
<term><varname>sql_inheritance</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>sql_inheritance</> configuration parameter</primary>
</indexterm>
<indexterm><primary>inheritance</></>
</term>
<listitem>
<para>
This setting controls whether undecorated table references are
considered to include inheritance child tables. The default is
<literal>on</>, which means child tables are included (thus,
a <literal>*</> suffix is assumed by default). If turned
<literal>off</>, child tables are not included (thus, an
<literal>ONLY</literal> prefix is assumed). The SQL standard
requires child tables to be included, so the <literal>off</> setting
is not spec-compliant, but it is provided for compatibility with
<productname>PostgreSQL</> releases prior to 7.1.
See <xref linkend="ddl-inherit"> for more information.
</para>
<para>
Turning <varname>sql_inheritance</> off is deprecated, because that
behavior has been found to be error-prone as well as contrary to SQL
standard. Discussions of inheritance behavior elsewhere in this
manual generally assume that it is <literal>on</>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-standard-conforming-strings" xreflabel="standard_conforming_strings">
<term><varname>standard_conforming_strings</varname> (<type>boolean</type>)
<indexterm><primary>strings</><secondary>standard conforming</></>
<indexterm>
<primary><varname>standard_conforming_strings</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
This controls whether ordinary string literals
(<literal>'...'</>) treat backslashes literally, as specified in
the SQL standard.
Beginning in <productname>PostgreSQL</productname> 9.1, the default is
<literal>on</> (prior releases defaulted to <literal>off</>).
Applications can check this
parameter to determine how string literals will be processed.
The presence of this parameter can also be taken as an indication
that the escape string syntax (<literal>E'...'</>) is supported.
Escape string syntax (<xref linkend="sql-syntax-strings-escape">)
should be used if an application desires
backslashes to be treated as escape characters.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-synchronize-seqscans" xreflabel="synchronize_seqscans">
<term><varname>synchronize_seqscans</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>synchronize_seqscans</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
This allows sequential scans of large tables to synchronize with each
other, so that concurrent scans read the same block at about the
same time and hence share the I/O workload. When this is enabled,
a scan might start in the middle of the table and then <quote>wrap
around</> the end to cover all rows, so as to synchronize with the
activity of scans already in progress. This can result in
unpredictable changes in the row ordering returned by queries that
have no <literal>ORDER BY</> clause. Setting this parameter to
<literal>off</> ensures the pre-8.3 behavior in which a sequential
scan always starts from the beginning of the table. The default
is <literal>on</>.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="runtime-config-compatible-clients">
<title>Platform and Client Compatibility</title>
<variablelist>
<varlistentry id="guc-transform-null-equals" xreflabel="transform_null_equals">
<term><varname>transform_null_equals</varname> (<type>boolean</type>)
<indexterm><primary>IS NULL</></>
<indexterm>
<primary><varname>transform_null_equals</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
When on, expressions of the form <literal><replaceable>expr</> =
NULL</literal> (or <literal>NULL =
<replaceable>expr</></literal>) are treated as
<literal><replaceable>expr</> IS NULL</literal>, that is, they
return true if <replaceable>expr</> evaluates to the null value,
and false otherwise. The correct SQL-spec-compliant behavior of
<literal><replaceable>expr</> = NULL</literal> is to always
return null (unknown). Therefore this parameter defaults to
<literal>off</>.
</para>
<para>
However, filtered forms in <productname>Microsoft
Access</productname> generate queries that appear to use
<literal><replaceable>expr</> = NULL</literal> to test for
null values, so if you use that interface to access the database you
might want to turn this option on. Since expressions of the
form <literal><replaceable>expr</> = NULL</literal> always
return the null value (using the SQL standard interpretation), they are not
very useful and do not appear often in normal applications so
this option does little harm in practice. But new users are
frequently confused about the semantics of expressions
involving null values, so this option is off by default.
</para>
<para>
Note that this option only affects the exact form <literal>= NULL</>,
not other comparison operators or other expressions
that are computationally equivalent to some expression
involving the equals operator (such as <literal>IN</literal>).
Thus, this option is not a general fix for bad programming.
