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postgres/doc/src/sgml/config.sgml
Tom Lane acfce502ba Create a GUC parameter temp_tablespaces that allows selection of the 
tablespace(s) in which to store temp tables and temporary files.  This is a
list to allow spreading the load across multiple tablespaces (a random list
element is chosen each time a temp object is to be created).  Temp files are
not stored in per-database pgsql_tmp/ directories anymore, but per-tablespace
directories.

Jaime Casanova and Albert Cervera, with review by Bernd Helmle and Tom Lane.
2007-06-03 17:08:34 +00:00

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<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.125 2007/06/03 17:05:29 tgl Exp $ -->
<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>
<para>
All parameter names are case-insensitive. Every parameter takes a
value of one of four types: Boolean, integer, floating point,
or string. 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
queried by referencing <literal>pg_settings.unit</>. For convenience,
a different unit can also be specified explicitly. Valid memory units
are <literal>kB</literal> (kilobytes), <literal>MB</literal>
(megabytes), and <literal>GB</literal> (gigabytes); 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>
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. (<application>initdb</>
installs a default copy there.) 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>) introduce comments
anywhere. 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><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. 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>SIGHUP</primary>
</indexterm>
The configuration file is reread whenever the main server process receives a
<systemitem>SIGHUP</> signal (which is most easily sent by means
of <literal>pg_ctl reload</>). 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.
</para>
<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-alteruser" endterm="sql-alteruser-title">
and <xref linkend="sql-alterdatabase" endterm="sql-alterdatabase-title">,
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" endterm="SQL-SET-title">
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 can
be modified via <command>SET</command> or <command>ALTER</> by superusers,
but not by ordinary users.
</para>
<para>
The <xref linkend="SQL-SHOW" endterm="SQL-SHOW-title">
command allows inspection of the current values of all parameters.
</para>
<para>
The virtual table <structname>pg_settings</structname>
(described in <xref linkend="view-pg-settings">) also allows
displaying and updating session run-time parameters. It 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.
</para>
</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>)</term>
<indexterm>
<primary><varname>data_directory</> configuration parameter</primary>
</indexterm>
<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>)</term>
<indexterm>
<primary><varname>config_file</> configuration parameter</primary>
</indexterm>
<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>)</term>
<indexterm>
<primary><varname>hba_file</> configuration parameter</primary>
</indexterm>
<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>)</term>
<indexterm>
<primary><varname>ident_file</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Specifies the configuration file for
<application>ident</> authentication
(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>)</term>
<indexterm>
<primary><varname>external_pid_file</> configuration parameter</primary>
</indexterm>
<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>)</term>
<indexterm>
<primary><varname>listen_addresses</> configuration parameter</primary>
</indexterm>
<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.
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 <quote>loopback</> connections to be made.
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>)</term>
<indexterm>
<primary><varname>port</> configuration parameter</primary>
</indexterm>
<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>)</term>
<indexterm>
<primary><varname>max_connections</> configuration parameter</primary>
</indexterm>
<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>
Increasing this parameter might cause <productname>PostgreSQL</>
to request more <systemitem class="osname">System V</> shared
memory or semaphores than your operating system's default configuration
allows. See <xref linkend="sysvipc"> for information on how to
adjust those parameters, if necessary.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-superuser-reserved-connections"
xreflabel="superuser_reserved_connections">
<term><varname>superuser_reserved_connections</varname>
(<type>integer</type>)</term>
<indexterm>
<primary><varname>superuser_reserved_connections</> configuration parameter</primary>
</indexterm>
<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.
</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-directory" xreflabel="unix_socket_directory">
<term><varname>unix_socket_directory</varname> (<type>string</type>)</term>
<indexterm>
<primary><varname>unix_socket_directory</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Specifies the directory of the Unix-domain socket on which the
server is to listen for
connections from client applications. The default is normally
<filename>/tmp</filename>, but can be changed at build time.
This parameter can only be set at server start.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-unix-socket-group" xreflabel="unix_socket_group">
<term><varname>unix_socket_group</varname> (<type>string</type>)</term>
<indexterm>
<primary><varname>unix_socket_group</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Sets the owning group of the Unix-domain socket. (The owning
user of the socket 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 selects the default
group for the current user. This parameter can only be set at
server start.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-unix-socket-permissions" xreflabel="unix_socket_permissions">
<term><varname>unix_socket_permissions</varname> (<type>integer</type>)</term>
<indexterm>
<primary><varname>unix_socket_permissions</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Sets the access permissions of the Unix-domain socket. Unix-domain
sockets use the usual Unix file system permission set.
The parameter value is expected to be a numeric mode
specification in the form 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 and 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>
</listitem>
</varlistentry>
<varlistentry id="guc-bonjour-name" xreflabel="bonjour_name">
<term><varname>bonjour_name</varname> (<type>string</type>)</term>
<indexterm>
<primary><varname>bonjour_name</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Specifies the <productname>Bonjour</productname> broadcast
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>)</term>
<indexterm>
<primary><varname>tcp_keepalives_idle</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
On systems that support the <symbol>TCP_KEEPIDLE</symbol> socket option, specifies the
number of seconds between sending keepalives on an otherwise idle
connection. A value of zero uses the system default. If <symbol>TCP_KEEPIDLE</symbol> is
not supported, this parameter must be zero. This parameter is ignored for
connections made via a Unix-domain socket.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-tcp-keepalives-interval" xreflabel="tcp_keepalives_interval">
<term><varname>tcp_keepalives_interval</varname> (<type>integer</type>)</term>
<indexterm>
<primary><varname>tcp_keepalives_interval</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
On systems that support the <symbol>TCP_KEEPINTVL</symbol> socket option, specifies how
long, in seconds, to wait for a response to a keepalive before
retransmitting. A value of zero uses the system default. If <symbol>TCP_KEEPINTVL</symbol>
is not supported, this parameter must be zero. This parameter is ignored
for connections made via a Unix-domain socket.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-tcp-keepalives-count" xreflabel="tcp_keepalives_count">
<term><varname>tcp_keepalives_count</varname> (<type>integer</type>)</term>
<indexterm>
<primary><varname>tcp_keepalives_count</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
On systems that support the <symbol>TCP_KEEPCNT</symbol> socket option, specifies how
many keepalives can be lost before the connection is considered dead.
A value of zero uses the system default. If <symbol>TCP_KEEPCNT</symbol> is not
supported, this parameter must be zero. This parameter is ignored
for connections made via a Unix-domain socket.
</para>
</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>)</term>
<indexterm><primary>timeout</><secondary>client authentication</></indexterm>
<indexterm><primary>client authentication</><secondary>timeout during</></indexterm>
<indexterm>
<primary><varname>authentication_timeout</> configuration parameter</primary>
</indexterm>
<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 breaks 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>)</term>
<indexterm>
<primary><varname>ssl</> configuration parameter</primary>
</indexterm>
<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.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-ssl-ciphers" xreflabel="ssl_ciphers">
<term><varname>ssl_ciphers</varname> (<type>string</type>)</term>
<indexterm>
<primary><varname>ssl_ciphers</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Specifies a list of <acronym>SSL</> ciphers that are allowed to be
used on secure connections. See the <application>openssl</>
manual page for a list of supported ciphers.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-password-encryption" xreflabel="password_encryption">
<term><varname>password_encryption</varname> (<type>boolean</type>)</term>
<indexterm>
<primary><varname>password_encryption</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
When a password is specified in <xref
linkend="sql-createuser" endterm="sql-createuser-title"> or
<xref linkend="sql-alteruser" endterm="sql-alteruser-title">
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>)</term>
<indexterm>
<primary><varname>krb_server_keyfile</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Sets the location of the Kerberos server key file. See
<xref linkend="kerberos-auth"> for details. This parameter
can only be set at server start.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-krb-srvname" xreflabel="krb_srvname">
<term><varname>krb_srvname</varname> (<type>string</type>)</term>
<indexterm>
<primary><varname>krb_srvname</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Sets the Kerberos service name. See <xref linkend="kerberos-auth">
for details. This parameter can only be set at server start.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-krb-server-hostname" xreflabel="krb_server_hostname">
<term><varname>krb_server_hostname</varname> (<type>string</type>)</term>
<indexterm>
<primary><varname>krb_server_hostname</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Sets the host name part of the service principal.
This, combined with <varname>krb_srvname</>, is used to generate
the complete service principal, that is
<varname>krb_srvname</><literal>/</><varname>krb_server_hostname</><literal>@</>REALM.
If not set, the default is the server host name. See <xref linkend="kerberos-auth">
for details. This parameter can only be set at server start.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-krb-caseins-users" xreflabel="krb_caseins_users">
<term><varname>krb_caseins_users</varname> (<type>boolean</type>)</term>
<indexterm>
<primary><varname>krb_caseins_users</varname> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Sets whether Kerberos user names should be treated case-insensitively.
