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postgres/doc/src/sgml/monitoring.sgml
Robert Haas 16a4e4aecd Extend the ProcSignal mechanism to support barriers. 
A new function EmitProcSignalBarrier() can be used to emit a global
barrier which all backends that participate in the ProcSignal
mechanism must absorb, and a new function WaitForProcSignalBarrier()
can be used to wait until all relevant backends have in fact
absorbed the barrier.

This can be used to coordinate global state changes, such as turning
checksums on while the system is running.

There's no real client of this mechanism yet, although two are
proposed, but an enum has to have at least one element, so this
includes a placeholder type (PROCSIGNAL_BARRIER_PLACEHOLDER) which
should be replaced by the first real client of this mechanism to
get committed.

Andres Freund and Robert Haas, reviewed by Daniel Gustafsson and,
in earlier versions, by Magnus Hagander.

Discussion: http://postgr.es/m/CA+TgmoZwDk=BguVDVa+qdA6SBKef=PKbaKDQALTC_9qoz1mJqg@mail.gmail.com
2019-12-19 14:56:20 -05:00

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<!-- doc/src/sgml/monitoring.sgml -->
<chapter id="monitoring">
<title>Monitoring Database Activity</title>
<indexterm zone="monitoring">
<primary>monitoring</primary>
<secondary>database activity</secondary>
</indexterm>
<indexterm zone="monitoring">
<primary>database activity</primary>
<secondary>monitoring</secondary>
</indexterm>
<para>
A database administrator frequently wonders, <quote>What is the system
doing right now?</quote>
This chapter discusses how to find that out.
</para>
<para>
Several tools are available for monitoring database activity and
analyzing performance. Most of this chapter is devoted to describing
<productname>PostgreSQL</productname>'s statistics collector,
but one should not neglect regular Unix monitoring programs such as
<command>ps</command>, <command>top</command>, <command>iostat</command>, and <command>vmstat</command>.
Also, once one has identified a
poorly-performing query, further investigation might be needed using
<productname>PostgreSQL</productname>'s <xref linkend="sql-explain"/> command.
<xref linkend="using-explain"/> discusses <command>EXPLAIN</command>
and other methods for understanding the behavior of an individual
query.
</para>
<sect1 id="monitoring-ps">
<title>Standard Unix Tools</title>
<indexterm zone="monitoring-ps">
<primary>ps</primary>
<secondary>to monitor activity</secondary>
</indexterm>
<para>
On most Unix platforms, <productname>PostgreSQL</productname> modifies its
command title as reported by <command>ps</command>, so that individual server
processes can readily be identified. A sample display is
<screen>
$ ps auxww | grep ^postgres
postgres 15551 0.0 0.1 57536 7132 pts/0 S 18:02 0:00 postgres -i
postgres 15554 0.0 0.0 57536 1184 ? Ss 18:02 0:00 postgres: background writer
postgres 15555 0.0 0.0 57536 916 ? Ss 18:02 0:00 postgres: checkpointer
postgres 15556 0.0 0.0 57536 916 ? Ss 18:02 0:00 postgres: walwriter
postgres 15557 0.0 0.0 58504 2244 ? Ss 18:02 0:00 postgres: autovacuum launcher
postgres 15558 0.0 0.0 17512 1068 ? Ss 18:02 0:00 postgres: stats collector
postgres 15582 0.0 0.0 58772 3080 ? Ss 18:04 0:00 postgres: joe runbug 127.0.0.1 idle
postgres 15606 0.0 0.0 58772 3052 ? Ss 18:07 0:00 postgres: tgl regression [local] SELECT waiting
postgres 15610 0.0 0.0 58772 3056 ? Ss 18:07 0:00 postgres: tgl regression [local] idle in transaction
</screen>
(The appropriate invocation of <command>ps</command> varies across different
platforms, as do the details of what is shown. This example is from a
recent Linux system.) The first process listed here is the
master server process. The command arguments
shown for it are the same ones used when it was launched. The next five
processes are background worker processes automatically launched by the
master process. (The <quote>stats collector</quote> process will not be present
if you have set the system not to start the statistics collector; likewise
the <quote>autovacuum launcher</quote> process can be disabled.)
Each of the remaining
processes is a server process handling one client connection. Each such
process sets its command line display in the form
<screen>
postgres: <replaceable>user</replaceable> <replaceable>database</replaceable> <replaceable>host</replaceable> <replaceable>activity</replaceable>
</screen>
The user, database, and (client) host items remain the same for
the life of the client connection, but the activity indicator changes.
The activity can be <literal>idle</literal> (i.e., waiting for a client command),
<literal>idle in transaction</literal> (waiting for client inside a <command>BEGIN</command> block),
or a command type name such as <literal>SELECT</literal>. Also,
<literal>waiting</literal> is appended if the server process is presently waiting
on a lock held by another session. In the above example we can infer
that process 15606 is waiting for process 15610 to complete its transaction
and thereby release some lock. (Process 15610 must be the blocker, because
there is no other active session. In more complicated cases it would be
necessary to look into the
<link linkend="view-pg-locks"><structname>pg_locks</structname></link>
system view to determine who is blocking whom.)
</para>
<para>
If <xref linkend="guc-cluster-name"/> has been configured the
cluster name will also be shown in <command>ps</command> output:
<screen>
$ psql -c 'SHOW cluster_name'
cluster_name
--------------
server1
(1 row)
$ ps aux|grep server1
postgres 27093 0.0 0.0 30096 2752 ? Ss 11:34 0:00 postgres: server1: background writer
...
</screen>
</para>
<para>
If you have turned off <xref linkend="guc-update-process-title"/> then the
activity indicator is not updated; the process title is set only once
when a new process is launched. On some platforms this saves a measurable
amount of per-command overhead; on others it's insignificant.
</para>
<tip>
<para>
<productname>Solaris</productname> requires special handling. You must
use <command>/usr/ucb/ps</command>, rather than
<command>/bin/ps</command>. You also must use two <option>w</option>
flags, not just one. In addition, your original invocation of the
<command>postgres</command> command must have a shorter
<command>ps</command> status display than that provided by each
server process. If you fail to do all three things, the <command>ps</command>
output for each server process will be the original <command>postgres</command>
command line.
</para>
</tip>
</sect1>
<sect1 id="monitoring-stats">
<title>The Statistics Collector</title>
<indexterm zone="monitoring-stats">
<primary>statistics</primary>
</indexterm>
<para>
<productname>PostgreSQL</productname>'s <firstterm>statistics collector</firstterm>
is a subsystem that supports collection and reporting of information about
server activity. Presently, the collector can count accesses to tables
and indexes in both disk-block and individual-row terms. It also tracks
the total number of rows in each table, and information about vacuum and
analyze actions for each table. It can also count calls to user-defined
functions and the total time spent in each one.
</para>
<para>
<productname>PostgreSQL</productname> also supports reporting dynamic
information about exactly what is going on in the system right now, such as
the exact command currently being executed by other server processes, and
which other connections exist in the system. This facility is independent
of the collector process.
</para>
<sect2 id="monitoring-stats-setup">
<title>Statistics Collection Configuration</title>
<para>
Since collection of statistics adds some overhead to query execution,
the system can be configured to collect or not collect information.
This is controlled by configuration parameters that are normally set in
<filename>postgresql.conf</filename>. (See <xref linkend="runtime-config"/> for
details about setting configuration parameters.)
</para>
<para>
The parameter <xref linkend="guc-track-activities"/> enables monitoring
of the current command being executed by any server process.
</para>
<para>
The parameter <xref linkend="guc-track-counts"/> controls whether
statistics are collected about table and index accesses.
</para>
<para>
The parameter <xref linkend="guc-track-functions"/> enables tracking of
usage of user-defined functions.
</para>
<para>
The parameter <xref linkend="guc-track-io-timing"/> enables monitoring
of block read and write times.
</para>
<para>
Normally these parameters are set in <filename>postgresql.conf</filename> so
that they apply to all server processes, but it is possible to turn
them on or off in individual sessions using the <xref
linkend="sql-set"/> command. (To prevent
ordinary users from hiding their activity from the administrator,
only superusers are allowed to change these parameters with
<command>SET</command>.)
</para>
<para>
The statistics collector transmits the collected information to other
<productname>PostgreSQL</productname> processes through temporary files.
These files are stored in the directory named by the
<xref linkend="guc-stats-temp-directory"/> parameter,
<filename>pg_stat_tmp</filename> by default.
For better performance, <varname>stats_temp_directory</varname> can be
pointed at a RAM-based file system, decreasing physical I/O requirements.
When the server shuts down cleanly, a permanent copy of the statistics
data is stored in the <filename>pg_stat</filename> subdirectory, so that
statistics can be retained across server restarts. When recovery is
performed at server start (e.g. after immediate shutdown, server crash,
and point-in-time recovery), all statistics counters are reset.
</para>
</sect2>
<sect2 id="monitoring-stats-views">
<title>Viewing Statistics</title>
<para>
Several predefined views, listed in <xref
linkend="monitoring-stats-dynamic-views-table"/>, are available to show
the current state of the system. There are also several other
views, listed in <xref
linkend="monitoring-stats-views-table"/>, available to show the results
of statistics collection. Alternatively, one can
build custom views using the underlying statistics functions, as discussed
in <xref linkend="monitoring-stats-functions"/>.
</para>
<para>
When using the statistics to monitor collected data, it is important
to realize that the information does not update instantaneously.
Each individual server process transmits new statistical counts to
the collector just before going idle; so a query or transaction still in
progress does not affect the displayed totals. Also, the collector itself
emits a new report at most once per <varname>PGSTAT_STAT_INTERVAL</varname>
milliseconds (500 ms unless altered while building the server). So the
displayed information lags behind actual activity. However, current-query
information collected by <varname>track_activities</varname> is
always up-to-date.
</para>
<para>
Another important point is that when a server process is asked to display
any of these statistics, it first fetches the most recent report emitted by
the collector process and then continues to use this snapshot for all
statistical views and functions until the end of its current transaction.
So the statistics will show static information as long as you continue the
current transaction. Similarly, information about the current queries of
all sessions is collected when any such information is first requested
within a transaction, and the same information will be displayed throughout
the transaction.
This is a feature, not a bug, because it allows you to perform several
queries on the statistics and correlate the results without worrying that
the numbers are changing underneath you. But if you want to see new
results with each query, be sure to do the queries outside any transaction
block. Alternatively, you can invoke
<function>pg_stat_clear_snapshot</function>(), which will discard the
current transaction's statistics snapshot (if any). The next use of
statistical information will cause a new snapshot to be fetched.
</para>
<para>
A transaction can also see its own statistics (as yet untransmitted to the
collector) in the views <structname>pg_stat_xact_all_tables</structname>,
<structname>pg_stat_xact_sys_tables</structname>,
<structname>pg_stat_xact_user_tables</structname>, and
<structname>pg_stat_xact_user_functions</structname>. These numbers do not act as
stated above; instead they update continuously throughout the transaction.
</para>
<para>
Some of the information in the dynamic statistics views shown in <xref
linkend="monitoring-stats-dynamic-views-table"/> is security restricted.
Ordinary users can only see all the information about their own sessions
(sessions belonging to a role that they are a member of). In rows about
other sessions, many columns will be null. Note, however, that the
existence of a session and its general properties such as its sessions user
and database are visible to all users. Superusers and members of the
built-in role <literal>pg_read_all_stats</literal> (see also <xref
linkend="default-roles"/>) can see all the information about all sessions.
</para>
<table id="monitoring-stats-dynamic-views-table">
<title>Dynamic Statistics Views</title>
<tgroup cols="2">
<thead>
<row>
<entry>View Name</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>
<structname>pg_stat_activity</structname>
<indexterm><primary>pg_stat_activity</primary></indexterm>
</entry>
<entry>
One row per server process, showing information related to
the current activity of that process, such as state and current query.
See <xref linkend="pg-stat-activity-view"/> for details.