</para>
<para>
Refer to <xref linkend="functions-comparison"> for related information.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
</sect1>
<sect1 id="runtime-config-error-handling">
<title>Error Handling</title>
<variablelist>
<varlistentry id="guc-exit-on-error" xreflabel="exit_on_error">
<term><varname>exit_on_error</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>exit_on_error</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
If true, any error will terminate the current session. By default,
this is set to false, so that only FATAL errors will terminate the
session.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-restart-after-crash" xreflabel="restart_after_crash">
<term><varname>restart_after_crash</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>restart_after_crash</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
When set to true, which is the default, <productname>PostgreSQL</>
will automatically reinitialize after a backend crash. Leaving this
value set to true is normally the best way to maximize the availability
of the database. However, in some circumstances, such as when
<productname>PostgreSQL</> is being invoked by clusterware, it may be
useful to disable the restart so that the clusterware can gain
control and take any actions it deems appropriate.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect1>
<sect1 id="runtime-config-preset">
<title>Preset Options</title>
<para>
The following <quote>parameters</> are read-only, and are determined
when <productname>PostgreSQL</productname> is compiled or when it is
installed. As such, they have been excluded from the sample
<filename>postgresql.conf</> file. These options report
various aspects of <productname>PostgreSQL</productname> behavior
that might be of interest to certain applications, particularly
administrative front-ends.
</para>
<variablelist>
<varlistentry id="guc-block-size" xreflabel="block_size">
<term><varname>block_size</varname> (<type>integer</type>)
<indexterm>
<primary><varname>block_size</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Reports the size of a disk block. It is determined by the value
of <literal>BLCKSZ</> when building the server. The default
value is 8192 bytes. The meaning of some configuration
variables (such as <xref linkend="guc-shared-buffers">) is
influenced by <varname>block_size</varname>. See <xref
linkend="runtime-config-resource"> for information.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-data-checksums" xreflabel="data_checksums">
<term><varname>data_checksums</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>data_checksums</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Reports whether data checksums are enabled for this cluster.
See <xref linkend="app-initdb-data-checksums"> for more information.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-debug-assertions" xreflabel="debug_assertions">
<term><varname>debug_assertions</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>debug_assertions</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Reports whether <productname>PostgreSQL</productname> has been built
with assertions enabled. That is the case if the
macro <symbol>USE_ASSERT_CHECKING</symbol> is defined
when <productname>PostgreSQL</productname> is built (accomplished
e.g. by the <command>configure</command> option
<option>--enable-cassert</option>). By
default <productname>PostgreSQL</productname> is built without
assertions.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-integer-datetimes" xreflabel="integer_datetimes">
<term><varname>integer_datetimes</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>integer_datetimes</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Reports whether <productname>PostgreSQL</> was built with
support for 64-bit-integer dates and times. This can be
disabled by configuring with <literal>--disable-integer-datetimes</>
when building <productname>PostgreSQL</>. The default value is
<literal>on</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-lc-collate" xreflabel="lc_collate">
<term><varname>lc_collate</varname> (<type>string</type>)
<indexterm>
<primary><varname>lc_collate</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Reports the locale in which sorting of textual data is done.
See <xref linkend="locale"> for more information.
This value is determined when a database is created.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-lc-ctype" xreflabel="lc_ctype">
<term><varname>lc_ctype</varname> (<type>string</type>)
<indexterm>
<primary><varname>lc_ctype</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Reports the locale that determines character classifications.
See <xref linkend="locale"> for more information.
This value is determined when a database is created.