The default is <literal>off</> (case sensitive). This parameter
can only be set at server start.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-db-user-namespace" xreflabel="db_user_namespace">
<term><varname>db_user_namespace</varname> (<type>boolean</type>)</term>
<indexterm>
<primary><varname>db_user_namespace</> configuration parameter</primary>
</indexterm>
<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. The <literal>@</> will be stripped off
before the user name is looked up by the server.
</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>)</term>
<indexterm>
<primary><varname>shared_buffers</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Sets the amount of memory the database server uses for shared
memory buffers. The default is typically 32 megabytes
(<literal>32MB</>), 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 and at least 16
kilobytes times <xref linkend="guc-max-connections">. (Non-default
values of <symbol>BLCKSZ</symbol> change the minimum.) However,
settings significantly higher than the minimum are usually needed
for good performance. Several tens of megabytes are recommended
for production installations. This parameter can only be set at
server start.
</para>
<para>
Increasing this parameter might cause <productname>PostgreSQL</>
to request more <systemitem class="osname">System V</> shared
memory than your operating system's default configuration
allows. See <xref linkend="sysvipc"> for information on how to
adjust those parameters, if necessary.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-temp-buffers" xreflabel="temp_buffers">
<term><varname>temp_buffers</varname> (<type>integer</type>)</term>
<indexterm>
<primary><varname>temp_buffers</> configuration parameter</primary>
</indexterm>
<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 up until the first use of temporary tables
within a 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 a lot of 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>)</term>
<indexterm>
<primary><varname>max_prepared_transactions</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Sets the maximum number of transactions that can be in the
<quote>prepared</> state simultaneously (see <xref
linkend="sql-prepare-transaction"
endterm="sql-prepare-transaction-title">).
Setting this parameter to zero disables the prepared-transaction
feature.
The default is five transactions.
This parameter can only be set at server start.
</para>
<para>
If you are not using prepared transactions, this parameter may as
well be set to zero. If you are using them, you will probably
want <varname>max_prepared_transactions</varname> to be at least
as large as <xref linkend="guc-max-connections">, to avoid unwanted
failures at the prepare step.
</para>
<para>
Increasing this parameter might cause <productname>PostgreSQL</>
to request more <systemitem class="osname">System V</> shared
memory than your operating system's default configuration
allows. See <xref linkend="sysvipc"> for information on how to
adjust those parameters, if necessary.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-work-mem" xreflabel="work_mem">
<term><varname>work_mem</varname> (<type>integer</type>)</term>
<indexterm>
<primary><varname>work_mem</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Specifies the amount of memory to be used by internal sort operations
and hash tables before switching to temporary disk files. The value is
defaults to one megabyte (<literal>1MB</>).
Note that for a complex query, several sort or hash operations might be
running in parallel; each one will be allowed to use as much memory
as this value specifies before it starts to put data into temporary
files. Also, several running sessions could be doing such operations
concurrently. So 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>)</term>
<indexterm>
<primary><varname>maintenance_work_mem</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Specifies the maximum amount of memory to be used in maintenance
operations, such as <command>VACUUM</command>, <command>CREATE
INDEX</>, and <command>ALTER TABLE ADD FOREIGN KEY</>. It defaults
to 16 megabytes (<literal>16MB</>). 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>
</listitem>
</varlistentry>
<varlistentry id="guc-max-stack-depth" xreflabel="max_stack_depth">
<term><varname>max_stack_depth</varname> (<type>integer</type>)</term>
<indexterm>
<primary><varname>max_stack_depth</> configuration parameter</primary>
</indexterm>
<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,
it will not let you set this variable to an unsafe value. However,
not all platforms provide the information, so caution is recommended
in selecting a value.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="runtime-config-resource-fsm">
<title>Free Space Map</title>
<indexterm>
<primary>free space map</primary>
</indexterm>
<para>
These parameters control the size of the shared <firstterm>free space
map</>, which tracks the locations of unused space in the database.
An undersized free space map can cause the database to consume
increasing amounts of disk space over time, because free space that
is not in the map cannot be re-used; instead <productname>PostgreSQL</>
will request more disk space from the operating system when it needs
to store new data.
The last few lines displayed by a database-wide <command>VACUUM VERBOSE</>
command can help in determining if the current settings are adequate.
A <literal>NOTICE</> message is also printed during such an operation
if the current settings are too low.
</para>
<para>
Increasing these parameters might cause <productname>PostgreSQL</>
to request more <systemitem class="osname">System V</> shared
memory than your operating system's default configuration
allows. See <xref linkend="sysvipc"> for information on how to
adjust those parameters, if necessary.
</para>
<variablelist>
<varlistentry id="guc-max-fsm-pages" xreflabel="max_fsm_pages">
<term><varname>max_fsm_pages</varname> (<type>integer</type>)</term>
<indexterm>
<primary><varname>max_fsm_pages</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Sets the maximum number of disk pages for which free space will
be tracked in the shared free-space map. Six bytes of shared memory
are consumed for each page slot. This setting must be at least
16 * <varname>max_fsm_relations</varname>. The default is chosen
by <application>initdb</> depending on the amount of available memory,
and can range from 20k to 200k pages.
This parameter can only be set at server start.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-max-fsm-relations" xreflabel="max_fsm_relations">
<term><varname>max_fsm_relations</varname> (<type>integer</type>)</term>
<indexterm>
<primary><varname>max_fsm_relations</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Sets the maximum number of relations (tables and indexes) for which
free space will be tracked in the shared free-space map. Roughly
seventy bytes of shared memory are consumed for each slot.
The default is one thousand relations.
This parameter can only be set at server start.
</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>)</term>
<indexterm>
<primary><varname>max_files_per_process</> configuration parameter</primary>
</indexterm>
<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 really support when a large number of 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>
<varlistentry id="guc-shared-preload-libraries" xreflabel="shared_preload_libraries">
<term><varname>shared_preload_libraries</varname> (<type>string</type>)</term>
<indexterm>
<primary><varname>shared_preload_libraries</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
This variable specifies one or more shared libraries that are
to be preloaded at server start. If more than one library is to be
loaded, separate their names with commas. For example,
<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.
This parameter can only be set at server start.
</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>
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.
</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 because some shared libraries may
need to perform certain operations that only take place at postmaster start
(for example, a shared library may need to reserve lightweight locks
or shared memory and you can't do that after the postmaster has started).
</para>
</note>
<para>
If a specified library is not found,
the server will fail to start.
</para>
<para>
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.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="runtime-config-resource-vacuum-cost">
<title id="runtime-config-resource-vacuum-cost-title">
Cost-Based Vacuum Delay
</title>
<para>
During the execution of <xref linkend="sql-vacuum"
endterm="sql-vacuum-title"> and <xref linkend="sql-analyze"
endterm="sql-analyze-title"> 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 while (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 in which it is not very
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. 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>)</term>
<indexterm>
<primary><varname>vacuum_cost_delay</> configuration parameter</primary>
</indexterm>
<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>
</listitem>
</varlistentry>
<varlistentry id="guc-vacuum-cost-page-hit" xreflabel="vacuum_cost_page_hit">
<term><varname>vacuum_cost_page_hit</varname> (<type>integer</type>)</term>
<indexterm>
<primary><varname>vacuum_cost_page_hit</> configuration parameter</primary>
</indexterm>
<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>)</term>
<indexterm>
<primary><varname>vacuum_cost_page_miss</> configuration parameter</primary>
</indexterm>
<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>)</term>
<indexterm>
<primary><varname>vacuum_cost_page_dirty</> configuration parameter</primary>
</indexterm>
<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>)</term>
<indexterm>
<primary><varname>vacuum_cost_limit</> configuration parameter</primary>
</indexterm>
<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>
Beginning in <productname>PostgreSQL</> 8.0, there is a separate server
process called the <firstterm>background writer</>, whose sole function
is to issue writes of <quote>dirty</> shared buffers. The intent is
that server processes handling user queries should seldom or never have
to wait for a write to occur, because the background writer will do it.
This arrangement also reduces the performance penalty associated with
checkpoints. The background writer will continuously trickle out dirty
pages to disk, so that only a few pages will need to be forced out when
checkpoint time arrives, instead of the storm of dirty-buffer writes that
formerly occurred at each checkpoint. However there is a net overall
increase in I/O load, because where a repeatedly-dirtied page might
before have been written only once per checkpoint interval, the
background writer might write it several times in the same interval.