</entry>
</row>
<row>
<entry><structname>pg_stat_replication</structname><indexterm><primary>pg_stat_replication</primary></indexterm></entry>
<entry>One row per WAL sender process, showing statistics about
replication to that sender's connected standby server.
See <xref linkend="pg-stat-replication-view"/> for details.
</entry>
</row>
<row>
<entry><structname>pg_stat_wal_receiver</structname><indexterm><primary>pg_stat_wal_receiver</primary></indexterm></entry>
<entry>Only one row, showing statistics about the WAL receiver from
that receiver's connected server.
See <xref linkend="pg-stat-wal-receiver-view"/> for details.
</entry>
</row>
<row>
<entry><structname>pg_stat_subscription</structname><indexterm><primary>pg_stat_subscription</primary></indexterm></entry>
<entry>At least one row per subscription, showing information about
the subscription workers.
See <xref linkend="pg-stat-subscription"/> for details.
</entry>
</row>
<row>
<entry><structname>pg_stat_ssl</structname><indexterm><primary>pg_stat_ssl</primary></indexterm></entry>
<entry>One row per connection (regular and replication), showing information about
SSL used on this connection.
See <xref linkend="pg-stat-ssl-view"/> for details.
</entry>
</row>
<row>
<entry><structname>pg_stat_gssapi</structname><indexterm><primary>pg_stat_gssapi</primary></indexterm></entry>
<entry>One row per connection (regular and replication), showing information about
GSSAPI authentication and encryption used on this connection.
See <xref linkend="pg-stat-gssapi-view"/> for details.
</entry>
</row>
<row>
<entry><structname>pg_stat_progress_create_index</structname><indexterm><primary>pg_stat_progress_create_index</primary></indexterm></entry>
<entry>One row for each backend running <command>CREATE INDEX</command> or <command>REINDEX</command>, showing
current progress.
See <xref linkend='create-index-progress-reporting'/>.
</entry>
</row>
<row>
<entry><structname>pg_stat_progress_vacuum</structname><indexterm><primary>pg_stat_progress_vacuum</primary></indexterm></entry>
<entry>One row for each backend (including autovacuum worker processes) running
<command>VACUUM</command>, showing current progress.
See <xref linkend='vacuum-progress-reporting'/>.
</entry>
</row>
<row>
<entry><structname>pg_stat_progress_cluster</structname><indexterm><primary>pg_stat_progress_cluster</primary></indexterm></entry>
<entry>One row for each backend running
<command>CLUSTER</command> or <command>VACUUM FULL</command>, showing current progress.
See <xref linkend='cluster-progress-reporting'/>.
</entry>
</row>
</tbody>
</tgroup>
</table>
<table id="monitoring-stats-views-table">
<title>Collected Statistics Views</title>
<tgroup cols="2">
<thead>
<row>
<entry>View Name</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><structname>pg_stat_archiver</structname><indexterm><primary>pg_stat_archiver</primary></indexterm></entry>
<entry>One row only, showing statistics about the
WAL archiver process's activity. See
<xref linkend="pg-stat-archiver-view"/> for details.
</entry>
</row>
<row>
<entry><structname>pg_stat_bgwriter</structname><indexterm><primary>pg_stat_bgwriter</primary></indexterm></entry>
<entry>One row only, showing statistics about the
background writer process's activity. See
<xref linkend="pg-stat-bgwriter-view"/> for details.
</entry>
</row>
<row>
<entry><structname>pg_stat_database</structname><indexterm><primary>pg_stat_database</primary></indexterm></entry>
<entry>One row per database, showing database-wide statistics. See
<xref linkend="pg-stat-database-view"/> for details.
</entry>
</row>
<row>
<entry><structname>pg_stat_database_conflicts</structname><indexterm><primary>pg_stat_database_conflicts</primary></indexterm></entry>
<entry>
One row per database, showing database-wide statistics about
query cancels due to conflict with recovery on standby servers.
See <xref linkend="pg-stat-database-conflicts-view"/> for details.
</entry>
</row>
<row>
<entry><structname>pg_stat_all_tables</structname><indexterm><primary>pg_stat_all_tables</primary></indexterm></entry>
<entry>
One row for each table in the current database, showing statistics
about accesses to that specific table.
See <xref linkend="pg-stat-all-tables-view"/> for details.
</entry>
</row>
<row>
<entry><structname>pg_stat_sys_tables</structname><indexterm><primary>pg_stat_sys_tables</primary></indexterm></entry>
<entry>Same as <structname>pg_stat_all_tables</structname>, except that only
system tables are shown.</entry>
</row>
<row>
<entry><structname>pg_stat_user_tables</structname><indexterm><primary>pg_stat_user_tables</primary></indexterm></entry>
<entry>Same as <structname>pg_stat_all_tables</structname>, except that only user
tables are shown.</entry>
</row>
<row>
<entry><structname>pg_stat_xact_all_tables</structname><indexterm><primary>pg_stat_xact_all_tables</primary></indexterm></entry>
<entry>Similar to <structname>pg_stat_all_tables</structname>, but counts actions
taken so far within the current transaction (which are <emphasis>not</emphasis>
yet included in <structname>pg_stat_all_tables</structname> and related views).
The columns for numbers of live and dead rows and vacuum and
analyze actions are not present in this view.</entry>
</row>
<row>
<entry><structname>pg_stat_xact_sys_tables</structname><indexterm><primary>pg_stat_xact_sys_tables</primary></indexterm></entry>
<entry>Same as <structname>pg_stat_xact_all_tables</structname>, except that only
system tables are shown.</entry>
</row>
<row>
<entry><structname>pg_stat_xact_user_tables</structname><indexterm><primary>pg_stat_xact_user_tables</primary></indexterm></entry>
<entry>Same as <structname>pg_stat_xact_all_tables</structname>, except that only
user tables are shown.</entry>
</row>
<row>
<entry><structname>pg_stat_all_indexes</structname><indexterm><primary>pg_stat_all_indexes</primary></indexterm></entry>
<entry>
One row for each index in the current database, showing statistics
about accesses to that specific index.
See <xref linkend="pg-stat-all-indexes-view"/> for details.
</entry>
</row>
<row>
<entry><structname>pg_stat_sys_indexes</structname><indexterm><primary>pg_stat_sys_indexes</primary></indexterm></entry>
<entry>Same as <structname>pg_stat_all_indexes</structname>, except that only
indexes on system tables are shown.</entry>
</row>
<row>
<entry><structname>pg_stat_user_indexes</structname><indexterm><primary>pg_stat_user_indexes</primary></indexterm></entry>
<entry>Same as <structname>pg_stat_all_indexes</structname>, except that only
indexes on user tables are shown.</entry>
</row>
<row>
<entry><structname>pg_statio_all_tables</structname><indexterm><primary>pg_statio_all_tables</primary></indexterm></entry>
<entry>
One row for each table in the current database, showing statistics
about I/O on that specific table.
See <xref linkend="pg-statio-all-tables-view"/> for details.
</entry>
</row>
<row>
<entry><structname>pg_statio_sys_tables</structname><indexterm><primary>pg_statio_sys_tables</primary></indexterm></entry>
<entry>Same as <structname>pg_statio_all_tables</structname>, except that only
system tables are shown.</entry>
</row>
<row>
<entry><structname>pg_statio_user_tables</structname><indexterm><primary>pg_statio_user_tables</primary></indexterm></entry>
<entry>Same as <structname>pg_statio_all_tables</structname>, except that only
user tables are shown.</entry>
</row>
<row>
<entry><structname>pg_statio_all_indexes</structname><indexterm><primary>pg_statio_all_indexes</primary></indexterm></entry>
<entry>
One row for each index in the current database,
showing statistics about I/O on that specific index.
See <xref linkend="pg-statio-all-indexes-view"/> for details.
</entry>
</row>
<row>
<entry><structname>pg_statio_sys_indexes</structname><indexterm><primary>pg_statio_sys_indexes</primary></indexterm></entry>
<entry>Same as <structname>pg_statio_all_indexes</structname>, except that only
indexes on system tables are shown.</entry>
</row>
<row>
<entry><structname>pg_statio_user_indexes</structname><indexterm><primary>pg_statio_user_indexes</primary></indexterm></entry>
<entry>Same as <structname>pg_statio_all_indexes</structname>, except that only
indexes on user tables are shown.</entry>
</row>
<row>
<entry><structname>pg_statio_all_sequences</structname><indexterm><primary>pg_statio_all_sequences</primary></indexterm></entry>
<entry>
One row for each sequence in the current database,
showing statistics about I/O on that specific sequence.
See <xref linkend="pg-statio-all-sequences-view"/> for details.
</entry>
</row>
<row>
<entry><structname>pg_statio_sys_sequences</structname><indexterm><primary>pg_statio_sys_sequences</primary></indexterm></entry>
<entry>Same as <structname>pg_statio_all_sequences</structname>, except that only
system sequences are shown. (Presently, no system sequences are defined,
so this view is always empty.)</entry>
</row>
<row>
<entry><structname>pg_statio_user_sequences</structname><indexterm><primary>pg_statio_user_sequences</primary></indexterm></entry>
<entry>Same as <structname>pg_statio_all_sequences</structname>, except that only
user sequences are shown.</entry>
</row>
<row>
<entry><structname>pg_stat_user_functions</structname><indexterm><primary>pg_stat_user_functions</primary></indexterm></entry>
<entry>
One row for each tracked function, showing statistics
about executions of that function. See
<xref linkend="pg-stat-user-functions-view"/> for details.
</entry>
</row>
<row>
<entry><structname>pg_stat_xact_user_functions</structname><indexterm><primary>pg_stat_xact_user_functions</primary></indexterm></entry>
<entry>Similar to <structname>pg_stat_user_functions</structname>, but counts only
calls during the current transaction (which are <emphasis>not</emphasis>
yet included in <structname>pg_stat_user_functions</structname>).</entry>
</row>
</tbody>
</tgroup>
</table>
<para>
The per-index statistics are particularly useful to determine which
indexes are being used and how effective they are.
</para>
<para>
The <structname>pg_statio_</structname> views are primarily useful to
determine the effectiveness of the buffer cache. When the number
of actual disk reads is much smaller than the number of buffer
hits, then the cache is satisfying most read requests without
invoking a kernel call. However, these statistics do not give the
entire story: due to the way in which <productname>PostgreSQL</productname>
handles disk I/O, data that is not in the
<productname>PostgreSQL</productname> buffer cache might still reside in the
kernel's I/O cache, and might therefore still be fetched without
requiring a physical read. Users interested in obtaining more
detailed information on <productname>PostgreSQL</productname> I/O behavior are
advised to use the <productname>PostgreSQL</productname> statistics collector
in combination with operating system utilities that allow insight
into the kernel's handling of I/O.
</para>
<table id="pg-stat-activity-view" xreflabel="pg_stat_activity">
<title><structname>pg_stat_activity</structname> View</title>
<tgroup cols="3">
<thead>
<row>
<entry>Column</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><structfield>datid</structfield></entry>
<entry><type>oid</type></entry>
<entry>OID of the database this backend is connected to</entry>
</row>
<row>
<entry><structfield>datname</structfield></entry>
<entry><type>name</type></entry>
<entry>Name of the database this backend is connected to</entry>
</row>
<row>
<entry><structfield>pid</structfield></entry>
<entry><type>integer</type></entry>
<entry>Process ID of this backend</entry>
</row>
<row>
<entry><structfield>usesysid</structfield></entry>
<entry><type>oid</type></entry>
<entry>OID of the user logged into this backend</entry>
</row>
<row>
<entry><structfield>usename</structfield></entry>
<entry><type>name</type></entry>
<entry>Name of the user logged into this backend</entry>
</row>
<row>
<entry><structfield>application_name</structfield></entry>
<entry><type>text</type></entry>
<entry>Name of the application that is connected
to this backend</entry>
</row>
<row>
<entry><structfield>client_addr</structfield></entry>
<entry><type>inet</type></entry>
<entry>IP address of the client connected to this backend.