Ordinarily this will be the same as <varname>lc_collate</varname>,
but for special applications it might be set differently.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-max-function-args" xreflabel="max_function_args">
<term><varname>max_function_args</varname> (<type>integer</type>)
<indexterm>
<primary><varname>max_function_args</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Reports the maximum number of function arguments. It is determined by
the value of <literal>FUNC_MAX_ARGS</> when building the server. The
default value is 100 arguments.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-max-identifier-length" xreflabel="max_identifier_length">
<term><varname>max_identifier_length</varname> (<type>integer</type>)
<indexterm>
<primary><varname>max_identifier_length</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Reports the maximum identifier length. It is determined as one
less than the value of <literal>NAMEDATALEN</> when building
the server. The default value of <literal>NAMEDATALEN</> is
64; therefore the default
<varname>max_identifier_length</varname> is 63 bytes, which
can be less than 63 characters when using multibyte encodings.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-max-index-keys" xreflabel="max_index_keys">
<term><varname>max_index_keys</varname> (<type>integer</type>)
<indexterm>
<primary><varname>max_index_keys</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Reports the maximum number of index keys. It is determined by
the value of <literal>INDEX_MAX_KEYS</> when building the server. The
default value is 32 keys.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-segment-size" xreflabel="segment_size">
<term><varname>segment_size</varname> (<type>integer</type>)
<indexterm>
<primary><varname>segment_size</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Reports the number of blocks (pages) that can be stored within a file
segment. It is determined by the value of <literal>RELSEG_SIZE</>
when building the server. The maximum size of a segment file in bytes
is equal to <varname>segment_size</> multiplied by
<varname>block_size</>; by default this is 1GB.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-server-encoding" xreflabel="server_encoding">
<term><varname>server_encoding</varname> (<type>string</type>)
<indexterm>
<primary><varname>server_encoding</> configuration parameter</primary>
</indexterm>
<indexterm><primary>character set</></>
</term>
<listitem>
<para>
Reports the database encoding (character set).
It is determined when the database is created. Ordinarily,
clients need only be concerned with the value of <xref
linkend="guc-client-encoding">.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-server-version" xreflabel="server_version">
<term><varname>server_version</varname> (<type>string</type>)
<indexterm>
<primary><varname>server_version</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Reports the version number of the server. It is determined by the
value of <literal>PG_VERSION</> when building the server.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-server-version-num" xreflabel="server_version_num">
<term><varname>server_version_num</varname> (<type>integer</type>)
<indexterm>
<primary><varname>server_version_num</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Reports the version number of the server as an integer. It is determined
by the value of <literal>PG_VERSION_NUM</> when building the server.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-wal-block-size" xreflabel="wal_block_size">
<term><varname>wal_block_size</varname> (<type>integer</type>)
<indexterm>
<primary><varname>wal_block_size</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Reports the size of a WAL disk block. It is determined by the value
of <literal>XLOG_BLCKSZ</> when building the server. The default value
is 8192 bytes.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-wal-segment-size" xreflabel="wal_segment_size">
<term><varname>wal_segment_size</varname> (<type>integer</type>)
<indexterm>
<primary><varname>wal_segment_size</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Reports the number of blocks (pages) in a WAL segment file.
The total size of a WAL segment file in bytes is equal to
<varname>wal_segment_size</> multiplied by <varname>wal_block_size</>;
by default this is 16MB. See <xref linkend="wal-configuration"> for
more information.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect1>
<sect1 id="runtime-config-custom">
<title>Customized Options</title>
<para>
This feature was designed to allow parameters not normally known to
<productname>PostgreSQL</productname> to be added by add-on modules
(such as procedural languages). This allows extension modules to be
configured in the standard ways.
</para>
<para>
Custom options have two-part names: an extension name, then a dot, then
the parameter name proper, much like qualified names in SQL. An example
is <literal>plpgsql.variable_conflict</>.
</para>
<para>
Because custom options may need to be set in processes that have not
loaded the relevant extension module, <productname>PostgreSQL</>
will accept a setting for any two-part parameter name. Such variables
are treated as placeholders and have no function until the module that
defines them is loaded. When an extension module is loaded, it will add
its variable definitions, convert any placeholder values according to
those definitions, and issue warnings for any unrecognized placeholders
that begin with its extension name.
</para>
</sect1>
<sect1 id="runtime-config-developer">
<title>Developer Options</title>
<para>
The following parameters are intended for work on the
<productname>PostgreSQL</productname> source code, and in some cases
to assist with recovery of severely damaged databases. There
should be no reason to use them on a production database.
As such, they have been excluded from the sample
<filename>postgresql.conf</> file. Note that many of these
parameters require special source compilation flags to work at all.
</para>
<variablelist>
<varlistentry id="guc-allow-system-table-mods" xreflabel="allow_system_table_mods">
<term><varname>allow_system_table_mods</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>allow_system_table_mods</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Allows modification of the structure of system tables.