In most situations a continuous low load is preferable to periodic
spikes, but 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>)</term>
<indexterm>
<primary><varname>bgwriter_delay</> configuration parameter</primary>
</indexterm>
<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. 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-percent" xreflabel="bgwriter_lru_percent">
<term><varname>bgwriter_lru_percent</varname> (<type>floating point</type>)</term>
<indexterm>
<primary><varname>bgwriter_lru_percent</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
To reduce the probability that server processes will need to issue
their own writes, the background writer tries to write buffers that
are likely to be recycled soon. In each round, it examines up to
<varname>bgwriter_lru_percent</> of the buffers that are nearest to
being recycled, and writes any that are dirty.
The default value is 1.0 (1% of the total number of shared 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-maxpages" xreflabel="bgwriter_lru_maxpages">
<term><varname>bgwriter_lru_maxpages</varname> (<type>integer</type>)</term>
<indexterm>
<primary><varname>bgwriter_lru_maxpages</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
In each round, no more than this many buffers will be written
as a result of scanning soon-to-be-recycled buffers.
The default value is five 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-all-percent" xreflabel="bgwriter_all_percent">
<term><varname>bgwriter_all_percent</varname> (<type>floating point</type>)</term>
<indexterm>
<primary><varname>bgwriter_all_percent</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
To reduce the amount of work that will be needed at checkpoint time,
the background writer also does a circular scan through the entire
buffer pool, writing buffers that are found to be dirty.
In each round, it examines up to
<varname>bgwriter_all_percent</> of the buffers for this purpose.
The default value is 0.333 (0.333% of the total number
of shared buffers). With the default <varname>bgwriter_delay</>
setting, this will allow the entire shared buffer pool to be scanned
about once per minute.
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-all-maxpages" xreflabel="bgwriter_all_maxpages">
<term><varname>bgwriter_all_maxpages</varname> (<type>integer</type>)</term>
<indexterm>
<primary><varname>bgwriter_all_maxpages</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
In each round, no more than this many buffers will be written
as a result of the scan of the entire buffer pool. (If this
limit is reached, the scan stops, and resumes at the next buffer
during the next round.)
The default value is five buffers.
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_all_percent</varname> and
<varname>bgwriter_all_maxpages</varname> reduce the extra I/O load
caused by the background writer, but leave more work to be done
at checkpoint time. To reduce load spikes at checkpoints,
increase these two values.
Similarly, smaller values of <varname>bgwriter_lru_percent</varname> and
<varname>bgwriter_lru_maxpages</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.
To disable background writing entirely,
set both <varname>maxpages</varname> values and/or both
<varname>percent</varname> values to zero.
</para>
</sect2>
</sect1>
<sect1 id="runtime-config-wal">
<title>Write Ahead Log</title>
<para>
See also <xref linkend="wal-configuration"> for details on WAL
tuning.
</para>
<sect2 id="runtime-config-wal-settings">
<title>Settings</title>
<variablelist>
<varlistentry id="guc-fsync" xreflabel="fsync">
<indexterm>
<primary><varname>fsync</> configuration parameter</primary>
</indexterm>
<term><varname>fsync</varname> (<type>boolean</type>)</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>
However, using <varname>fsync</varname> results in a
performance penalty: when a transaction is committed,
<productname>PostgreSQL</productname> must wait for the
operating system to flush the write-ahead log to disk. When
<varname>fsync</varname> is disabled, the operating system is
allowed to do its best in buffering, ordering, and delaying
writes. This can result in significantly improved performance.
However, if the system crashes, the results of the last few
committed transactions might be lost in part or whole. In the
worst case, unrecoverable data corruption might occur.
(Crashes of the database software itself are <emphasis>not</>
a risk factor here. Only an operating-system-level crash
creates a risk of corruption.)
</para>
<para>
Due to the risks involved, there is no universally correct
setting for <varname>fsync</varname>. Some administrators
always disable <varname>fsync</varname>, while others only
turn it off during initial bulk data loads, where there is a clear
restart point if something goes wrong. Others
always leave <varname>fsync</varname> enabled. The default is
to enable <varname>fsync</varname>, for maximum reliability.
If you trust your operating system, your hardware, and your
utility company (or your battery backup), you can consider
disabling <varname>fsync</varname>.
</para>
<para>
This parameter 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-wal-sync-method" xreflabel="wal_sync_method">
<term><varname>wal_sync_method</varname> (<type>string</type>)</term>
<indexterm>
<primary><varname>wal_sync_method</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Method used for forcing WAL updates out to disk.
If <varname>fsync</varname> is off then this setting is irrelevant,
since 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_writethrough</> (call <function>fsync()</> at each commit, forcing write-through of any disk write cache)
</para>
</listitem>
<listitem>
<para>
<literal>fsync</> (call <function>fsync()</> at each commit)
</para>
</listitem>
<listitem>
<para>
<literal>open_sync</> (write WAL files with <function>open()</> option <symbol>O_SYNC</>)
</para>
</listitem>
</itemizedlist>
<para>
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.
The <literal>open_</>* options also use <literal>O_DIRECT</> if available.
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">
<indexterm>
<primary><varname>full_page_writes</> configuration parameter</primary>
</indexterm>
<term><varname>full_page_writes</varname> (<type>boolean</type>)</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 a price
in 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 a corrupt database after an operating system crash
or power failure. The risks are similar to turning off
<varname>fsync</>, though smaller. It might be safe to turn off
this parameter if you have hardware (such as a battery-backed disk
controller) or file-system software that reduces
the risk of partial page writes to an acceptably low level (e.g., ReiserFS 4).
</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-buffers" xreflabel="wal_buffers">
<term><varname>wal_buffers</varname> (<type>integer</type>)</term>
<indexterm>
<primary><varname>wal_buffers</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
The amount of memory used in shared memory for WAL data. The
default is 64 kilobytes (<literal>64kB</>). The setting need only
be large enough to hold the amount of WAL data generated by one
typical transaction, since the data is written out to disk at
every transaction commit. This parameter can only be set at server
start.
</para>
<para>
Increasing this parameter might cause <productname>PostgreSQL</>
to request more <systemitem class="osname">System V</> shared
memory than your operating system's default configuration
allows. See <xref linkend="sysvipc"> for information on how to
adjust those parameters, if necessary.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-commit-delay" xreflabel="commit_delay">
<term><varname>commit_delay</varname> (<type>integer</type>)</term>
<indexterm>
<primary><varname>commit_delay</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Time delay between writing a commit record to the WAL buffer
and flushing the buffer out to disk, in microseconds. A
nonzero delay can allow multiple transactions to be committed
with only one <function>fsync()</function> system call, if
system load is high enough that additional transactions become
ready to commit within the given interval. But the delay is
just wasted if no other transactions become ready to
commit. Therefore, the delay is only performed if at least
<varname>commit_siblings</varname> other transactions are
active at the instant that a server process has written its
commit record. The default is zero (no delay).
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-commit-siblings" xreflabel="commit_siblings">
<term><varname>commit_siblings</varname> (<type>integer</type>)</term>
<indexterm>
<primary><varname>commit_siblings</> configuration parameter</primary>
</indexterm>
<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>)</term>
<indexterm>
<primary><varname>checkpoint_segments</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Maximum distance between automatic WAL checkpoints, in log
file segments (each segment is normally 16 megabytes). The
default is three segments.
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>)</term>
<indexterm>
<primary><varname>checkpoint_timeout</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Maximum time between automatic WAL checkpoints, in
seconds. The default is five minutes (<literal>5min</>).
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>)</term>
<indexterm>
<primary><varname>checkpoint_warning</> configuration parameter</primary>
</indexterm>
<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.
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-command" xreflabel="archive_command">
<term><varname>archive_command</varname> (<type>string</type>)</term>
<indexterm>
<primary><varname>archive_command</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
The shell command to execute to archive a completed segment of
the WAL file series. If this is an empty string (the default),
WAL archiving is disabled. Any <literal>%p</> in the string is
replaced by the path name of the file to archive, and any
<literal>%f</> is replaced by the file name only.
(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. For more information see <xref
linkend="backup-archiving-wal">.
This parameter can only be set in the <filename>postgresql.conf</>
file or on the server command line.
</para>
<para>
It is important for the command to return a zero exit status if
and only if it succeeds. Examples:
<programlisting>
archive_command = 'cp "%p" /mnt/server/archivedir/"%f"'
archive_command = 'copy "%p" /mnt/server/archivedir/"%f"' # Windows
</programlisting>
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-archive-timeout" xreflabel="archive_timeout">
<term><varname>archive_timeout</varname> (<type>integer</type>)</term>
<indexterm>
<primary><varname>archive_timeout</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
The <xref linkend="guc-archive-command"> is only invoked on
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 put a limit on 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. 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. 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-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 temporary solution can be found by using one
of these configuration parameters to force the optimizer to
choose a different plan. Turning one of these settings off
permanently is seldom a good idea, however.