If this field is null, it indicates either that the client is
connected via a Unix socket on the server machine or that this is an
internal process such as autovacuum.
</entry>
</row>
<row>
<entry><structfield>client_hostname</structfield></entry>
<entry><type>text</type></entry>
<entry>Host name of the connected client, as reported by a
reverse DNS lookup of <structfield>client_addr</structfield>. This field will
only be non-null for IP connections, and only when <xref
linkend="guc-log-hostname"/> is enabled.
</entry>
</row>
<row>
<entry><structfield>client_port</structfield></entry>
<entry><type>integer</type></entry>
<entry>TCP port number that the client is using for communication
with this backend, or <literal>-1</literal> if a Unix socket is used.
If this field is null, it indicates that this is an internal server process.
</entry>
</row>
<row>
<entry><structfield>backend_start</structfield></entry>
<entry><type>timestamp with time zone</type></entry>
<entry>Time when this process was started. For client backends,
this is the time the client connected to the server.
</entry>
</row>
<row>
<entry><structfield>xact_start</structfield></entry>
<entry><type>timestamp with time zone</type></entry>
<entry>Time when this process' current transaction was started, or null
if no transaction is active. If the current
query is the first of its transaction, this column is equal to the
<structfield>query_start</structfield> column.
</entry>
</row>
<row>
<entry><structfield>query_start</structfield></entry>
<entry><type>timestamp with time zone</type></entry>
<entry>Time when the currently active query was started, or if
<structfield>state</structfield> is not <literal>active</literal>, when the last query
was started
</entry>
</row>
<row>
<entry><structfield>state_change</structfield></entry>
<entry><type>timestamp with time zone</type></entry>
<entry>Time when the <structfield>state</structfield> was last changed</entry>
</row>
<row>
<entry><structfield>wait_event_type</structfield></entry>
<entry><type>text</type></entry>
<entry>The type of event for which the backend is waiting, if any;
otherwise NULL. Possible values are:
<itemizedlist>
<listitem>
<para>
<literal>LWLock</literal>: The backend is waiting for a lightweight lock.
Each such lock protects a particular data structure in shared memory.
<literal>wait_event</literal> will contain a name identifying the purpose
of the lightweight lock. (Some locks have specific names; others
are part of a group of locks each with a similar purpose.)
</para>
</listitem>
<listitem>
<para>
<literal>Lock</literal>: The backend is waiting for a heavyweight lock.
Heavyweight locks, also known as lock manager locks or simply locks,
primarily protect SQL-visible objects such as tables. However,
they are also used to ensure mutual exclusion for certain internal
operations such as relation extension. <literal>wait_event</literal> will
identify the type of lock awaited.
</para>
</listitem>
<listitem>
<para>
<literal>BufferPin</literal>: The server process is waiting to access to
a data buffer during a period when no other process can be
examining that buffer. Buffer pin waits can be protracted if
another process holds an open cursor which last read data from the
buffer in question.
</para>
</listitem>
<listitem>
<para>
<literal>Activity</literal>: The server process is idle. This is used by
system processes waiting for activity in their main processing loop.
<literal>wait_event</literal> will identify the specific wait point.
</para>
</listitem>
<listitem>
<para>
<literal>Extension</literal>: The server process is waiting for activity
in an extension module. This category is useful for modules to
track custom waiting points.
</para>
</listitem>
<listitem>
<para>
<literal>Client</literal>: The server process is waiting for some activity
on a socket from user applications, and that the server expects
something to happen that is independent from its internal processes.
<literal>wait_event</literal> will identify the specific wait point.
</para>
</listitem>
<listitem>
<para>
<literal>IPC</literal>: The server process is waiting for some activity
from another process in the server. <literal>wait_event</literal> will
identify the specific wait point.
</para>
</listitem>
<listitem>
<para>
<literal>Timeout</literal>: The server process is waiting for a timeout
to expire. <literal>wait_event</literal> will identify the specific wait
point.
</para>
</listitem>
<listitem>
<para>
<literal>IO</literal>: The server process is waiting for a IO to complete.
<literal>wait_event</literal> will identify the specific wait point.
</para>
</listitem>
</itemizedlist>
</entry>
</row>
<row>
<entry><structfield>wait_event</structfield></entry>
<entry><type>text</type></entry>
<entry>Wait event name if backend is currently waiting, otherwise NULL.
See <xref linkend="wait-event-table"/> for details.
</entry>
</row>
<row>
<entry><structfield>state</structfield></entry>
<entry><type>text</type></entry>
<entry>Current overall state of this backend.
Possible values are:
<itemizedlist>
<listitem>
<para>
<literal>active</literal>: The backend is executing a query.
</para>
</listitem>
<listitem>
<para>
<literal>idle</literal>: The backend is waiting for a new client command.
</para>
</listitem>
<listitem>
<para>
<literal>idle in transaction</literal>: The backend is in a transaction,
but is not currently executing a query.
</para>
</listitem>
<listitem>
<para>
<literal>idle in transaction (aborted)</literal>: This state is similar to
<literal>idle in transaction</literal>, except one of the statements in
the transaction caused an error.
</para>
</listitem>
<listitem>
<para>
<literal>fastpath function call</literal>: The backend is executing a
fast-path function.
</para>
</listitem>
<listitem>
<para>
<literal>disabled</literal>: This state is reported if <xref
linkend="guc-track-activities"/> is disabled in this backend.
</para>
</listitem>
</itemizedlist>
</entry>
</row>
<row>
<entry><structfield>backend_xid</structfield></entry>
<entry><type>xid</type></entry>
<entry>Top-level transaction identifier of this backend, if any.</entry>
</row>
<row>
<entry><structfield>backend_xmin</structfield></entry>
<entry><type>xid</type></entry>
<entry>The current backend's <literal>xmin</literal> horizon.</entry>
</row>
<row>
<entry><structfield>query</structfield></entry>
<entry><type>text</type></entry>
<entry>Text of this backend's most recent query. If
<structfield>state</structfield> is <literal>active</literal> this field shows the
currently executing query. In all other states, it shows the last query
that was executed. By default the query text is truncated at 1024
characters; this value can be changed via the parameter
<xref linkend="guc-track-activity-query-size"/>.
</entry>
</row>
<row>
<entry><structfield>backend_type</structfield></entry>
<entry><type>text</type></entry>
<entry>Type of current backend. Possible types are
<literal>autovacuum launcher</literal>, <literal>autovacuum worker</literal>,
<literal>logical replication launcher</literal>,
<literal>logical replication worker</literal>,
<literal>parallel worker</literal>, <literal>background writer</literal>,
<literal>client backend</literal>, <literal>checkpointer</literal>,
<literal>startup</literal>, <literal>walreceiver</literal>,
<literal>walsender</literal> and <literal>walwriter</literal>.
In addition, background workers registered by extensions may have
additional types.
</entry>
</row>
</tbody>
</tgroup>
</table>
<para>
The <structname>pg_stat_activity</structname> view will have one row
per server process, showing information related to
the current activity of that process.
</para>
<note>
<para>
The <structfield>wait_event</structfield> and <structfield>state</structfield> columns are
independent. If a backend is in the <literal>active</literal> state,
it may or may not be <literal>waiting</literal> on some event. If the state
is <literal>active</literal> and <structfield>wait_event</structfield> is non-null, it
means that a query is being executed, but is being blocked somewhere
in the system.
</para>
</note>
<table id="wait-event-table">
<title><structname>wait_event</structname> Description</title>
<tgroup cols="3">
<thead>
<row>
<entry>Wait Event Type</entry>
<entry>Wait Event Name</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry morerows="64"><literal>LWLock</literal></entry>
<entry><literal>ShmemIndexLock</literal></entry>
<entry>Waiting to find or allocate space in shared memory.</entry>
</row>
<row>
<entry><literal>OidGenLock</literal></entry>
<entry>Waiting to allocate or assign an OID.</entry>
</row>
<row>
<entry><literal>XidGenLock</literal></entry>
<entry>Waiting to allocate or assign a transaction id.</entry>
</row>
<row>
<entry><literal>ProcArrayLock</literal></entry>
<entry>Waiting to get a snapshot or clearing a transaction id at
transaction end.</entry>
</row>
<row>
<entry><literal>SInvalReadLock</literal></entry>
<entry>Waiting to retrieve or remove messages from shared invalidation
queue.</entry>
</row>
<row>
<entry><literal>SInvalWriteLock</literal></entry>
<entry>Waiting to add a message in shared invalidation queue.</entry>
</row>
<row>
<entry><literal>WALBufMappingLock</literal></entry>
<entry>Waiting to replace a page in WAL buffers.</entry>
</row>
<row>
<entry><literal>WALWriteLock</literal></entry>
<entry>Waiting for WAL buffers to be written to disk.</entry>
</row>
<row>
<entry><literal>ControlFileLock</literal></entry>
<entry>Waiting to read or update the control file or creation of a
new WAL file.</entry>
</row>
<row>
<entry><literal>CheckpointLock</literal></entry>
<entry>Waiting to perform checkpoint.</entry>
</row>
<row>
<entry><literal>CLogControlLock</literal></entry>
<entry>Waiting to read or update transaction status.</entry>
</row>
<row>
<entry><literal>SubtransControlLock</literal></entry>
<entry>Waiting to read or update sub-transaction information.</entry>
</row>
<row>
<entry><literal>MultiXactGenLock</literal></entry>
<entry>Waiting to read or update shared multixact state.</entry>
</row>
<row>
<entry><literal>MultiXactOffsetControlLock</literal></entry>
<entry>Waiting to read or update multixact offset mappings.</entry>
</row>
<row>
<entry><literal>MultiXactMemberControlLock</literal></entry>
<entry>Waiting to read or update multixact member mappings.</entry>
</row>
<row>
<entry><literal>RelCacheInitLock</literal></entry>
<entry>Waiting to read or write relation cache initialization
file.</entry>
</row>
<row>
<entry><literal>CheckpointerCommLock</literal></entry>
<entry>Waiting to manage fsync requests.</entry>
</row>
<row>
<entry><literal>TwoPhaseStateLock</literal></entry>
<entry>Waiting to read or update the state of prepared transactions.</entry>
</row>
<row>
<entry><literal>TablespaceCreateLock</literal></entry>
<entry>Waiting to create or drop the tablespace.</entry>
</row>
<row>
<entry><literal>BtreeVacuumLock</literal></entry>
<entry>Waiting to read or update vacuum-related information for a
B-tree index.</entry>
</row>
<row>
<entry><literal>AddinShmemInitLock</literal></entry>
<entry>Waiting to manage space allocation in shared memory.</entry>
</row>
<row>
<entry><literal>AutovacuumLock</literal></entry>
<entry>Autovacuum worker or launcher waiting to update or
read the current state of autovacuum workers.</entry>
</row>
<row>
<entry><literal>AutovacuumScheduleLock</literal></entry>
<entry>Waiting to ensure that the table it has selected for a vacuum
still needs vacuuming.
</entry>
</row>
<row>
<entry><literal>SyncScanLock</literal></entry>
<entry>Waiting to get the start location of a scan on a table for
synchronized scans.</entry>
</row>
<row>
<entry><literal>RelationMappingLock</literal></entry>
<entry>Waiting to update the relation map file used to store catalog
to filenode mapping.