This is used by <command>initdb</command>.
This parameter can only be set at server start.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-ignore-system-indexes" xreflabel="ignore_system_indexes">
<term><varname>ignore_system_indexes</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>ignore_system_indexes</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Ignore system indexes when reading system tables (but still
update the indexes when modifying the tables). This is useful
when recovering from damaged system indexes.
This parameter cannot be changed after session start.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-post-auth-delay" xreflabel="post_auth_delay">
<term><varname>post_auth_delay</varname> (<type>integer</type>)
<indexterm>
<primary><varname>post_auth_delay</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
If nonzero, a delay of this many seconds occurs when a new
server process is started, after it conducts the
authentication procedure. This is intended to give developers an
opportunity to attach to the server process with a debugger.
This parameter cannot be changed after session start.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-pre-auth-delay" xreflabel="pre_auth_delay">
<term><varname>pre_auth_delay</varname> (<type>integer</type>)
<indexterm>
<primary><varname>pre_auth_delay</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
If nonzero, a delay of this many seconds occurs just after a
new server process is forked, before it conducts the
authentication procedure. This is intended to give developers an
opportunity to attach to the server process with a debugger to
trace down misbehavior in authentication.
This parameter can only be set in the <filename>postgresql.conf</>
file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-trace-notify" xreflabel="trace_notify">
<term><varname>trace_notify</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>trace_notify</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Generates a great amount of debugging output for the
<command>LISTEN</command> and <command>NOTIFY</command>
commands. <xref linkend="guc-client-min-messages"> or
<xref linkend="guc-log-min-messages"> must be
<literal>DEBUG1</literal> or lower to send this output to the
client or server logs, respectively.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-trace-recovery-messages" xreflabel="trace_recovery_messages">
<term><varname>trace_recovery_messages</varname> (<type>enum</type>)
<indexterm>
<primary><varname>trace_recovery_messages</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Enables logging of recovery-related debugging output that otherwise
would not be logged. This parameter allows the user to override the
normal setting of <xref linkend="guc-log-min-messages">, but only for
specific messages. This is intended for use in debugging Hot Standby.
Valid values are <literal>DEBUG5</>, <literal>DEBUG4</>,
<literal>DEBUG3</>, <literal>DEBUG2</>, <literal>DEBUG1</>, and
<literal>LOG</>. The default, <literal>LOG</>, does not affect
logging decisions at all. The other values cause recovery-related
debug messages of that priority or higher to be logged as though they
had <literal>LOG</> priority; for common settings of
<varname>log_min_messages</> this results in unconditionally sending
them to the server log.
This parameter can only be set in the <filename>postgresql.conf</>
file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-trace-sort" xreflabel="trace_sort">
<term><varname>trace_sort</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>trace_sort</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
If on, emit information about resource usage during sort operations.
This parameter is only available if the <symbol>TRACE_SORT</symbol> macro
was defined when <productname>PostgreSQL</productname> was compiled.
(However, <symbol>TRACE_SORT</symbol> is currently defined by default.)
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>trace_locks</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>trace_locks</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
If on, emit information about lock usage. Information dumped
includes the type of lock operation, the type of lock and the unique
identifier of the object being locked or unlocked. Also included
are bit masks for the lock types already granted on this object as
well as for the lock types awaited on this object. For each lock
type a count of the number of granted locks and waiting locks is
also dumped as well as the totals. An example of the log file output
is shown here:
<screen>
LOG: LockAcquire: new: lock(0xb7acd844) id(24688,24696,0,0,0,1)
grantMask(0) req(0,0,0,0,0,0,0)=0 grant(0,0,0,0,0,0,0)=0
wait(0) type(AccessShareLock)
LOG: GrantLock: lock(0xb7acd844) id(24688,24696,0,0,0,1)
grantMask(2) req(1,0,0,0,0,0,0)=1 grant(1,0,0,0,0,0,0)=1
wait(0) type(AccessShareLock)
LOG: UnGrantLock: updated: lock(0xb7acd844) id(24688,24696,0,0,0,1)
grantMask(0) req(0,0,0,0,0,0,0)=0 grant(0,0,0,0,0,0,0)=0
wait(0) type(AccessShareLock)
LOG: CleanUpLock: deleting: lock(0xb7acd844) id(24688,24696,0,0,0,1)
grantMask(0) req(0,0,0,0,0,0,0)=0 grant(0,0,0,0,0,0,0)=0
wait(0) type(INVALID)
</screen>
Details of the structure being dumped may be found in
<filename>src/include/storage/lock.h</filename>.