Better ways to improve the quality of the
plans chosen by the optimizer include adjusting the <xref
linkend="runtime-config-query-constants"
endterm="runtime-config-query-constants-title">, running <xref
linkend="sql-analyze" endterm="sql-analyze-title"> more
frequently, 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>)</term>
<indexterm>
<primary>bitmap scan</primary>
</indexterm>
<indexterm>
<primary><varname>enable_bitmapscan</> configuration parameter</primary>
</indexterm>
<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>)</term>
<indexterm>
<primary><varname>enable_hashagg</> configuration parameter</primary>
</indexterm>
<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>)</term>
<indexterm>
<primary><varname>enable_hashjoin</> configuration parameter</primary>
</indexterm>
<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>)</term>
<indexterm>
<primary>index scan</primary>
</indexterm>
<indexterm>
<primary><varname>enable_indexscan</> configuration parameter</primary>
</indexterm>
<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-mergejoin" xreflabel="enable_mergejoin">
<term><varname>enable_mergejoin</varname> (<type>boolean</type>)</term>
<indexterm>
<primary><varname>enable_mergejoin</> configuration parameter</primary>
</indexterm>
<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>)</term>
<indexterm>
<primary><varname>enable_nestloop</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Enables or disables the query planner's use of nested-loop join
plans. It's not possible 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>)</term>
<indexterm>
<primary>sequential scan</primary>
</indexterm>
<indexterm>
<primary><varname>enable_seqscan</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Enables or disables the query planner's use of sequential scan
plan types. It's not possible 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>)</term>
<indexterm>
<primary><varname>enable_sort</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Enables or disables the query planner's use of explicit sort
steps. It's not possible 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>)</term>
<indexterm>
<primary><varname>enable_tidscan</> configuration parameter</primary>
</indexterm>
<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 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. Traditionally, these variables have been
referenced to sequential page fetches as the unit of cost; 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 get. 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>)</term>
<indexterm>
<primary><varname>seq_page_cost</> configuration parameter</primary>
</indexterm>
<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.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-random-page-cost" xreflabel="random_page_cost">
<term><varname>random_page_cost</varname> (<type>floating point</type>)</term>
<indexterm>
<primary><varname>random_page_cost</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Sets the planner's estimate of the cost of a
non-sequentially-fetched disk page. The default is 4.0.
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>
<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>)</term>
<indexterm>
<primary><varname>cpu_tuple_cost</> configuration parameter</primary>
</indexterm>
<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>)</term>
<indexterm>
<primary><varname>cpu_index_tuple_cost</> configuration parameter</primary>
</indexterm>
<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>)</term>
<indexterm>
<primary><varname>cpu_operator_cost</> configuration parameter</primary>
</indexterm>
<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>)</term>
<indexterm>
<primary><varname>effective_cache_size</> configuration parameter</primary>
</indexterm>
<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 default is 128 megabytes (<literal>128MB</>).
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="runtime-config-query-geqo">
<title>Genetic Query Optimizer</title>
<variablelist>
<varlistentry id="guc-geqo" xreflabel="geqo">
<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><varname>geqo</varname> (<type>boolean</type>)</term>
<listitem>
<para>
Enables or disables genetic query optimization, which is an
algorithm that attempts to do query planning without
exhaustive searching. This is on by default. The
<varname>geqo_threshold</varname> variable provides a more
granular way to disable GEQO for certain classes of queries.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-geqo-threshold" xreflabel="geqo_threshold">
<term><varname>geqo_threshold</varname> (<type>integer</type>)</term>
<indexterm>
<primary><varname>geqo_threshold</> configuration parameter</primary>
</indexterm>
<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 deterministic, exhaustive planner, but for queries with
many tables the deterministic planner takes too long.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-geqo-effort" xreflabel="geqo_effort">
<term><varname>geqo_effort</varname>
(<type>integer</type>)</term>
<indexterm>
<primary><varname>geqo_effort</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Controls the trade off between planning time and query plan
efficiency 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>)</term>
<indexterm>
<primary><varname>geqo_pool_size</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Controls the pool size used by GEQO. The pool size 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
default 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>)</term>
<indexterm>
<primary><varname>geqo_generations</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Controls the number of generations used by GEQO. Generations
specifies 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 default 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>)</term>
<indexterm>
<primary><varname>geqo_selection_bias</> configuration parameter</primary>
</indexterm>
<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>
</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>)</term>
<indexterm>
<primary><varname>default_statistics_target</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Sets the default statistics target for table columns that have
not had 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 10. 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>boolean</type>)</term>
<indexterm>
<primary>constraint exclusion</primary>
</indexterm>
<indexterm>
<primary><varname>constraint_exclusion</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Enables or disables the query planner's use of table constraints to
optimize queries. The default is <literal>off</>.
</para>
<para>
When this parameter is <literal>on</>, the planner compares
query conditions with table <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. This can
improve performance when inheritance is used to build
partitioned tables.
</para>
<para>
Currently, <varname>constraint_exclusion</> is disabled by
default because the constraint checks are relatively
expensive, and in many circumstances will yield no savings.
It is recommended to turn this on only if you are actually
using partitioned tables designed to take advantage of the
feature.
</para>
<para>
Refer to <xref linkend="ddl-partitioning"> for more information
on using constraint exclusion and partitioning.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-from-collapse-limit" xreflabel="from_collapse_limit">
<term><varname>from_collapse_limit</varname> (<type>integer</type>)</term>
<indexterm>
<primary><varname>from_collapse_limit</> configuration parameter</primary>
</indexterm>
<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. It is usually
wise to keep this less than <xref linkend="guc-geqo-threshold">.
For more information see <xref linkend="explicit-joins">.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-join-collapse-limit" xreflabel="join_collapse_limit">
<term><varname>join_collapse_limit</varname> (<type>integer</type>)</term>
<indexterm>
<primary><varname>join_collapse_limit</> configuration parameter</primary>
</indexterm>
<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. 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>
</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>)</term>
<indexterm>
<primary><varname>log_destination</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
<productname>PostgreSQL</productname> supports several methods
for logging server messages, including
<systemitem>stderr</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>
</listitem>
</varlistentry>
<varlistentry id="guc-redirect-stderr" xreflabel="redirect_stderr">
<term><varname>redirect_stderr</varname> (<type>boolean</type>)</term>
<indexterm>
<primary><varname>redirect_stderr</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
This parameter allows messages sent to <application>stderr</> to be
captured and redirected into log files.
This method, in combination with logging to <application>stderr</>,
is often more useful than
logging to <application>syslog</>, since some types of messages
might not appear in <application>syslog</> output (a common example
is dynamic-linker failure messages).
This parameter can only be set at server start.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-log-directory" xreflabel="log_directory">
<term><varname>log_directory</varname> (<type>string</type>)</term>
<indexterm>
<primary><varname>log_directory</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
When <varname>redirect_stderr</> 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.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-log-filename" xreflabel="log_filename">
<term><varname>log_filename</varname> (<type>string</type>)</term>
<indexterm>
<primary><varname>log_filename</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
When <varname>redirect_stderr</varname> is enabled, this parameter
sets the file names of the created log files. The value
is treated as a <systemitem>strftime</systemitem> pattern,
so <literal>%</literal>-escapes
can be used to specify time-varying file names.
If no <literal>%</literal>-escapes are present,
<productname>PostgreSQL</productname> will
append the epoch of the new log file's open time. For example,
if <varname>log_filename</varname> were <literal>server_log</literal>, then the
chosen file name would be <literal>server_log.1093827753</literal>
for a log starting at Sun Aug 29 19:02:33 2004 MST.
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>)</term>
<indexterm>
<primary><varname>log_rotation_age</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
When <varname>redirect_stderr</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>)</term>
<indexterm>
<primary><varname>log_rotation_size</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
When <varname>redirect_stderr</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>)</term>
<indexterm>
<primary><varname>log_truncate_on_rotation</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
When <varname>redirect_stderr</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>string</type>)</term>
<indexterm>
<primary><varname>syslog_facility</> configuration parameter</primary>
</indexterm>
<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>)</term>
<indexterm>
<primary><varname>syslog_identity</> configuration parameter</primary>
</indexterm>
<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>
</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>string</type>)</term>
<indexterm>
<primary><varname>client_min_messages</> configuration parameter</primary>
</indexterm>
<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>string</type>)</term>
<indexterm>
<primary><varname>log_min_messages</> configuration parameter</primary>
</indexterm>
<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>NOTICE</>. 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-error-verbosity" xreflabel="log_error_verbosity">
<term><varname>log_error_verbosity</varname> (<type>string</type>)</term>
<indexterm>
<primary><varname>log_error_verbosity</> configuration parameter</primary>
</indexterm>
<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.