</entry>
</row>
<row>
<entry><literal>AsyncCtlLock</literal></entry>
<entry>Waiting to read or update shared notification state.</entry>
</row>
<row>
<entry><literal>AsyncQueueLock</literal></entry>
<entry>Waiting to read or update notification messages.</entry>
</row>
<row>
<entry><literal>SerializableXactHashLock</literal></entry>
<entry>Waiting to retrieve or store information about serializable
transactions.</entry>
</row>
<row>
<entry><literal>SerializableFinishedListLock</literal></entry>
<entry>Waiting to access the list of finished serializable
transactions.</entry>
</row>
<row>
<entry><literal>SerializablePredicateLockListLock</literal></entry>
<entry>Waiting to perform an operation on a list of locks held by
serializable transactions.</entry>
</row>
<row>
<entry><literal>OldSerXidLock</literal></entry>
<entry>Waiting to read or record conflicting serializable
transactions.</entry>
</row>
<row>
<entry><literal>SyncRepLock</literal></entry>
<entry>Waiting to read or update information about synchronous
replicas.</entry>
</row>
<row>
<entry><literal>BackgroundWorkerLock</literal></entry>
<entry>Waiting to read or update background worker state.</entry>
</row>
<row>
<entry><literal>DynamicSharedMemoryControlLock</literal></entry>
<entry>Waiting to read or update dynamic shared memory state.</entry>
</row>
<row>
<entry><literal>AutoFileLock</literal></entry>
<entry>Waiting to update the <filename>postgresql.auto.conf</filename> file.</entry>
</row>
<row>
<entry><literal>ReplicationSlotAllocationLock</literal></entry>
<entry>Waiting to allocate or free a replication slot.</entry>
</row>
<row>
<entry><literal>ReplicationSlotControlLock</literal></entry>
<entry>Waiting to read or update replication slot state.</entry>
</row>
<row>
<entry><literal>CommitTsControlLock</literal></entry>
<entry>Waiting to read or update transaction commit timestamps.</entry>
</row>
<row>
<entry><literal>CommitTsLock</literal></entry>
<entry>Waiting to read or update the last value set for the
transaction timestamp.</entry>
</row>
<row>
<entry><literal>ReplicationOriginLock</literal></entry>
<entry>Waiting to setup, drop or use replication origin.</entry>
</row>
<row>
<entry><literal>MultiXactTruncationLock</literal></entry>
<entry>Waiting to read or truncate multixact information.</entry>
</row>
<row>
<entry><literal>OldSnapshotTimeMapLock</literal></entry>
<entry>Waiting to read or update old snapshot control information.</entry>
</row>
<row>
<entry><literal>LogicalRepWorkerLock</literal></entry>
<entry>Waiting for action on logical replication worker to finish.</entry>
</row>
<row>
<entry><literal>CLogTruncationLock</literal></entry>
<entry>Waiting to execute <function>txid_status</function> or update
the oldest transaction id available to it.</entry>
</row>
<row>
<entry><literal>clog</literal></entry>
<entry>Waiting for I/O on a clog (transaction status) buffer.</entry>
</row>
<row>
<entry><literal>commit_timestamp</literal></entry>
<entry>Waiting for I/O on commit timestamp buffer.</entry>
</row>
<row>
<entry><literal>subtrans</literal></entry>
<entry>Waiting for I/O a subtransaction buffer.</entry>
</row>
<row>
<entry><literal>multixact_offset</literal></entry>
<entry>Waiting for I/O on a multixact offset buffer.</entry>
</row>
<row>
<entry><literal>multixact_member</literal></entry>
<entry>Waiting for I/O on a multixact_member buffer.</entry>
</row>
<row>
<entry><literal>async</literal></entry>
<entry>Waiting for I/O on an async (notify) buffer.</entry>
</row>
<row>
<entry><literal>oldserxid</literal></entry>
<entry>Waiting for I/O on an oldserxid buffer.</entry>
</row>
<row>
<entry><literal>wal_insert</literal></entry>
<entry>Waiting to insert WAL into a memory buffer.</entry>
</row>
<row>
<entry><literal>buffer_content</literal></entry>
<entry>Waiting to read or write a data page in memory.</entry>
</row>
<row>
<entry><literal>buffer_io</literal></entry>
<entry>Waiting for I/O on a data page.</entry>
</row>
<row>
<entry><literal>replication_origin</literal></entry>
<entry>Waiting to read or update the replication progress.</entry>
</row>
<row>
<entry><literal>replication_slot_io</literal></entry>
<entry>Waiting for I/O on a replication slot.</entry>
</row>
<row>
<entry><literal>proc</literal></entry>
<entry>Waiting to read or update the fast-path lock information.</entry>
</row>
<row>
<entry><literal>buffer_mapping</literal></entry>
<entry>Waiting to associate a data block with a buffer in the buffer
pool.</entry>
</row>
<row>
<entry><literal>lock_manager</literal></entry>
<entry>Waiting to add or examine locks for backends, or waiting to
join or exit a locking group (used by parallel query).</entry>
</row>
<row>
<entry><literal>predicate_lock_manager</literal></entry>
<entry>Waiting to add or examine predicate lock information.</entry>
</row>
<row>
<entry><literal>serializable_xact</literal></entry>
<entry>Waiting to perform an operation on a serializable transaction
in a parallel query.</entry>
</row>
<row>
<entry><literal>parallel_query_dsa</literal></entry>
<entry>Waiting for parallel query dynamic shared memory allocation lock.</entry>
</row>
<row>
<entry><literal>tbm</literal></entry>
<entry>Waiting for TBM shared iterator lock.</entry>
</row>
<row>
<entry><literal>parallel_append</literal></entry>
<entry>Waiting to choose the next subplan during Parallel Append plan
execution.</entry>
</row>
<row>
<entry><literal>parallel_hash_join</literal></entry>
<entry>Waiting to allocate or exchange a chunk of memory or update
counters during Parallel Hash plan execution.</entry>
</row>
<row>
<entry morerows="9"><literal>Lock</literal></entry>
<entry><literal>relation</literal></entry>
<entry>Waiting to acquire a lock on a relation.</entry>
</row>
<row>
<entry><literal>extend</literal></entry>
<entry>Waiting to extend a relation.</entry>
</row>
<row>
<entry><literal>page</literal></entry>
<entry>Waiting to acquire a lock on page of a relation.</entry>
</row>
<row>
<entry><literal>tuple</literal></entry>
<entry>Waiting to acquire a lock on a tuple.</entry>
</row>
<row>
<entry><literal>transactionid</literal></entry>
<entry>Waiting for a transaction to finish.</entry>
</row>
<row>
<entry><literal>virtualxid</literal></entry>
<entry>Waiting to acquire a virtual xid lock.</entry>
</row>
<row>
<entry><literal>speculative token</literal></entry>
<entry>Waiting to acquire a speculative insertion lock.</entry>
</row>
<row>
<entry><literal>object</literal></entry>
<entry>Waiting to acquire a lock on a non-relation database object.</entry>
</row>
<row>
<entry><literal>userlock</literal></entry>
<entry>Waiting to acquire a user lock.</entry>
</row>
<row>
<entry><literal>advisory</literal></entry>
<entry>Waiting to acquire an advisory user lock.</entry>
</row>
<row>
<entry><literal>BufferPin</literal></entry>
<entry><literal>BufferPin</literal></entry>
<entry>Waiting to acquire a pin on a buffer.</entry>
</row>
<row>
<entry morerows="13"><literal>Activity</literal></entry>
<entry><literal>ArchiverMain</literal></entry>
<entry>Waiting in main loop of the archiver process.</entry>
</row>
<row>
<entry><literal>AutoVacuumMain</literal></entry>
<entry>Waiting in main loop of autovacuum launcher process.</entry>
</row>
<row>
<entry><literal>BgWriterHibernate</literal></entry>
<entry>Waiting in background writer process, hibernating.</entry>
</row>
<row>
<entry><literal>BgWriterMain</literal></entry>
<entry>Waiting in main loop of background writer process background worker.</entry>
</row>
<row>
<entry><literal>CheckpointerMain</literal></entry>
<entry>Waiting in main loop of checkpointer process.</entry>
</row>
<row>
<entry><literal>LogicalApplyMain</literal></entry>
<entry>Waiting in main loop of logical apply process.</entry>
</row>
<row>
<entry><literal>LogicalLauncherMain</literal></entry>
<entry>Waiting in main loop of logical launcher process.</entry>
</row>
<row>
<entry><literal>PgStatMain</literal></entry>
<entry>Waiting in main loop of the statistics collector process.</entry>
</row>
<row>
<entry><literal>RecoveryWalAll</literal></entry>
<entry>Waiting for WAL from any kind of source (local, archive or stream) at recovery.</entry>
</row>
<row>
<entry><literal>RecoveryWalStream</literal></entry>
<entry>Waiting for WAL from a stream at recovery.</entry>
</row>
<row>
<entry><literal>SysLoggerMain</literal></entry>
<entry>Waiting in main loop of syslogger process.</entry>
</row>
<row>
<entry><literal>WalReceiverMain</literal></entry>
<entry>Waiting in main loop of WAL receiver process.</entry>
</row>
<row>
<entry><literal>WalSenderMain</literal></entry>
<entry>Waiting in main loop of WAL sender process.</entry>
</row>
<row>
<entry><literal>WalWriterMain</literal></entry>
<entry>Waiting in main loop of WAL writer process.</entry>
</row>
<row>
<entry morerows="7"><literal>Client</literal></entry>
<entry><literal>ClientRead</literal></entry>
<entry>Waiting to read data from the client.</entry>
</row>
<row>
<entry><literal>ClientWrite</literal></entry>
<entry>Waiting to write data to the client.</entry>
</row>
<row>
<entry><literal>LibPQWalReceiverConnect</literal></entry>
<entry>Waiting in WAL receiver to establish connection to remote server.</entry>
</row>
<row>
<entry><literal>LibPQWalReceiverReceive</literal></entry>
<entry>Waiting in WAL receiver to receive data from remote server.</entry>
</row>
<row>
<entry><literal>SSLOpenServer</literal></entry>
<entry>Waiting for SSL while attempting connection.</entry>
</row>
<row>
<entry><literal>WalReceiverWaitStart</literal></entry>
<entry>Waiting for startup process to send initial data for streaming replication.</entry>
</row>
<row>
<entry><literal>WalSenderWaitForWAL</literal></entry>
<entry>Waiting for WAL to be flushed in WAL sender process.</entry>
</row>
<row>
<entry><literal>WalSenderWriteData</literal></entry>
<entry>Waiting for any activity when processing replies from WAL receiver in WAL sender process.</entry>
</row>
<row>
<entry><literal>Extension</literal></entry>
<entry><literal>Extension</literal></entry>
<entry>Waiting in an extension.</entry>
</row>
<row>
<entry morerows="36"><literal>IPC</literal></entry>
<entry><literal>BgWorkerShutdown</literal></entry>
<entry>Waiting for background worker to shut down.</entry>
</row>
<row>
<entry><literal>BgWorkerStartup</literal></entry>
<entry>Waiting for background worker to start up.</entry>
</row>
<row>
<entry><literal>BtreePage</literal></entry>
<entry>Waiting for the page number needed to continue a parallel B-tree scan to become available.</entry>
</row>
<row>
<entry><literal>CheckpointDone</literal></entry>
<entry>Waiting for a checkpoint to complete.</entry>
</row>
<row>
<entry><literal>CheckpointStart</literal></entry>
<entry>Waiting for a checkpoint to start.</entry>
</row>
<row>
<entry><literal>ClogGroupUpdate</literal></entry>
<entry>Waiting for group leader to update transaction status at transaction end.</entry>
</row>
<row>
<entry><literal>ExecuteGather</literal></entry>
<entry>Waiting for activity from child process when executing <literal>Gather</literal> node.</entry>
</row>
<row>
<entry><literal>Hash/Batch/Allocating</literal></entry>
<entry>Waiting for an elected Parallel Hash participant to allocate a hash table.</entry>
</row>
<row>
<entry><literal>Hash/Batch/Electing</literal></entry>
<entry>Electing a Parallel Hash participant to allocate a hash table.</entry>
</row>
<row>
<entry><literal>Hash/Batch/Loading</literal></entry>
<entry>Waiting for other Parallel Hash participants to finish loading a hash table.</entry>
</row>
<row>
<entry><literal>Hash/Build/Allocating</literal></entry>
<entry>Waiting for an elected Parallel Hash participant to allocate the initial hash table.</entry>
</row>
<row>
<entry><literal>Hash/Build/Electing</literal></entry>
<entry>Electing a Parallel Hash participant to allocate the initial hash table.</entry>
</row>
<row>
<entry><literal>Hash/Build/HashingInner</literal></entry>
<entry>Waiting for other Parallel Hash participants to finish hashing the inner relation.</entry>
</row>
<row>
<entry><literal>Hash/Build/HashingOuter</literal></entry>
<entry>Waiting for other Parallel Hash participants to finish partitioning the outer relation.</entry>
</row>
<row>
<entry><literal>Hash/GrowBatches/Allocating</literal></entry>
<entry>Waiting for an elected Parallel Hash participant to allocate more batches.</entry>
</row>
<row>
<entry><literal>Hash/GrowBatches/Deciding</literal></entry>
<entry>Electing a Parallel Hash participant to decide on future batch growth.</entry>
</row>
<row>
<entry><literal>Hash/GrowBatches/Electing</literal></entry>
<entry>Electing a Parallel Hash participant to allocate more batches.</entry>
</row>
<row>
<entry><literal>Hash/GrowBatches/Finishing</literal></entry>
<entry>Waiting for an elected Parallel Hash participant to decide on future batch growth.</entry>
</row>
<row>
<entry><literal>Hash/GrowBatches/Repartitioning</literal></entry>
<entry>Waiting for other Parallel Hash participants to finishing repartitioning.</entry>
</row>
<row>
<entry><literal>Hash/GrowBuckets/Allocating</literal></entry>