</para>
<para>
This parameter is only available if the <symbol>LOCK_DEBUG</symbol>
macro was defined when <productname>PostgreSQL</productname> was
compiled.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>trace_lwlocks</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>trace_lwlocks</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
If on, emit information about lightweight lock usage. Lightweight
locks are intended primarily to provide mutual exclusion of access
to shared-memory data structures.
</para>
<para>
This parameter is only available if the <symbol>LOCK_DEBUG</symbol>
macro was defined when <productname>PostgreSQL</productname> was
compiled.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>trace_userlocks</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>trace_userlocks</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
If on, emit information about user lock usage. Output is the same
as for <symbol>trace_locks</symbol>, only for advisory locks.
</para>
<para>
This parameter is only available if the <symbol>LOCK_DEBUG</symbol>
macro was defined when <productname>PostgreSQL</productname> was
compiled.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>trace_lock_oidmin</varname> (<type>integer</type>)
<indexterm>
<primary><varname>trace_lock_oidmin</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
If set, do not trace locks for tables below this OID. (use to avoid
output on system tables)
</para>
<para>
This parameter is only available if the <symbol>LOCK_DEBUG</symbol>
macro was defined when <productname>PostgreSQL</productname> was
compiled.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>trace_lock_table</varname> (<type>integer</type>)
<indexterm>
<primary><varname>trace_lock_table</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Unconditionally trace locks on this table (OID).
</para>
<para>
This parameter is only available if the <symbol>LOCK_DEBUG</symbol>
macro was defined when <productname>PostgreSQL</productname> was
compiled.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>debug_deadlocks</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>debug_deadlocks</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
If set, dumps information about all current locks when a
deadlock timeout occurs.
</para>
<para>
This parameter is only available if the <symbol>LOCK_DEBUG</symbol>
macro was defined when <productname>PostgreSQL</productname> was
compiled.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>log_btree_build_stats</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>log_btree_build_stats</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
If set, logs system resource usage statistics (memory and CPU) on
various B-tree operations.
</para>
<para>
This parameter is only available if the <symbol>BTREE_BUILD_STATS</symbol>
macro was defined when <productname>PostgreSQL</productname> was
compiled.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-wal-debug" xreflabel="wal_debug">
<term><varname>wal_debug</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>wal_debug</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
If on, emit WAL-related debugging output. This parameter is
only available if the <symbol>WAL_DEBUG</symbol> macro was
defined when <productname>PostgreSQL</productname> was
compiled.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-ignore-checksum-failure" xreflabel="ignore_checksum_failure">
<term><varname>ignore_checksum_failure</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>ignore_checksum_failure</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Only has effect if <xref linkend="app-initdb-data-checksums"> are enabled.
</para>
<para>
Detection of a checksum failure during a read normally causes
<productname>PostgreSQL</> to report an error, aborting the current
transaction. Setting <varname>ignore_checksum_failure</> to on causes
the system to ignore the failure (but still report a warning), and
continue processing. This behavior may <emphasis>cause crashes, propagate
or hide corruption, or other serious problems</>. However, it may allow
you to get past the error and retrieve undamaged tuples that might still be
present in the table if the block header is still sane. If the header is
corrupt an error will be reported even if this option is enabled. The
default setting is <literal>off</>, and it can only be changed by a superuser.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-zero-damaged-pages" xreflabel="zero_damaged_pages">
<term><varname>zero_damaged_pages</varname> (<type>boolean</type>)
<indexterm>
<primary><varname>zero_damaged_pages</> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Detection of a damaged page header normally causes
<productname>PostgreSQL</> to report an error, aborting the current
transaction. Setting <varname>zero_damaged_pages</> to on causes
the system to instead report a warning, zero out the damaged
page in memory, and continue processing. This behavior <emphasis>will destroy data</>,
namely all the rows on the damaged page. However, it does allow you to get
past the error and retrieve rows from any undamaged pages that might
be present in the table. It is useful for recovering data if
corruption has occurred due to a hardware or software error. You should
generally not set this on until you have given up hope of recovering
data from the damaged pages of a table. Zeroed-out pages are not
forced to disk so it is recommended to recreate the table or
the index before turning this parameter off again. The
default setting is <literal>off</>, and it can only be changed
by a superuser.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect1>
<sect1 id="runtime-config-short">
<title>Short Options</title>
<para>
For convenience there are also single letter command-line option
switches available for some parameters. They are described in
<xref linkend="runtime-config-short-table">. Some of these
options exist for historical reasons, and their presence as a
single-letter option does not necessarily indicate an endorsement
to use the option heavily.