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>string</type>)</term>
<indexterm>
<primary><varname>log_min_error_statement</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Controls whether or not the SQL statement that causes an error
condition will be 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>)</term>
<indexterm>
<primary><varname>log_min_duration_statement</> configuration parameter</primary>
</indexterm>
<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>
<varlistentry id="guc-silent-mode" xreflabel="silent_mode">
<term><varname>silent_mode</varname> (<type>boolean</type>)</term>
<indexterm>
<primary><varname>silent_mode</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Runs the server silently. If this parameter is set, the server
will automatically run in background and any controlling
terminals are disassociated.
The server's standard output and standard error are redirected
to <literal>/dev/null</>, so any messages sent to them will be lost.
Unless <application>syslog</> logging is selected or
<varname>redirect_stderr</> is enabled, using this parameter
is discouraged because it makes it impossible to see error messages.
This parameter can only be set at server start.
</para>
</listitem>
</varlistentry>
</variablelist>
<para>
Here is a list of the various message severity levels used in
these settings:
<variablelist>
<varlistentry>
<term><literal>DEBUG[1-5]</literal></term>
<listitem>
<para>
Provides information for use by developers.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>INFO</literal></term>
<listitem>
<para>
Provides information implicitly requested by the user,
e.g., during <command>VACUUM VERBOSE</>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>NOTICE</literal></term>
<listitem>
<para>
Provides information that might be helpful to users, e.g.,
truncation of long identifiers and the creation of indexes as part
of primary keys.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>WARNING</literal></term>
<listitem>
<para>
Provides warnings to the user, e.g., <command>COMMIT</>
outside a transaction block.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>ERROR</literal></term>
<listitem>
<para>
Reports an error that caused the current command to abort.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>LOG</literal></term>
<listitem>
<para>
Reports information of interest to administrators, e.g.,
checkpoint activity.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>FATAL</literal></term>
<listitem>
<para>
Reports an error that caused the current session to abort.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>PANIC</literal></term>
<listitem>
<para>
Reports an error that caused all sessions to abort.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</sect2>
<sect2 id="runtime-config-logging-what">
<title>What To Log</title>
<variablelist>
<varlistentry>
<term><varname>debug_print_parse</varname> (<type>boolean</type>)</term>
<term><varname>debug_print_rewritten</varname> (<type>boolean</type>)</term>
<term><varname>debug_print_plan</varname> (<type>boolean</type>)</term>
<term><varname>debug_pretty_print</varname> (<type>boolean</type>)</term>
<indexterm>
<primary><varname>debug_print_parse</> configuration parameter</primary>
</indexterm>
<indexterm>
<primary><varname>debug_print_rewritten</> configuration parameter</primary>
</indexterm>
<indexterm>
<primary><varname>debug_print_plan</> configuration parameter</primary>
</indexterm>
<indexterm>
<primary><varname>debug_pretty_print</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
These parameters enable various debugging output to be emitted.
For each executed query, they print
the resulting parse tree, the query rewriter output, or the
execution plan. <varname>debug_pretty_print</varname> indents
these displays to produce a more readable but much longer
output format. <varname>client_min_messages</varname> or
<varname>log_min_messages</varname> must be
<literal>DEBUG1</literal> or lower to actually send this output
to the client or the server log, respectively.
These parameters are off by default.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-log-connections" xreflabel="log_connections">
<term><varname>log_connections</varname> (<type>boolean</type>)</term>
<indexterm>
<primary><varname>log_connections</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
This outputs a line to the server log detailing each successful
connection. This is off by default, although it is probably very
useful. 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.
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-disconnections" xreflabel="log_disconnections">
<term><varname>log_disconnections</varname> (<type>boolean</type>)</term>
<indexterm>
<primary><varname>log_disconnections</> configuration parameter</primary>
</indexterm>
<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 can only be set in the <filename>postgresql.conf</>
file or on the server command line.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-log-duration" xreflabel="log_duration">
<term><varname>log_duration</varname> (<type>boolean</type>)</term>
<indexterm>
<primary><varname>log_duration</> configuration parameter</primary>
</indexterm>
<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-line-prefix" xreflabel="log_line_prefix">
<term><varname>log_line_prefix</varname> (<type>string</type>)</term>
<indexterm>
<primary><varname>log_line_prefix</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
This is a <function>printf</>-style string that is output at the
beginning of each log line. The default is an empty string.
Each recognized escape is replaced as outlined
below - anything else that looks like an escape is ignored. Other
characters are copied straight to the log line. Some escapes are
only recognized by session processes, and do not apply to
background processes such as the main server process. <application>Syslog</>
produces its own
time stamp and process ID information, so you probably do not want to
use those escapes if you are using <application>syslog</>.
This parameter can only be set in the <filename>postgresql.conf</>
file or on the server command line.
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Escape</entry>
<entry>Effect</entry>
<entry>Session only</entry>
</row>
</thead>
<tbody>
<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 (no milliseconds, no timezone on Windows)</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: This is the command that generated the log line.</entry>
<entry>yes</entry>
</row>
<row>
<entry><literal>%c</literal></entry>
<entry>Session ID: A unique identifier for each session.
It is 2 4-byte hexadecimal numbers (without leading zeros)
separated by a dot. The numbers
are the session start time and the process ID, so this can also
be used as a space saving way of printing these items.</entry>
<entry>yes</entry>
</row>
<row>
<entry><literal>%l</literal></entry>
<entry>Number of the log line for each process, starting at 1</entry>
<entry>no</entry>
</row>
<row>
<entry><literal>%s</literal></entry>
<entry>Session start time stamp</entry>
<entry>yes</entry>
</row>
<row>
<entry><literal>%x</literal></entry>
<entry>Transaction ID</entry>
<entry>yes</entry>
</row>
<row>
<entry><literal>%q</literal></entry>
<entry>Does not produce any 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>
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-log-statement" xreflabel="log_statement">
<term><varname>log_statement</varname> (<type>string</type>)</term>
<indexterm>
<primary><varname>log_statement</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Controls which SQL statements are logged. Valid values are
<literal>none</>, <literal>ddl</>, <literal>mod</>, and
<literal>all</>. <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-hostname" xreflabel="log_hostname">
<term><varname>log_hostname</varname> (<type>boolean</type>)</term>
<indexterm>
<primary><varname>log_hostname</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
By default, connection log messages only show the IP address of the
connecting host. Turning on this parameter 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-lock-waits" xreflabel="log_lock_waits">
<term><varname>log_lock_waits</varname> (<type>boolean</type>)</term>
<indexterm>
<primary><varname>log_lock_waits</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Controls whether a log message is produced when a statement 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-temp-files" xreflabel="log_temp_files">
<term><varname>log_temp_files</varname> (<type>integer</type>)</term>
<indexterm>
<primary><varname>log_temp_files</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Controls whether temporary files are logged when deleted.
A value of zero logs all temporary files, and positive
values log only files whose size is equal or greater than
the specified number of kilobytes. Temporary files can be
created for sorts, hashes, and temporary results. The
default is <literal>-1</> (off).
</para>
</listitem>
</varlistentry>
</variablelist>
</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 a server-wide statistics collection feature.
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>
<note>
<para>
As of <productname>PostgreSQL</productname> 8.2,
<varname>stats_command_string</varname> controls a separate data
collection mechanism that can be turned on or off independently
of whether the statistics-collection subprocess is running.
The subprocess is only needed to support collection of
block-level or row-level statistics.
</para>
</note>
<variablelist>
<varlistentry id="guc-stats-command-string" xreflabel="stats_command_string">
<term><varname>stats_command_string</varname> (<type>boolean</type>)</term>
<indexterm>
<primary><varname>stats_command_string</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Enables the collection of information on the currently
executing command of each session, along with the time at
which 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-update-process-title" xreflabel="update_process_title">
<term><varname>update_process_title</varname> (<type>boolean</type>)</term>
<indexterm>
<primary><varname>update_process_title</> configuration parameter</primary>
</indexterm>
<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 using the <application>Process
Explorer</>. Only superusers can change this setting.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-stats-start-collector" xreflabel="stats_start_collector">
<term><varname>stats_start_collector</varname> (<type>boolean</type>)</term>
<indexterm>
<primary><varname>stats_start_collector</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Controls whether the server should start the
statistics-collection subprocess. This is on by default, but
can be turned off if you know you have no interest in
collecting statistics or running autovacuum.
This parameter can only be set at server start, because the collection
subprocess cannot be started or stopped on-the-fly. (However, the
extent to which statistics are actually gathered can be changed while
the server is running, so long as the subprocess exists.)
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-stats-block-level" xreflabel="stats_block_level">
<term><varname>stats_block_level</varname> (<type>boolean</type>)</term>
<indexterm>
<primary><varname>stats_block_level</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Enables the collection of block-level statistics on database
activity. This parameter is off by default.