<entry>Waiting for an elected Parallel Hash participant to finish allocating more buckets.</entry>
</row>
<row>
<entry><literal>Hash/GrowBuckets/Electing</literal></entry>
<entry>Electing a Parallel Hash participant to allocate more buckets.</entry>
</row>
<row>
<entry><literal>Hash/GrowBuckets/Reinserting</literal></entry>
<entry>Waiting for other Parallel Hash participants to finish inserting tuples into new buckets.</entry>
</row>
<row>
<entry><literal>LogicalSyncData</literal></entry>
<entry>Waiting for logical replication remote server to send data for initial table synchronization.</entry>
</row>
<row>
<entry><literal>LogicalSyncStateChange</literal></entry>
<entry>Waiting for logical replication remote server to change state.</entry>
</row>
<row>
<entry><literal>MessageQueueInternal</literal></entry>
<entry>Waiting for other process to be attached in shared message queue.</entry>
</row>
<row>
<entry><literal>MessageQueuePutMessage</literal></entry>
<entry>Waiting to write a protocol message to a shared message queue.</entry>
</row>
<row>
<entry><literal>MessageQueueReceive</literal></entry>
<entry>Waiting to receive bytes from a shared message queue.</entry>
</row>
<row>
<entry><literal>MessageQueueSend</literal></entry>
<entry>Waiting to send bytes to a shared message queue.</entry>
</row>
<row>
<entry><literal>ParallelBitmapScan</literal></entry>
<entry>Waiting for parallel bitmap scan to become initialized.</entry>
</row>
<row>
<entry><literal>ParallelCreateIndexScan</literal></entry>
<entry>Waiting for parallel <command>CREATE INDEX</command> workers to finish heap scan.</entry>
</row>
<row>
<entry><literal>ParallelFinish</literal></entry>
<entry>Waiting for parallel workers to finish computing.</entry>
</row>
<row>
<entry><literal>ProcArrayGroupUpdate</literal></entry>
<entry>Waiting for group leader to clear transaction id at transaction end.</entry>
</row>
<row>
<entry><literal>Promote</literal></entry>
<entry>Waiting for standby promotion.</entry>
</row>
<row>
<entry><literal>ReplicationOriginDrop</literal></entry>
<entry>Waiting for a replication origin to become inactive to be dropped.</entry>
</row>
<row>
<entry><literal>ReplicationSlotDrop</literal></entry>
<entry>Waiting for a replication slot to become inactive to be dropped.</entry>
</row>
<row>
<entry><literal>SafeSnapshot</literal></entry>
<entry>Waiting for a snapshot for a <literal>READ ONLY DEFERRABLE</literal> transaction.</entry>
</row>
<row>
<entry><literal>SyncRep</literal></entry>
<entry>Waiting for confirmation from remote server during synchronous replication.</entry>
</row>
<row>
<entry morerows="2"><literal>Timeout</literal></entry>
<entry><literal>BaseBackupThrottle</literal></entry>
<entry>Waiting during base backup when throttling activity.</entry>
</row>
<row>
<entry><literal>PgSleep</literal></entry>
<entry>Waiting in process that called <function>pg_sleep</function>.</entry>
</row>
<row>
<entry><literal>RecoveryApplyDelay</literal></entry>
<entry>Waiting to apply WAL at recovery because it is delayed.</entry>
</row>
<row>
<entry morerows="67"><literal>IO</literal></entry>
<entry><literal>BufFileRead</literal></entry>
<entry>Waiting for a read from a buffered file.</entry>
</row>
<row>
<entry><literal>BufFileWrite</literal></entry>
<entry>Waiting for a write to a buffered file.</entry>
</row>
<row>
<entry><literal>ControlFileRead</literal></entry>
<entry>Waiting for a read from the control file.</entry>
</row>
<row>
<entry><literal>ControlFileSync</literal></entry>
<entry>Waiting for the control file to reach stable storage.</entry>
</row>
<row>
<entry><literal>ControlFileSyncUpdate</literal></entry>
<entry>Waiting for an update to the control file to reach stable storage.</entry>
</row>
<row>
<entry><literal>ControlFileWrite</literal></entry>
<entry>Waiting for a write to the control file.</entry>
</row>
<row>
<entry><literal>ControlFileWriteUpdate</literal></entry>
<entry>Waiting for a write to update the control file.</entry>
</row>
<row>
<entry><literal>CopyFileRead</literal></entry>
<entry>Waiting for a read during a file copy operation.</entry>
</row>
<row>
<entry><literal>CopyFileWrite</literal></entry>
<entry>Waiting for a write during a file copy operation.</entry>
</row>
<row>
<entry><literal>DataFileExtend</literal></entry>
<entry>Waiting for a relation data file to be extended.</entry>
</row>
<row>
<entry><literal>DataFileFlush</literal></entry>
<entry>Waiting for a relation data file to reach stable storage.</entry>
</row>
<row>
<entry><literal>DataFileImmediateSync</literal></entry>
<entry>Waiting for an immediate synchronization of a relation data file to stable storage.</entry>
</row>
<row>
<entry><literal>DataFilePrefetch</literal></entry>
<entry>Waiting for an asynchronous prefetch from a relation data file.</entry>
</row>
<row>
<entry><literal>DataFileRead</literal></entry>
<entry>Waiting for a read from a relation data file.</entry>
</row>
<row>
<entry><literal>DataFileSync</literal></entry>
<entry>Waiting for changes to a relation data file to reach stable storage.</entry>
</row>
<row>
<entry><literal>DataFileTruncate</literal></entry>
<entry>Waiting for a relation data file to be truncated.</entry>
</row>
<row>
<entry><literal>DataFileWrite</literal></entry>
<entry>Waiting for a write to a relation data file.</entry>
</row>
<row>
<entry><literal>DSMFillZeroWrite</literal></entry>
<entry>Waiting to write zero bytes to a dynamic shared memory backing file.</entry>
</row>
<row>
<entry><literal>LockFileAddToDataDirRead</literal></entry>
<entry>Waiting for a read while adding a line to the data directory lock file.</entry>
</row>
<row>
<entry><literal>LockFileAddToDataDirSync</literal></entry>
<entry>Waiting for data to reach stable storage while adding a line to the data directory lock file.</entry>
</row>
<row>
<entry><literal>LockFileAddToDataDirWrite</literal></entry>
<entry>Waiting for a write while adding a line to the data directory lock file.</entry>
</row>
<row>
<entry><literal>LockFileCreateRead</literal></entry>
<entry>Waiting to read while creating the data directory lock file.</entry>
</row>
<row>
<entry><literal>LockFileCreateSync</literal></entry>
<entry>Waiting for data to reach stable storage while creating the data directory lock file.</entry>
</row>
<row>
<entry><literal>LockFileCreateWrite</literal></entry>
<entry>Waiting for a write while creating the data directory lock file.</entry>
</row>
<row>
<entry><literal>LockFileReCheckDataDirRead</literal></entry>
<entry>Waiting for a read during recheck of the data directory lock file.</entry>
</row>
<row>
<entry><literal>LogicalRewriteCheckpointSync</literal></entry>
<entry>Waiting for logical rewrite mappings to reach stable storage during a checkpoint.</entry>
</row>
<row>
<entry><literal>LogicalRewriteMappingSync</literal></entry>
<entry>Waiting for mapping data to reach stable storage during a logical rewrite.</entry>
</row>
<row>
<entry><literal>LogicalRewriteMappingWrite</literal></entry>
<entry>Waiting for a write of mapping data during a logical rewrite.</entry>
</row>
<row>
<entry><literal>LogicalRewriteSync</literal></entry>
<entry>Waiting for logical rewrite mappings to reach stable storage.</entry>
</row>
<row>
<entry><literal>LogicalRewriteWrite</literal></entry>
<entry>Waiting for a write of logical rewrite mappings.</entry>
</row>
<row>
<entry><literal>ProcSignalBarrier</literal></entry>
<entry>Waiting for a barrier event to be processed by all backends.</entry>
</row>
<row>
<entry><literal>RelationMapRead</literal></entry>
<entry>Waiting for a read of the relation map file.</entry>
</row>
<row>
<entry><literal>RelationMapSync</literal></entry>
<entry>Waiting for the relation map file to reach stable storage.</entry>
</row>
<row>
<entry><literal>RelationMapWrite</literal></entry>
<entry>Waiting for a write to the relation map file.</entry>
</row>
<row>
<entry><literal>ReorderBufferRead</literal></entry>
<entry>Waiting for a read during reorder buffer management.</entry>
</row>
<row>
<entry><literal>ReorderBufferWrite</literal></entry>
<entry>Waiting for a write during reorder buffer management.</entry>
</row>
<row>
<entry><literal>ReorderLogicalMappingRead</literal></entry>
<entry>Waiting for a read of a logical mapping during reorder buffer management.</entry>
</row>
<row>
<entry><literal>ReplicationSlotRead</literal></entry>
<entry>Waiting for a read from a replication slot control file.</entry>
</row>
<row>
<entry><literal>ReplicationSlotRestoreSync</literal></entry>
<entry>Waiting for a replication slot control file to reach stable storage while restoring it to memory.</entry>
</row>
<row>
<entry><literal>ReplicationSlotSync</literal></entry>
<entry>Waiting for a replication slot control file to reach stable storage.</entry>
</row>
<row>
<entry><literal>ReplicationSlotWrite</literal></entry>
<entry>Waiting for a write to a replication slot control file.</entry>
</row>
<row>
<entry><literal>SLRUFlushSync</literal></entry>
<entry>Waiting for SLRU data to reach stable storage during a checkpoint or database shutdown.</entry>
</row>
<row>
<entry><literal>SLRURead</literal></entry>
<entry>Waiting for a read of an SLRU page.</entry>
</row>
<row>
<entry><literal>SLRUSync</literal></entry>
<entry>Waiting for SLRU data to reach stable storage following a page write.</entry>
</row>
<row>
<entry><literal>SLRUWrite</literal></entry>
<entry>Waiting for a write of an SLRU page.</entry>
</row>
<row>
<entry><literal>SnapbuildRead</literal></entry>
<entry>Waiting for a read of a serialized historical catalog snapshot.</entry>
</row>
<row>
<entry><literal>SnapbuildSync</literal></entry>
<entry>Waiting for a serialized historical catalog snapshot to reach stable storage.</entry>
</row>
<row>
<entry><literal>SnapbuildWrite</literal></entry>
<entry>Waiting for a write of a serialized historical catalog snapshot.</entry>
</row>
<row>
<entry><literal>TimelineHistoryFileSync</literal></entry>
<entry>Waiting for a timeline history file received via streaming replication to reach stable storage.</entry>
</row>
<row>
<entry><literal>TimelineHistoryFileWrite</literal></entry>
<entry>Waiting for a write of a timeline history file received via streaming replication.</entry>
</row>
<row>
<entry><literal>TimelineHistoryRead</literal></entry>
<entry>Waiting for a read of a timeline history file.</entry>
</row>
<row>
<entry><literal>TimelineHistorySync</literal></entry>
<entry>Waiting for a newly created timeline history file to reach stable storage.</entry>
</row>
<row>
<entry><literal>TimelineHistoryWrite</literal></entry>
<entry>Waiting for a write of a newly created timeline history file.</entry>
</row>
<row>
<entry><literal>TwophaseFileRead</literal></entry>
<entry>Waiting for a read of a two phase state file.</entry>
</row>
<row>
<entry><literal>TwophaseFileSync</literal></entry>
<entry>Waiting for a two phase state file to reach stable storage.</entry>
</row>
<row>
<entry><literal>TwophaseFileWrite</literal></entry>
<entry>Waiting for a write of a two phase state file.</entry>
</row>
<row>
<entry><literal>WALBootstrapSync</literal></entry>
<entry>Waiting for WAL to reach stable storage during bootstrapping.</entry>
</row>
<row>
<entry><literal>WALBootstrapWrite</literal></entry>
<entry>Waiting for a write of a WAL page during bootstrapping.</entry>
</row>
<row>
<entry><literal>WALCopyRead</literal></entry>
<entry>Waiting for a read when creating a new WAL segment by copying an existing one.</entry>
</row>
<row>
<entry><literal>WALCopySync</literal></entry>
<entry>Waiting a new WAL segment created by copying an existing one to reach stable storage.</entry>
</row>
<row>
<entry><literal>WALCopyWrite</literal></entry>
<entry>Waiting for a write when creating a new WAL segment by copying an existing one.</entry>
</row>
<row>
<entry><literal>WALInitSync</literal></entry>
<entry>Waiting for a newly initialized WAL file to reach stable storage.</entry>
</row>
<row>
<entry><literal>WALInitWrite</literal></entry>
<entry>Waiting for a write while initializing a new WAL file.</entry>
</row>
<row>
<entry><literal>WALRead</literal></entry>
<entry>Waiting for a read from a WAL file.</entry>
</row>
<row>
<entry><literal>WALSenderTimelineHistoryRead</literal></entry>
<entry>Waiting for a read from a timeline history file during walsender timeline command.</entry>
</row>
<row>
<entry><literal>WALSync</literal></entry>
<entry>Waiting for a WAL file to reach stable storage.</entry>
</row>
<row>
<entry><literal>WALSyncMethodAssign</literal></entry>
<entry>Waiting for data to reach stable storage while assigning WAL sync method.</entry>
</row>
<row>
<entry><literal>WALWrite</literal></entry>
<entry>Waiting for a write to a WAL file.</entry>
</row>
</tbody>
</tgroup>
</table>
<note>
<para>
For tranches registered by extensions, the name is specified by extension
and this will be displayed as <structfield>wait_event</structfield>. It is quite
possible that user has registered the tranche in one of the backends (by
having allocation in dynamic shared memory) in which case other backends
won't have that information, so we display <literal>extension</literal> for such
cases.