</para>
<table id="runtime-config-short-table">
<title>Short Option Key</title>
<tgroup cols="2">
<thead>
<row>
<entry>Short Option</entry>
<entry>Equivalent</entry>
</row>
</thead>
<tbody>
<row>
<entry><option>-B <replaceable>x</replaceable></option></entry>
<entry><literal>shared_buffers = <replaceable>x</replaceable></></entry>
</row>
<row>
<entry><option>-d <replaceable>x</replaceable></option></entry>
<entry><literal>log_min_messages = DEBUG<replaceable>x</replaceable></></entry>
</row>
<row>
<entry><option>-e</option></entry>
<entry><literal>datestyle = euro</></entry>
</row>
<row>
<entry>
<option>-fb</option>, <option>-fh</option>, <option>-fi</option>,
<option>-fm</option>, <option>-fn</option>, <option>-fo</option>,
<option>-fs</option>, <option>-ft</option>
</entry>
<entry>
<literal>enable_bitmapscan = off</>,
<literal>enable_hashjoin = off</>,
<literal>enable_indexscan = off</>,
<literal>enable_mergejoin = off</>,
<literal>enable_nestloop = off</>,
<literal>enable_indexonlyscan = off</>,
<literal>enable_seqscan = off</>,
<literal>enable_tidscan = off</>
</entry>
</row>
<row>
<entry><option>-F</option></entry>
<entry><literal>fsync = off</></entry>
</row>
<row>
<entry><option>-h <replaceable>x</replaceable></option></entry>
<entry><literal>listen_addresses = <replaceable>x</replaceable></></entry>
</row>
<row>
<entry><option>-i</option></entry>
<entry><literal>listen_addresses = '*'</></entry>
</row>
<row>
<entry><option>-k <replaceable>x</replaceable></option></entry>
<entry><literal>unix_socket_directories = <replaceable>x</replaceable></></entry>
</row>
<row>
<entry><option>-l</option></entry>
<entry><literal>ssl = on</></entry>
</row>
<row>
<entry><option>-N <replaceable>x</replaceable></option></entry>
<entry><literal>max_connections = <replaceable>x</replaceable></></entry>
</row>
<row>
<entry><option>-O</option></entry>
<entry><literal>allow_system_table_mods = on</></entry>
</row>
<row>
<entry><option>-p <replaceable>x</replaceable></option></entry>
<entry><literal>port = <replaceable>x</replaceable></></entry>
</row>
<row>
<entry><option>-P</option></entry>
<entry><literal>ignore_system_indexes = on</></entry>
</row>
<row>
<entry><option>-s</option></entry>
<entry><literal>log_statement_stats = on</></entry>
</row>
<row>
<entry><option>-S <replaceable>x</replaceable></option></entry>
<entry><literal>work_mem = <replaceable>x</replaceable></></entry>
</row>
<row>
<entry><option>-tpa</option>, <option>-tpl</option>, <option>-te</option></entry>
<entry><literal>log_parser_stats = on</>,
<literal>log_planner_stats = on</>,
<literal>log_executor_stats = on</></entry>
</row>
<row>
<entry><option>-W <replaceable>x</replaceable></option></entry>
<entry><literal>post_auth_delay = <replaceable>x</replaceable></></entry>
</row>
</tbody>
</tgroup>
</table>
</sect1>
</chapter>
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