Only superusers can change this setting.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-stats-row-level" xreflabel="stats_row_level">
<term><varname>stats_row_level</varname> (<type>boolean</type>)</term>
<indexterm>
<primary><varname>stats_row_level</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Enables the collection of row-level 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-stats-reset-on-server-start" xreflabel="stats_reset_on_server_start">
<term><varname>stats_reset_on_server_start</varname> (<type>boolean</type>)</term>
<indexterm>
<primary><varname>stats_reset_on_server_start</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
If on, collected block-level and row-level statistics are zeroed out
whenever the server is restarted. If off, statistics are accumulated
across server restarts. This parameter is off by default.
This parameter can only be set at server start.
</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>)</term>
<term><varname>log_parser_stats</varname> (<type>boolean</type>)</term>
<term><varname>log_planner_stats</varname> (<type>boolean</type>)</term>
<term><varname>log_executor_stats</varname> (<type>boolean</type>)</term>
<indexterm>
<primary><varname>log_statement_stats</> configuration parameter</primary>
</indexterm>
<indexterm>
<primary><varname>log_parser_stats</> configuration parameter</primary>
</indexterm>
<indexterm>
<primary><varname>log_planner_stats</> configuration parameter</primary>
</indexterm>
<indexterm>
<primary><varname>log_executor_stats</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
For each query, write performance statistics of the respective
module to the server log. This is a crude profiling
instrument. <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>)</term>
<indexterm>
<primary><varname>autovacuum</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Controls whether the server should run the
autovacuum launcher daemon. This is on by default.
<varname>stats_start_collector</> and <varname>stats_row_level</>
must also be turned on 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 periodically launch autovacuum processes in order to
prevent transaction ID wraparound. See <xref
linkend="vacuum-for-wraparound"> for more information.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-autovacuum-max-workers" xreflabel="autovacuum_max_workers">
<term><varname>autovacuum_max_workers</varname> (<type>integer</type>)</term>
<indexterm>
<primary><varname>autovacuum_max_workers</> configuration parameter</primary>
</indexterm>
<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 (<literal>3</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-log-autovacuum" xreflabel="log_autovacuum">
<term><varname>log_autovacuum</varname> (<type>integer</type>)</term>
<indexterm>
<primary><varname>log_autovacuum</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Causes actions executed by autovacuum to be logged if it ran for at
least the specified number of milliseconds. Setting this to zero prints
all action durations. Minus-one (the default) disables logging
autovacuum action durations. For example, if you set it to
<literal>250ms</literal> then all vacuums and analyzes that run
250ms or longer will be logged. 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-naptime" xreflabel="autovacuum_naptime">
<term><varname>autovacuum_naptime</varname> (<type>integer</type>)</term>
<indexterm>
<primary><varname>autovacuum_naptime</> configuration parameter</primary>
</indexterm>
<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>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-autovacuum-vacuum-threshold" xreflabel="autovacuum_vacuum_threshold">
<term><varname>autovacuum_vacuum_threshold</varname> (<type>integer</type>)</term>
<indexterm>
<primary><varname>autovacuum_vacuum_threshold</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Specifies the minimum number of updated or deleted tuples needed
to trigger a <command>VACUUM</> in any one table.
The default is 500 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 entries in
<structname>pg_autovacuum</>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-autovacuum-analyze-threshold" xreflabel="autovacuum_analyze_threshold">
<term><varname>autovacuum_analyze_threshold</varname> (<type>integer</type>)</term>
<indexterm>
<primary><varname>autovacuum_analyze_threshold</> configuration parameter</primary>
</indexterm>
<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 250 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 entries in
<structname>pg_autovacuum</>.
</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>)</term>
<indexterm>
<primary><varname>autovacuum_vacuum_scale_factor</> configuration parameter</primary>
</indexterm>
<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 entries in
<structname>pg_autovacuum</>.
</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>)</term>
<indexterm>
<primary><varname>autovacuum_analyze_scale_factor</> configuration parameter</primary>
</indexterm>
<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 entries in
<structname>pg_autovacuum</>.
</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>)</term>
<indexterm>
<primary><varname>autovacuum_freeze_max_age</> configuration parameter</primary>
</indexterm>
<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.
The default is 200 million transactions.
This parameter can only be set at server start, but the setting
can be reduced for individual tables by entries in
<structname>pg_autovacuum</>.
For more information see <xref linkend="vacuum-for-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>)</term>
<indexterm>
<primary><varname>autovacuum_vacuum_cost_delay</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Specifies the cost delay value that will be used in automatic
<command>VACUUM</> operations. If <literal>-1</> is
specified (which is the default), the regular
<xref linkend="guc-vacuum-cost-delay"> value will be used.
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 entries in
<structname>pg_autovacuum</>.
</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>)</term>
<indexterm>
<primary><varname>autovacuum_vacuum_cost_limit</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Specifies the cost limit value that will be used in automatic
<command>VACUUM</> operations. If <literal>-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 entries in
<structname>pg_autovacuum</>.
</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>)</term>
<indexterm>
<primary><varname>search_path</> configuration parameter</primary>
</indexterm>
<indexterm><primary>path</><secondary>for schemas</></>
<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 component. 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> has to be a comma-separated
list of schema names. If one of the list items is
the special value <literal>$user</literal>, then the schema
having the name returned by <function>SESSION_USER</> is substituted, if there
is such a schema. (If not, <literal>$user</literal> is ignored.)
</para>
<para>
The system catalog schema, <literal>pg_catalog</>, is always
searched, whether it is mentioned in the path or not. If it is
mentioned in the path then it will be searched in the specified
order. If <literal>pg_catalog</> is not in the path then it will
be searched <emphasis>before</> searching any of the path items.
</para>
<para>
Likewise, the current session's temporary-table schema,
<literal>pg_temp_<replaceable>nnn</></>, is always searched if it
exists. It can be explicitly listed in the path by using the
alias <literal>pg_temp</>. If it is not listed in the path then
it is searched first (before even <literal>pg_catalog</>). However,
the temporary schema is only searched for relation (table, view,
sequence, etc) and data type names. It will never be searched for
function or operator names.
</para>
<para>
When objects are created without specifying a particular target
schema, they will be placed in the first schema listed
in the search path. An error is reported if the search path is
empty.
</para>
<para>
The default value for this parameter is
<literal>'"$user", public'</literal> (where the second part will be
ignored if there is no schema named <literal>public</>).
This 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()</>. This is not quite the same as
examining the value of <varname>search_path</varname>, since
<function>current_schemas()</> shows how the requests
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>)</term>
<indexterm>
<primary><varname>default_tablespace</> configuration parameter</primary>
</indexterm>
<indexterm><primary>tablespace</><secondary>default</></>
<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>
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>)</term>
<indexterm>
<primary><varname>temp_tablespaces</> configuration parameter</primary>
</indexterm>
<indexterm><primary>tablespace</><secondary>temporary</></>
<listitem>
<para>
This variable specifies tablespace(s) 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 tablespace(s).
</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.
</para>
<para>
If any element of the list is an empty string or does not match the
name of any existing tablespace, <productname>PostgreSQL</> will
automatically use the default tablespace of the current database
instead. If a nondefault tablespace
is specified, the user must have <literal>CREATE</> privilege
for it, or creation attempts will fail.
</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>)</term>
<indexterm>
<primary><varname>check_function_bodies</> configuration parameter</primary>
</indexterm>
<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"
endterm="sql-createfunction-title">. Disabling validation is
occasionally useful to avoid problems such as forward references
when restoring function definitions from a dump.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-default-transaction-isolation" xreflabel="default_transaction_isolation">
<indexterm>
<primary>transaction isolation level</primary>
</indexterm>
<indexterm>
<primary><varname>default_transaction_isolation</> configuration parameter</primary>
</indexterm>
<term><varname>default_transaction_isolation</varname> (<type>string</type>)</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"
endterm="sql-set-transaction-title"> for more information.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-default-transaction-read-only" xreflabel="default_transaction_read_only">
<indexterm>
<primary>read-only transaction</primary>
</indexterm>
<indexterm>
<primary><varname>default_transaction_read_only</> configuration parameter</primary>
</indexterm>
<term><varname>default_transaction_read_only</varname> (<type>boolean</type>)</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"
endterm="sql-set-transaction-title"> for more information.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-statement-timeout" xreflabel="statement_timeout">
<term><varname>statement_timeout</varname> (<type>integer</type>)</term>
<indexterm>
<primary><varname>statement_timeout</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Abort any statement that takes over 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 off the
limitation.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-session-replication-role" xreflabel="session_replication_role">
<term><varname>session_replication_role</varname> (<type>string</type>)</term>
<indexterm>
<primary><varname>session_replication_role</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Controls the trigger and rule firing for the current session.
See <xref linkend="sql-altertable" endterm="sql-altertable-title"> for the different options to
enable or disable triggers and rules. Setting the variable requires
superuser privilege and can only be done before any query plans have
been cached. Possible values are <literal>origin</>,
<literal>replica</> and <literal>local</>.