</para>
</note>
<para>
Here is an example of how wait events can be viewed
<programlisting>
SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event is NOT NULL;
pid | wait_event_type | wait_event
------+-----------------+---------------
2540 | Lock | relation
6644 | LWLock | ProcArrayLock
(2 rows)
</programlisting>
</para>
<table id="pg-stat-replication-view" xreflabel="pg_stat_replication">
<title><structname>pg_stat_replication</structname> View</title>
<tgroup cols="3">
<thead>
<row>
<entry>Column</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><structfield>pid</structfield></entry>
<entry><type>integer</type></entry>
<entry>Process ID of a WAL sender process</entry>
</row>
<row>
<entry><structfield>usesysid</structfield></entry>
<entry><type>oid</type></entry>
<entry>OID of the user logged into this WAL sender process</entry>
</row>
<row>
<entry><structfield>usename</structfield></entry>
<entry><type>name</type></entry>
<entry>Name of the user logged into this WAL sender process</entry>
</row>
<row>
<entry><structfield>application_name</structfield></entry>
<entry><type>text</type></entry>
<entry>Name of the application that is connected
to this WAL sender</entry>
</row>
<row>
<entry><structfield>client_addr</structfield></entry>
<entry><type>inet</type></entry>
<entry>IP address of the client connected to this WAL sender.
If this field is null, it indicates that the client is
connected via a Unix socket on the server machine.
</entry>
</row>
<row>
<entry><structfield>client_hostname</structfield></entry>
<entry><type>text</type></entry>
<entry>Host name of the connected client, as reported by a
reverse DNS lookup of <structfield>client_addr</structfield>. This field will
only be non-null for IP connections, and only when <xref
linkend="guc-log-hostname"/> is enabled.
</entry>
</row>
<row>
<entry><structfield>client_port</structfield></entry>
<entry><type>integer</type></entry>
<entry>TCP port number that the client is using for communication
with this WAL sender, or <literal>-1</literal> if a Unix socket is used
</entry>
</row>
<row>
<entry><structfield>backend_start</structfield></entry>
<entry><type>timestamp with time zone</type></entry>
<entry>Time when this process was started, i.e., when the
client connected to this WAL sender
</entry>
</row>
<row>
<entry><structfield>backend_xmin</structfield></entry>
<entry><type>xid</type></entry>
<entry>This standby's <literal>xmin</literal> horizon reported
by <xref linkend="guc-hot-standby-feedback"/>.</entry>
</row>
<row>
<entry><structfield>state</structfield></entry>
<entry><type>text</type></entry>
<entry>Current WAL sender state.
Possible values are:
<itemizedlist>
<listitem>
<para>
<literal>startup</literal>: This WAL sender is starting up.
</para>
</listitem>
<listitem>
<para>
<literal>catchup</literal>: This WAL sender's connected standby is
catching up with the primary.
</para>
</listitem>
<listitem>
<para>
<literal>streaming</literal>: This WAL sender is streaming changes
after its connected standby server has caught up with the primary.
</para>
</listitem>
<listitem>
<para>
<literal>backup</literal>: This WAL sender is sending a backup.
</para>
</listitem>
<listitem>
<para>
<literal>stopping</literal>: This WAL sender is stopping.
</para>
</listitem>
</itemizedlist>
</entry>
</row>
<row>
<entry><structfield>sent_lsn</structfield></entry>
<entry><type>pg_lsn</type></entry>
<entry>Last write-ahead log location sent on this connection</entry>
</row>
<row>
<entry><structfield>write_lsn</structfield></entry>
<entry><type>pg_lsn</type></entry>
<entry>Last write-ahead log location written to disk by this standby
server</entry>
</row>
<row>
<entry><structfield>flush_lsn</structfield></entry>
<entry><type>pg_lsn</type></entry>
<entry>Last write-ahead log location flushed to disk by this standby
server</entry>
</row>
<row>
<entry><structfield>replay_lsn</structfield></entry>
<entry><type>pg_lsn</type></entry>
<entry>Last write-ahead log location replayed into the database on this
standby server</entry>
</row>
<row>
<entry><structfield>write_lag</structfield></entry>
<entry><type>interval</type></entry>
<entry>Time elapsed between flushing recent WAL locally and receiving
notification that this standby server has written it (but not yet
flushed it or applied it). This can be used to gauge the delay that
<literal>synchronous_commit</literal> level
<literal>remote_write</literal> incurred while committing if this
server was configured as a synchronous standby.</entry>
</row>
<row>
<entry><structfield>flush_lag</structfield></entry>
<entry><type>interval</type></entry>
<entry>Time elapsed between flushing recent WAL locally and receiving
notification that this standby server has written and flushed it
(but not yet applied it). This can be used to gauge the delay that
<literal>synchronous_commit</literal> level
<literal>on</literal> incurred while committing if this
server was configured as a synchronous standby.</entry>
</row>
<row>
<entry><structfield>replay_lag</structfield></entry>
<entry><type>interval</type></entry>
<entry>Time elapsed between flushing recent WAL locally and receiving
notification that this standby server has written, flushed and
applied it. This can be used to gauge the delay that
<literal>synchronous_commit</literal> level
<literal>remote_apply</literal> incurred while committing if this
server was configured as a synchronous standby.</entry>
</row>
<row>
<entry><structfield>sync_priority</structfield></entry>
<entry><type>integer</type></entry>
<entry>Priority of this standby server for being chosen as the
synchronous standby in a priority-based synchronous replication.
This has no effect in a quorum-based synchronous replication.</entry>
</row>
<row>
<entry><structfield>sync_state</structfield></entry>
<entry><type>text</type></entry>
<entry>Synchronous state of this standby server.
Possible values are:
<itemizedlist>
<listitem>
<para>
<literal>async</literal>: This standby server is asynchronous.
</para>
</listitem>
<listitem>
<para>
<literal>potential</literal>: This standby server is now asynchronous,
but can potentially become synchronous if one of current
synchronous ones fails.
</para>
</listitem>
<listitem>
<para>
<literal>sync</literal>: This standby server is synchronous.
</para>
</listitem>
<listitem>
<para>
<literal>quorum</literal>: This standby server is considered as a candidate
for quorum standbys.
</para>
</listitem>
</itemizedlist>
</entry>
</row>
<row>
<entry><structfield>reply_time</structfield></entry>
<entry><type>timestamp with time zone</type></entry>
<entry>Send time of last reply message received from standby server</entry>
</row>
<row>
<entry><structfield>spill_bytes</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Amount of decoded transaction data spilled to disk.</entry>
</row>
<row>
<entry><structfield>spill_txns</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of transactions spilled to disk after the memory used by
logical decoding exceeds <literal>logical_decoding_work_mem</literal>. The
counter gets incremented both for toplevel transactions and
subtransactions.</entry>
</row>
<row>
<entry><structfield>spill_count</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of times transactions were spilled to disk. Transactions
may get spilled repeatedly, and this counter gets incremented on every
such invocation.</entry>
</row>
</tbody>
</tgroup>
</table>
<para>
The <structname>pg_stat_replication</structname> view will contain one row
per WAL sender process, showing statistics about replication to that
sender's connected standby server. Only directly connected standbys are
listed; no information is available about downstream standby servers.
</para>
<para>
The lag times reported in the <structname>pg_stat_replication</structname>
view are measurements of the time taken for recent WAL to be written,
flushed and replayed and for the sender to know about it. These times
represent the commit delay that was (or would have been) introduced by each
synchronous commit level, if the remote server was configured as a
synchronous standby. For an asynchronous standby, the
<structfield>replay_lag</structfield> column approximates the delay
before recent transactions became visible to queries. If the standby
server has entirely caught up with the sending server and there is no more
WAL activity, the most recently measured lag times will continue to be
displayed for a short time and then show NULL.
</para>
<para>
Lag times work automatically for physical replication. Logical decoding
plugins may optionally emit tracking messages; if they do not, the tracking
mechanism will simply display NULL lag.
</para>
<note>
<para>
The reported lag times are not predictions of how long it will take for
the standby to catch up with the sending server assuming the current
rate of replay. Such a system would show similar times while new WAL is
being generated, but would differ when the sender becomes idle. In
particular, when the standby has caught up completely,
<structname>pg_stat_replication</structname> shows the time taken to
write, flush and replay the most recent reported WAL location rather than
zero as some users might expect. This is consistent with the goal of
measuring synchronous commit and transaction visibility delays for
recent write transactions.
To reduce confusion for users expecting a different model of lag, the
lag columns revert to NULL after a short time on a fully replayed idle
system. Monitoring systems should choose whether to represent this
as missing data, zero or continue to display the last known value.