</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>)</term>
<indexterm>
<primary><varname>vacuum_freeze_min_age</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Specifies the cutoff age (in transactions) that <command>VACUUM</>
should use to decide whether to replace transaction IDs with
<literal>FrozenXID</> while scanning a table.
The default is 100 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-xmlbinary" xreflabel="xmlbinary">
<term><varname>xmlbinary</varname> (<type>string</type>)</term>
<indexterm>
<primary><varname>xmlbinary</> configuration parameter</primary>
</indexterm>
<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>string</type>)</term>
<indexterm>
<primary><varname>xmloption</> configuration parameter</primary>
</indexterm>
<indexterm>
<primary><varname>SET XML OPTION</></primary>
</indexterm>
<indexterm>
<primary>XML option</primary>
</indexterm>
<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>)</term>
<indexterm>
<primary><varname>DateStyle</> configuration parameter</primary>
</indexterm>
<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-timezone" xreflabel="timezone">
<term><varname>timezone</varname> (<type>string</type>)</term>
<indexterm>
<primary><varname>timezone</> configuration parameter</primary>
</indexterm>
<indexterm><primary>time zone</></>
<listitem>
<para>
Sets the time zone for displaying and interpreting time stamps.
The default is <literal>'unknown'</>, which means to use whatever
the system environment specifies as the time zone. See <xref
linkend="datatype-datetime"> for more
information.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-timezone-abbreviations" xreflabel="timezone_abbreviations">
<term><varname>timezone_abbreviations</varname> (<type>string</type>)</term>
<indexterm>
<primary><varname>timezone_abbreviations</> configuration parameter</primary>
</indexterm>
<indexterm><primary>time zone names</></>
<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 'Australia' and 'India', 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">
<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><varname>extra_float_digits</varname> (<type>integer</type>)</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 2, 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.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-client-encoding" xreflabel="client_encoding">
<term><varname>client_encoding</varname> (<type>string</type>)</term>
<indexterm>
<primary><varname>client_encoding</> configuration parameter</primary>
</indexterm>
<indexterm><primary>character set</></>
<listitem>
<para>
Sets the client-side encoding (character set).
The default is to use the database encoding.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-lc-messages" xreflabel="lc_messages">
<term><varname>lc_messages</varname> (<type>string</type>)</term>
<indexterm>
<primary><varname>lc_messages</> configuration parameter</primary>
</indexterm>
<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.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-lc-monetary" xreflabel="lc_monetary">
<term><varname>lc_monetary</varname> (<type>string</type>)</term>
<indexterm>
<primary><varname>lc_monetary</> configuration parameter</primary>
</indexterm>
<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>)</term>
<indexterm>
<primary><varname>lc_numeric</> configuration parameter</primary>
</indexterm>
<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>)</term>
<indexterm>
<primary><varname>lc_time</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Sets the locale to use for formatting date and time values.
(Currently, this setting does nothing, but it might in the
future.) 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>
</variablelist>
</sect2>
<sect2 id="runtime-config-client-other">
<title>Other Defaults</title>
<variablelist>
<varlistentry id="guc-explain-pretty-print" xreflabel="explain_pretty_print">
<term><varname>explain_pretty_print</varname> (<type>boolean</type>)</term>
<indexterm>
<primary><varname>explain_pretty_print</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Determines whether <command>EXPLAIN VERBOSE</> uses the
indented or non-indented format for displaying detailed
query-tree dumps. The default is <literal>on</>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-dynamic-library-path" xreflabel="dynamic_library_path">
<term><varname>dynamic_library_path</varname> (<type>string</type>)</term>
<indexterm>
<primary><varname>dynamic_library_path</> configuration parameter</primary>
</indexterm>
<indexterm><primary>dynamic loading</></>
<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> has to 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>'$libdir'</literal>. If the value is set to an empty
string, the automatic path search is turned off.
</para>
<para>
This parameter can be changed at run time by superusers, but a
setting done that way will only persist until the end of the
client connection, so this method should be reserved for
development purposes. The recommended way to set this parameter
is in the <filename>postgresql.conf</filename> configuration
file.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-gin-fuzzy-search-limit" xreflabel="gin_fuzzy_search_limit">
<term><varname>gin_fuzzy_search_limit</varname> (<type>integer</type>)</term>
<indexterm>
<primary><varname>gin_fuzzy_search_limit</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Soft upper limit of the size of the set returned by GIN index. For more
information see <xref linkend="gin-tips">.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-local-preload-libraries" xreflabel="local_preload_libraries">
<term><varname>local_preload_libraries</varname> (<type>string</type>)</term>
<indexterm>
<primary><varname>local_preload_libraries</> configuration parameter</primary>
</indexterm>
<indexterm>
<primary><filename>$libdir/plugins</></primary>
</indexterm>
<listitem>
<para>
This variable specifies one or more shared libraries that are
to be preloaded at connection start. If more than one library
is to be loaded, separate their names with commas.
This parameter cannot be changed after the start of a particular
session.
</para>
<para>
Because this is not a superuser-only option, 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>
There is no performance advantage to loading a library at session
start rather than when it is first used. Rather, 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, debugging could
be enabled for all sessions under a given user name by setting
this parameter with <command>ALTER USER SET</>.
</para>
<para>
If a specified library is not found,
the connection attempt will fail.
</para>
<para>
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.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
</sect1>
<sect1 id="runtime-config-locks">
<title>Lock Management</title>
<variablelist>
<varlistentry id="guc-deadlock-timeout" xreflabel="deadlock_timeout">
<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><varname>deadlock_timeout</varname> (<type>integer</type>)</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 slow, 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 starting the check 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. Set <xref linkend="guc-log-lock-waits"> to log deadlock
checks. 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.
</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>)</term>
<indexterm>
<primary><varname>max_locks_per_transaction</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
The shared lock table is created to track 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 clients that touch many different
tables in a single transaction. This parameter can only be set at
server start.
</para>
<para>
Increasing this parameter might cause <productname>PostgreSQL</>
to request more <systemitem class="osname">System V</> shared
memory than your operating system's default configuration
allows. See <xref linkend="sysvipc"> for information on how to
adjust those parameters, if necessary.
</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-add-missing-from" xreflabel="add_missing_from">
<term><varname>add_missing_from</varname> (<type>boolean</type>)</term>
<indexterm><primary>FROM</><secondary>missing</></>
<indexterm>
<primary><varname>add_missing_from</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
When on, tables that are referenced by a query will be
automatically added to the <literal>FROM</> clause if not
already present. This behavior does not comply with the SQL
standard and many people dislike it because it can mask mistakes
(such as referencing a table where you should have referenced
its alias). The default is <literal>off</>. This variable can be
enabled for compatibility with releases of
<productname>PostgreSQL</> prior to 8.1, where this behavior was
allowed by default.
</para>
<para>
Note that even when this variable is enabled, a warning
message will be emitted for each implicit <literal>FROM</>
entry referenced by a query. Users are encouraged to update
their applications to not rely on this behavior, by adding all
tables referenced by a query to the query's <literal>FROM</>
clause (or its <literal>USING</> clause in the case of
<command>DELETE</>).
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-array-nulls" xreflabel="array_nulls">
<term><varname>array_nulls</varname> (<type>boolean</type>)</term>
<indexterm>
<primary><varname>array_nulls</> configuration parameter</primary>
</indexterm>
<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 backwards 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>string</type>)</term>
<indexterm><primary>strings</><secondary>backslash quotes</></>
<indexterm>
<primary><varname>backslash_quote</> configuration parameter</primary>
</indexterm>
<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 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>)</term>
<indexterm>
<primary><varname>default_with_oids</> configuration parameter</primary>
</indexterm>
<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>. In <productname>PostgreSQL</>
8.1 <varname>default_with_oids</> is <literal>off</> by default; in
prior versions of <productname>PostgreSQL</productname>, 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>)</term>
<indexterm><primary>strings</><secondary>escape warning</></>
<indexterm>
<primary><varname>escape_string_warning</> configuration parameter</primary>
</indexterm>
<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 will change
in a future release for SQL compatibility. This variable can
be enabled to help detect applications that will break.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-regex-flavor" xreflabel="regex_flavor">
<term><varname>regex_flavor</varname> (<type>string</type>)</term>
<indexterm><primary>regular expressions</></>
<indexterm>
<primary><varname>regex_flavor</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
The regular expression <quote>flavor</> can be set to
<literal>advanced</>, <literal>extended</>, or <literal>basic</>.
The default is <literal>advanced</>. The <literal>extended</>
setting might be useful for exact backwards compatibility with
pre-7.4 releases of <productname>PostgreSQL</>. See
<xref linkend="posix-syntax-details"> for details.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-sql-inheritance" xreflabel="sql_inheritance">
<term><varname>sql_inheritance</varname> (<type>boolean</type>)</term>
<indexterm>
<primary><varname>sql_inheritance</> configuration parameter</primary>
</indexterm>
<indexterm><primary>inheritance</></>
<listitem>
<para>
This controls the inheritance semantics. If turned <literal>off</>,
subtables are not included by various commands by default; basically
an implied <literal>ONLY</literal> key word. This was added for
compatibility with releases prior to 7.1. See
<xref linkend="ddl-inherit"> for more information.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-standard-conforming-strings" xreflabel="standard_conforming_strings">
<term><varname>standard_conforming_strings</varname> (<type>boolean</type>)</term>
<indexterm><primary>strings</><secondary>standard conforming</></>
<indexterm>
<primary><varname>standard_conforming_strings</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
This controls whether ordinary string literals
(<literal>'...'</>) treat backslashes literally, as specified in
the SQL standard.
The default is currently <literal>off</>, causing
<productname>PostgreSQL</productname> to have its historical
behavior of treating backslashes as escape characters.
The default will change to <literal>on</> in a future release
to improve compatibility with the standard.
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 should be used if an application desires
backslashes to be treated as escape characters.
</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>)</term>
<indexterm><primary>IS NULL</></>
<indexterm>
<primary><varname>transform_null_equals</> configuration parameter</primary>
</indexterm>
<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 correct 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 not on 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-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>)</term>
<indexterm>
<primary><varname>block_size</> configuration parameter</primary>
</indexterm>
<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-integer-datetimes" xreflabel="integer_datetimes">
<term><varname>integer_datetimes</varname> (<type>boolean</type>)</term>
<indexterm>
<primary><varname>integer_datetimes</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Reports whether <productname>PostgreSQL</productname> was built
with support for 64-bit-integer dates and times. It is set by
configuring with <literal>--enable-integer-datetimes</literal>
when building <productname>PostgreSQL</productname>. The
default value is <literal>off</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-lc-collate" xreflabel="lc_collate">
<term><varname>lc_collate</varname> (<type>string</type>)</term>
<indexterm>
<primary><varname>lc_collate</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Reports the locale in which sorting of textual data is done.
See <xref linkend="locale"> for more information.
The value is determined when the database cluster is initialized.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-lc-ctype" xreflabel="lc_ctype">
<term><varname>lc_ctype</varname> (<type>string</type>)</term>
<indexterm>
<primary><varname>lc_ctype</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Reports the locale that determines character classifications.
See <xref linkend="locale"> for more information.
The value is determined when the database cluster is initialized.
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>)</term>
<indexterm>
<primary><varname>max_function_args</> configuration parameter</primary>
</indexterm>
<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>)</term>
<indexterm>
<primary><varname>max_identifier_length</> configuration parameter</primary>
</indexterm>
<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.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-max-index-keys" xreflabel="max_index_keys">
<term><varname>max_index_keys</varname> (<type>integer</type>)</term>
<indexterm>
<primary><varname>max_index_keys</> configuration parameter</primary>
</indexterm>
<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-server-encoding" xreflabel="server_encoding">
<term><varname>server_encoding</varname> (<type>string</type>)</term>
<indexterm>
<primary><varname>server_encoding</> configuration parameter</primary>
</indexterm>
<indexterm><primary>character set</></>
<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>)</term>
<indexterm>
<primary><varname>server_version</> configuration parameter</primary>
</indexterm>
<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>)</term>
<indexterm>
<primary><varname>server_version_num</> configuration parameter</primary>
</indexterm>
<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>
</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 add-on modules to be
configured in the standard ways.
</para>
<variablelist>
<varlistentry id="guc-custom-variable-classes" xreflabel="custom_variable_classes">
<term><varname>custom_variable_classes</varname> (<type>string</type>)</term>
<indexterm>
<primary><varname>custom_variable_classes</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
This variable specifies one or several class names to be used for
custom variables, in the form of a comma-separated list. A custom
variable is a variable not normally known
to <productname>PostgreSQL</productname> proper but used by some
add-on module. Such variables must have names consisting of a class
name, a dot, and a variable name. <varname>custom_variable_classes</>
specifies all the class names in use in a particular installation.
This parameter can only be set in the <filename>postgresql.conf</>
file or on the server command line.
</para>
</listitem>
</varlistentry>
</variablelist>
<para>
The difficulty with setting custom variables in
<filename>postgresql.conf</> is that the file must be read before add-on
modules have been loaded, and so custom variables would ordinarily be
rejected as unknown. When <varname>custom_variable_classes</> is set,
the server will accept definitions of arbitrary variables within each
specified class. These variables will be treated as placeholders and
will have no function until the module that defines them is loaded. When a
module for a specific class is loaded, it will add the proper variable
definitions for its class name, convert any placeholder
values according to those definitions, and issue warnings for any
placeholders of its class that remain (which presumably would be
misspelled configuration variables).
</para>
<para>
Here is an example of what <filename>postgresql.conf</> might contain
when using custom variables:
<programlisting>
custom_variable_classes = 'plr,plperl'
plr.path = '/usr/lib/R'
plperl.use_strict = true
plruby.use_strict = true # generates error: unknown class name
</programlisting>
</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, and in some cases
to assist with recovery of severely damaged databases. There
should be no reason to use them in a production database setup.
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>)</term>
<indexterm>
<primary><varname>allow_system_table_mods</varname> configuration parameter</primary>
</indexterm>
<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-debug-assertions" xreflabel="debug_assertions">
<term><varname>debug_assertions</varname> (<type>boolean</type>)</term>
<indexterm>
<primary><varname>debug_assertions</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Turns on various assertion checks. This is a debugging aid. If
you are experiencing strange problems or crashes you might want
to turn this on, as it might expose programming mistakes. To use
this parameter, the macro <symbol>USE_ASSERT_CHECKING</symbol>
must be defined when <productname>PostgreSQL</productname> is
built (accomplished by the <command>configure</command> option
<option>--enable-cassert</option>). Note that
<varname>debug_assertions</varname> defaults to <literal>on</>
if <productname>PostgreSQL</productname> has been built with
assertions enabled.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-ignore-system-indexes" xreflabel="ignore_system_indexes">
<term><varname>ignore_system_indexes</varname> (<type>boolean</type>)</term>
<indexterm>
<primary><varname>ignore_system_indexes</varname> configuration parameter</primary>
</indexterm>
<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>)</term>
<indexterm>
<primary><varname>post_auth_delay</> configuration parameter</primary>
</indexterm>
<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 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>)</term>
<indexterm>
<primary><varname>pre_auth_delay</> configuration parameter</primary>
</indexterm>
<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 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>)</term>
<indexterm>
<primary><varname>trace_notify</> configuration parameter</primary>
</indexterm>
<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 log, respectively.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-trace-sort" xreflabel="trace_sort">
<term><varname>trace_sort</varname> (<type>boolean</type>)</term>
<indexterm>
<primary><varname>trace_sort</> configuration parameter</primary>
</indexterm>
<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>)</term>
<term><varname>trace_lwlocks</varname> (<type>boolean</type>)</term>
<term><varname>trace_userlocks</varname> (<type>boolean</type>)</term>
<term><varname>trace_lock_oidmin</varname> (<type>boolean</type>)</term>
<term><varname>trace_lock_table</varname> (<type>boolean</type>)</term>
<term><varname>debug_deadlocks</varname> (<type>boolean</type>)</term>
<term><varname>log_btree_build_stats</varname> (<type>boolean</type>)</term>
<listitem>
<para>
Various other code tracing and debugging options.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-wal-debug" xreflabel="wal_debug">
<term><varname>wal_debug</varname> (<type>boolean</type>)</term>
<indexterm>
<primary><varname>wal_debug</> configuration parameter</primary>
</indexterm>
<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-zero-damaged-pages" xreflabel="zero_damaged_pages">
<term><varname>zero_damaged_pages</varname> (<type>boolean</type>)</term>
<indexterm>
<primary><varname>zero_damaged_pages</> configuration parameter</primary>
</indexterm>
<listitem>
<para>
Detection of a damaged page header normally causes
<productname>PostgreSQL</> to report an error, aborting the current
command. Setting <varname>zero_damaged_pages</> to on causes
the system to instead report a warning, zero out the damaged page,
and continue processing. This behavior <emphasis>will destroy data</>,
namely all the rows on the damaged page. But it allows you to get
past the error and retrieve rows from any undamaged pages that might
be present in the table. So it is useful for recovering data if
corruption has occurred due to hardware or software error. You should
generally not set this on until you have given up hope of recovering
data from the damaged page(s) of a table. 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>-A <replaceable>x</replaceable></option></entry>
<entry><literal>debug_assertions = <replaceable>x</replaceable></></entry>
</row>
<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>-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_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_directory = <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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