</para>
</note>
<table id="pg-stat-wal-receiver-view" xreflabel="pg_stat_wal_receiver">
<title><structname>pg_stat_wal_receiver</structname> View</title>
<tgroup cols="3">
<thead>
<row>
<entry>Column</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><structfield>pid</structfield></entry>
<entry><type>integer</type></entry>
<entry>Process ID of the WAL receiver process</entry>
</row>
<row>
<entry><structfield>status</structfield></entry>
<entry><type>text</type></entry>
<entry>Activity status of the WAL receiver process</entry>
</row>
<row>
<entry><structfield>receive_start_lsn</structfield></entry>
<entry><type>pg_lsn</type></entry>
<entry>First write-ahead log location used when WAL receiver is
started</entry>
</row>
<row>
<entry><structfield>receive_start_tli</structfield></entry>
<entry><type>integer</type></entry>
<entry>First timeline number used when WAL receiver is started</entry>
</row>
<row>
<entry><structfield>received_lsn</structfield></entry>
<entry><type>pg_lsn</type></entry>
<entry>Last write-ahead log location already received and flushed to
disk, the initial value of this field being the first log location used
when WAL receiver is started</entry>
</row>
<row>
<entry><structfield>received_tli</structfield></entry>
<entry><type>integer</type></entry>
<entry>Timeline number of last write-ahead log location received and
flushed to disk, the initial value of this field being the timeline
number of the first log location used when WAL receiver is started
</entry>
</row>
<row>
<entry><structfield>last_msg_send_time</structfield></entry>
<entry><type>timestamp with time zone</type></entry>
<entry>Send time of last message received from origin WAL sender</entry>
</row>
<row>
<entry><structfield>last_msg_receipt_time</structfield></entry>
<entry><type>timestamp with time zone</type></entry>
<entry>Receipt time of last message received from origin WAL sender</entry>
</row>
<row>
<entry><structfield>latest_end_lsn</structfield></entry>
<entry><type>pg_lsn</type></entry>
<entry>Last write-ahead log location reported to origin WAL sender</entry>
</row>
<row>
<entry><structfield>latest_end_time</structfield></entry>
<entry><type>timestamp with time zone</type></entry>
<entry>Time of last write-ahead log location reported to origin WAL sender</entry>
</row>
<row>
<entry><structfield>slot_name</structfield></entry>
<entry><type>text</type></entry>
<entry>Replication slot name used by this WAL receiver</entry>
</row>
<row>
<entry><structfield>sender_host</structfield></entry>
<entry><type>text</type></entry>
<entry>
Host of the <productname>PostgreSQL</productname> instance
this WAL receiver is connected to. This can be a host name,
an IP address, or a directory path if the connection is via
Unix socket. (The path case can be distinguished because it
will always be an absolute path, beginning with <literal>/</literal>.)
</entry>
</row>
<row>
<entry><structfield>sender_port</structfield></entry>
<entry><type>integer</type></entry>
<entry>
Port number of the <productname>PostgreSQL</productname> instance
this WAL receiver is connected to.
</entry>
</row>
<row>
<entry><structfield>conninfo</structfield></entry>
<entry><type>text</type></entry>
<entry>
Connection string used by this WAL receiver,
with security-sensitive fields obfuscated.
</entry>
</row>
</tbody>
</tgroup>
</table>
<para>
The <structname>pg_stat_wal_receiver</structname> view will contain only
one row, showing statistics about the WAL receiver from that receiver's
connected server.
</para>
<table id="pg-stat-subscription" xreflabel="pg_stat_subscription">
<title><structname>pg_stat_subscription</structname> View</title>
<tgroup cols="3">
<thead>
<row>
<entry>Column</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><structfield>subid</structfield></entry>
<entry><type>oid</type></entry>
<entry>OID of the subscription</entry>
</row>
<row>
<entry><structfield>subname</structfield></entry>
<entry><type>text</type></entry>
<entry>Name of the subscription</entry>
</row>
<row>
<entry><structfield>pid</structfield></entry>
<entry><type>integer</type></entry>
<entry>Process ID of the subscription worker process</entry>
</row>
<row>
<entry><structfield>relid</structfield></entry>
<entry><type>Oid</type></entry>
<entry>OID of the relation that the worker is synchronizing; null for the
main apply worker</entry>
</row>
<row>
<entry><structfield>received_lsn</structfield></entry>
<entry><type>pg_lsn</type></entry>
<entry>Last write-ahead log location received, the initial value of
this field being 0</entry>
</row>
<row>
<entry><structfield>last_msg_send_time</structfield></entry>
<entry><type>timestamp with time zone</type></entry>
<entry>Send time of last message received from origin WAL sender</entry>
</row>
<row>
<entry><structfield>last_msg_receipt_time</structfield></entry>
<entry><type>timestamp with time zone</type></entry>
<entry>Receipt time of last message received from origin WAL sender
</entry>
</row>
<row>
<entry><structfield>latest_end_lsn</structfield></entry>
<entry><type>pg_lsn</type></entry>
<entry>Last write-ahead log location reported to origin WAL sender
</entry>
</row>
<row>
<entry><structfield>latest_end_time</structfield></entry>
<entry><type>timestamp with time zone</type></entry>
<entry>Time of last write-ahead log location reported to origin WAL
sender</entry>
</row>
</tbody>
</tgroup>
</table>
<para>
The <structname>pg_stat_subscription</structname> view will contain one
row per subscription for main worker (with null PID if the worker is
not running), and additional rows for workers handling the initial data
copy of the subscribed tables.
</para>
<table id="pg-stat-ssl-view" xreflabel="pg_stat_ssl">
<title><structname>pg_stat_ssl</structname> View</title>
<tgroup cols="3">
<thead>
<row>
<entry>Column</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><structfield>pid</structfield></entry>
<entry><type>integer</type></entry>
<entry>Process ID of a backend or WAL sender process</entry>
</row>
<row>
<entry><structfield>ssl</structfield></entry>
<entry><type>boolean</type></entry>
<entry>True if SSL is used on this connection</entry>
</row>
<row>
<entry><structfield>version</structfield></entry>
<entry><type>text</type></entry>
<entry>Version of SSL in use, or NULL if SSL is not in use
on this connection</entry>
</row>
<row>
<entry><structfield>cipher</structfield></entry>
<entry><type>text</type></entry>
<entry>Name of SSL cipher in use, or NULL if SSL is not in use
on this connection</entry>
</row>
<row>
<entry><structfield>bits</structfield></entry>
<entry><type>integer</type></entry>
<entry>Number of bits in the encryption algorithm used, or NULL
if SSL is not used on this connection</entry>
</row>
<row>
<entry><structfield>compression</structfield></entry>
<entry><type>boolean</type></entry>
<entry>True if SSL compression is in use, false if not,
or NULL if SSL is not in use on this connection</entry>
</row>
<row>
<entry><structfield>client_dn</structfield></entry>
<entry><type>text</type></entry>
<entry>Distinguished Name (DN) field from the client certificate
used, or NULL if no client certificate was supplied or if SSL
is not in use on this connection. This field is truncated if the
DN field is longer than <symbol>NAMEDATALEN</symbol> (64 characters
in a standard build).
</entry>
</row>
<row>
<entry><structfield>client_serial</structfield></entry>
<entry><type>numeric</type></entry>
<entry>Serial number of the client certificate, or NULL if no client
certificate was supplied or if SSL is not in use on this connection. The
combination of certificate serial number and certificate issuer uniquely
identifies a certificate (unless the issuer erroneously reuses serial
numbers).</entry>
</row>
<row>
<entry><structfield>issuer_dn</structfield></entry>
<entry><type>text</type></entry>
<entry>DN of the issuer of the client certificate, or NULL if no client
certificate was supplied or if SSL is not in use on this connection.
This field is truncated like <structfield>client_dn</structfield>.</entry>
</row>
</tbody>
</tgroup>
</table>
<para>
The <structname>pg_stat_ssl</structname> view will contain one row per
backend or WAL sender process, showing statistics about SSL usage on
this connection. It can be joined to <structname>pg_stat_activity</structname>
or <structname>pg_stat_replication</structname> on the
<structfield>pid</structfield> column to get more details about the
connection.
</para>
<table id="pg-stat-gssapi-view" xreflabel="pg_stat_gssapi">
<title><structname>pg_stat_gssapi</structname> View</title>
<tgroup cols="3">
<thead>
<row>
<entry>Column</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><structfield>pid</structfield></entry>
<entry><type>integer</type></entry>
<entry>Process ID of a backend</entry>
</row>
<row>
<entry><structfield>gss_authenticated</structfield></entry>
<entry><type>boolean</type></entry>
<entry>True if GSSAPI authentication was used for this connection</entry>
</row>
<row>
<entry><structfield>principal</structfield></entry>
<entry><type>text</type></entry>
<entry>Principal used to authenticate this connection, or NULL
if GSSAPI was not used to authenticate this connection. This
field is truncated if the principal is longer than
<symbol>NAMEDATALEN</symbol> (64 characters in a standard build).
</entry>
</row>
<row>
<entry><structfield>encrypted</structfield></entry>
<entry><type>boolean</type></entry>
<entry>True if GSSAPI encryption is in use on this connection</entry>
</row>
</tbody>
</tgroup>
</table>
<para>
The <structname>pg_stat_gssapi</structname> view will contain one row per
backend, showing information about GSSAPI usage on this connection. It can
be joined to <structname>pg_stat_activity</structname> or
<structname>pg_stat_replication</structname> on the
<structfield>pid</structfield> column to get more details about the
connection.
</para>
<table id="pg-stat-archiver-view" xreflabel="pg_stat_archiver">
<title><structname>pg_stat_archiver</structname> View</title>
<tgroup cols="3">
<thead>
<row>
<entry>Column</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><structfield>archived_count</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of WAL files that have been successfully archived</entry>
</row>
<row>
<entry><structfield>last_archived_wal</structfield></entry>
<entry><type>text</type></entry>
<entry>Name of the last WAL file successfully archived</entry>
</row>
<row>
<entry><structfield>last_archived_time</structfield></entry>
<entry><type>timestamp with time zone</type></entry>
<entry>Time of the last successful archive operation</entry>
</row>
<row>
<entry><structfield>failed_count</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of failed attempts for archiving WAL files</entry>
</row>
<row>
<entry><structfield>last_failed_wal</structfield></entry>
<entry><type>text</type></entry>
<entry>Name of the WAL file of the last failed archival operation</entry>
</row>
<row>
<entry><structfield>last_failed_time</structfield></entry>
<entry><type>timestamp with time zone</type></entry>
<entry>Time of the last failed archival operation</entry>
</row>
<row>
<entry><structfield>stats_reset</structfield></entry>
<entry><type>timestamp with time zone</type></entry>
<entry>Time at which these statistics were last reset</entry>
</row>
</tbody>
</tgroup>
</table>
<para>
The <structname>pg_stat_archiver</structname> view will always have a
single row, containing data about the archiver process of the cluster.
</para>
<table id="pg-stat-bgwriter-view" xreflabel="pg_stat_bgwriter">
<title><structname>pg_stat_bgwriter</structname> View</title>
<tgroup cols="3">
<thead>
<row>
<entry>Column</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><structfield>checkpoints_timed</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of scheduled checkpoints that have been performed</entry>
</row>
<row>
<entry><structfield>checkpoints_req</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of requested checkpoints that have been performed</entry>
</row>
<row>
<entry><structfield>checkpoint_write_time</structfield></entry>
<entry><type>double precision</type></entry>
<entry>
Total amount of time that has been spent in the portion of
checkpoint processing where files are written to disk, in milliseconds
</entry>
</row>
<row>
<entry><structfield>checkpoint_sync_time</structfield></entry>
<entry><type>double precision</type></entry>
<entry>
Total amount of time that has been spent in the portion of
checkpoint processing where files are synchronized to disk, in
milliseconds
</entry>
</row>
<row>
<entry><structfield>buffers_checkpoint</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of buffers written during checkpoints</entry>
</row>
<row>
<entry><structfield>buffers_clean</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of buffers written by the background writer</entry>
</row>
<row>
<entry><structfield>maxwritten_clean</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of times the background writer stopped a cleaning
scan because it had written too many buffers</entry>
</row>
<row>
<entry><structfield>buffers_backend</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of buffers written directly by a backend</entry>
</row>
<row>
<entry><structfield>buffers_backend_fsync</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of times a backend had to execute its own
<function>fsync</function> call (normally the background writer handles those
even when the backend does its own write)</entry>
</row>
<row>
<entry><structfield>buffers_alloc</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of buffers allocated</entry>
</row>
<row>
<entry><structfield>stats_reset</structfield></entry>
<entry><type>timestamp with time zone</type></entry>
<entry>Time at which these statistics were last reset</entry>
</row>
</tbody>
</tgroup>
</table>
<para>
The <structname>pg_stat_bgwriter</structname> view will always have a
single row, containing global data for the cluster.
</para>
<table id="pg-stat-database-view" xreflabel="pg_stat_database">
<title><structname>pg_stat_database</structname> View</title>
<tgroup cols="3">
<thead>
<row>
<entry>Column</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><structfield>datid</structfield></entry>
<entry><type>oid</type></entry>
<entry>OID of this database, or 0 for objects belonging to a shared
relation</entry>
</row>
<row>
<entry><structfield>datname</structfield></entry>
<entry><type>name</type></entry>
<entry>Name of this database, or <literal>NULL</literal> for the shared
objects.</entry>
</row>
<row>
<entry><structfield>numbackends</structfield></entry>
<entry><type>integer</type></entry>
<entry>Number of backends currently connected to this database, or
<literal>NULL</literal> for the shared objects. This is the only column
in this view that returns a value reflecting current state; all other
columns return the accumulated values since the last reset.</entry>
</row>
<row>
<entry><structfield>xact_commit</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of transactions in this database that have been
committed</entry>
</row>
<row>
<entry><structfield>xact_rollback</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of transactions in this database that have been
rolled back</entry>
</row>
<row>
<entry><structfield>blks_read</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of disk blocks read in this database</entry>
</row>
<row>
<entry><structfield>blks_hit</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of times disk blocks were found already in the buffer
cache, so that a read was not necessary (this only includes hits in the
PostgreSQL buffer cache, not the operating system's file system cache)
</entry>
</row>
<row>
<entry><structfield>tup_returned</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of rows returned by queries in this database</entry>
</row>
<row>
<entry><structfield>tup_fetched</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of rows fetched by queries in this database</entry>
</row>
<row>
<entry><structfield>tup_inserted</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of rows inserted by queries in this database</entry>
</row>
<row>
<entry><structfield>tup_updated</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of rows updated by queries in this database</entry>
</row>
<row>
<entry><structfield>tup_deleted</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of rows deleted by queries in this database</entry>
</row>
<row>
<entry><structfield>conflicts</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of queries canceled due to conflicts with recovery
in this database. (Conflicts occur only on standby servers; see
<xref linkend="pg-stat-database-conflicts-view"/> for details.)
</entry>
</row>
<row>
<entry><structfield>temp_files</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of temporary files created by queries in this database.
All temporary files are counted, regardless of why the temporary file
was created (e.g., sorting or hashing), and regardless of the
<xref linkend="guc-log-temp-files"/> setting.
</entry>
</row>
<row>
<entry><structfield>temp_bytes</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Total amount of data written to temporary files by queries in
this database. All temporary files are counted, regardless of why
the temporary file was created, and
regardless of the <xref linkend="guc-log-temp-files"/> setting.
</entry>
</row>
<row>
<entry><structfield>deadlocks</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of deadlocks detected in this database</entry>
</row>
<row>
<entry><structfield>checksum_failures</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of data page checksum failures detected in this
database (or on a shared object), or NULL if data checksums are not
enabled.</entry>
</row>
<row>
<entry><structfield>checksum_last_failure</structfield></entry>
<entry><type>timestamp with time zone</type></entry>
<entry>Time at which the last data page checksum failure was detected in
this database (or on a shared object), or NULL if data checksums are not
enabled.</entry>
</row>
<row>
<entry><structfield>blk_read_time</structfield></entry>
<entry><type>double precision</type></entry>
<entry>Time spent reading data file blocks by backends in this database,
in milliseconds</entry>
</row>
<row>
<entry><structfield>blk_write_time</structfield></entry>
<entry><type>double precision</type></entry>
<entry>Time spent writing data file blocks by backends in this database,
in milliseconds</entry>
</row>
<row>
<entry><structfield>stats_reset</structfield></entry>
<entry><type>timestamp with time zone</type></entry>
<entry>Time at which these statistics were last reset</entry>
</row>
</tbody>
</tgroup>
</table>
<para>
The <structname>pg_stat_database</structname> view will contain one row
for each database in the cluster, plus one for the shared objects, showing
database-wide statistics.
</para>
<table id="pg-stat-database-conflicts-view" xreflabel="pg_stat_database_conflicts">
<title><structname>pg_stat_database_conflicts</structname> View</title>
<tgroup cols="3">
<thead>
<row>
<entry>Column</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><structfield>datid</structfield></entry>
<entry><type>oid</type></entry>
<entry>OID of a database</entry>
</row>
<row>
<entry><structfield>datname</structfield></entry>
<entry><type>name</type></entry>
<entry>Name of this database</entry>
</row>
<row>
<entry><structfield>confl_tablespace</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of queries in this database that have been canceled due to
dropped tablespaces</entry>
</row>
<row>
<entry><structfield>confl_lock</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of queries in this database that have been canceled due to
lock timeouts</entry>
</row>
<row>
<entry><structfield>confl_snapshot</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of queries in this database that have been canceled due to
old snapshots</entry>
</row>
<row>
<entry><structfield>confl_bufferpin</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of queries in this database that have been canceled due to
pinned buffers</entry>
</row>
<row>
<entry><structfield>confl_deadlock</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of queries in this database that have been canceled due to
deadlocks</entry>
</row>
</tbody>
</tgroup>
</table>
<para>
The <structname>pg_stat_database_conflicts</structname> view will contain
one row per database, showing database-wide statistics about
query cancels occurring due to conflicts with recovery on standby servers.
This view will only contain information on standby servers, since
conflicts do not occur on master servers.
</para>
<table id="pg-stat-all-tables-view" xreflabel="pg_stat_all_tables">
<title><structname>pg_stat_all_tables</structname> View</title>
<tgroup cols="3">
<thead>
<row>
<entry>Column</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><structfield>relid</structfield></entry>
<entry><type>oid</type></entry>
<entry>OID of a table</entry>
</row>
<row>
<entry><structfield>schemaname</structfield></entry>
<entry><type>name</type></entry>
<entry>Name of the schema that this table is in</entry>
</row>
<row>
<entry><structfield>relname</structfield></entry>
<entry><type>name</type></entry>
<entry>Name of this table</entry>
</row>
<row>
<entry><structfield>seq_scan</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of sequential scans initiated on this table</entry>
</row>
<row>
<entry><structfield>seq_tup_read</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of live rows fetched by sequential scans</entry>
</row>
<row>
<entry><structfield>idx_scan</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of index scans initiated on this table</entry>
</row>
<row>
<entry><structfield>idx_tup_fetch</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of live rows fetched by index scans</entry>
</row>
<row>
<entry><structfield>n_tup_ins</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of rows inserted</entry>
</row>
<row>
<entry><structfield>n_tup_upd</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of rows updated (includes HOT updated rows)</entry>
</row>
<row>
<entry><structfield>n_tup_del</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of rows deleted</entry>
</row>
<row>
<entry><structfield>n_tup_hot_upd</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of rows HOT updated (i.e., with no separate index
update required)</entry>
</row>
<row>
<entry><structfield>n_live_tup</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Estimated number of live rows</entry>
</row>
<row>
<entry><structfield>n_dead_tup</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Estimated number of dead rows</entry>
</row>
<row>
<entry><structfield>n_mod_since_analyze</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Estimated number of rows modified since this table was last analyzed</entry>
</row>
<row>
<entry><structfield>last_vacuum</structfield></entry>
<entry><type>timestamp with time zone</type></entry>
<entry>Last time at which this table was manually vacuumed
(not counting <command>VACUUM FULL</command>)</entry>
</row>
<row>
<entry><structfield>last_autovacuum</structfield></entry>
<entry><type>timestamp with time zone</type></entry>
<entry>Last time at which this table was vacuumed by the autovacuum
daemon</entry>
</row>
<row>
<entry><structfield>last_analyze</structfield></entry>
<entry><type>timestamp with time zone</type></entry>
<entry>Last time at which this table was manually analyzed</entry>
</row>
<row>
<entry><structfield>last_autoanalyze</structfield></entry>
<entry><type>timestamp with time zone</type></entry>
<entry>Last time at which this table was analyzed by the autovacuum
daemon</entry>
</row>
<row>
<entry><structfield>vacuum_count</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of times this table has been manually vacuumed
(not counting <command>VACUUM FULL</command>)</entry>
</row>
<row>
<entry><structfield>autovacuum_count</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of times this table has been vacuumed by the autovacuum
daemon</entry>
</row>
<row>
<entry><structfield>analyze_count</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of times this table has been manually analyzed</entry>
</row>
<row>
<entry><structfield>autoanalyze_count</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of times this table has been analyzed by the autovacuum
daemon</entry>
</row>
</tbody>
</tgroup>
</table>
<para>
The <structname>pg_stat_all_tables</structname> view will contain
one row for each table in the current database (including TOAST
tables), showing statistics about accesses to that specific table. The
<structname>pg_stat_user_tables</structname> and
<structname>pg_stat_sys_tables</structname> views
contain the same information,
but filtered to only show user and system tables respectively.
</para>
<table id="pg-stat-all-indexes-view" xreflabel="pg_stat_all_indexes">
<title><structname>pg_stat_all_indexes</structname> View</title>
<tgroup cols="3">
<thead>
<row>
<entry>Column</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><structfield>relid</structfield></entry>
<entry><type>oid</type></entry>
<entry>OID of the table for this index</entry>
</row>
<row>
<entry><structfield>indexrelid</structfield></entry>
<entry><type>oid</type></entry>
<entry>OID of this index</entry>
</row>
<row>
<entry><structfield>schemaname</structfield></entry>
<entry><type>name</type></entry>
<entry>Name of the schema this index is in</entry>
</row>
<row>
<entry><structfield>relname</structfield></entry>
<entry><type>name</type></entry>
<entry>Name of the table for this index</entry>
</row>
<row>
<entry><structfield>indexrelname</structfield></entry>
<entry><type>name</type></entry>
<entry>Name of this index</entry>
</row>
<row>
<entry><structfield>idx_scan</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of index scans initiated on this index</entry>
</row>
<row>
<entry><structfield>idx_tup_read</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of index entries returned by scans on this index</entry>
</row>
<row>
<entry><structfield>idx_tup_fetch</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of live table rows fetched by simple index scans using this
index</entry>
</row>
</tbody>
</tgroup>
</table>
<para>
The <structname>pg_stat_all_indexes</structname> view will contain
one row for each index in the current database,
showing statistics about accesses to that specific index. The
<structname>pg_stat_user_indexes</structname> and
<structname>pg_stat_sys_indexes</structname> views
contain the same information,
but filtered to only show user and system indexes respectively.
</para>
<para>
Indexes can be used by simple index scans, <quote>bitmap</quote> index scans,
and the optimizer. In a bitmap scan
the output of several indexes can be combined via AND or OR rules,
so it is difficult to associate individual heap row fetches
with specific indexes when a bitmap scan is used. Therefore, a bitmap
scan increments the
<structname>pg_stat_all_indexes</structname>.<structfield>idx_tup_read</structfield>
count(s) for the index(es) it uses, and it increments the
<structname>pg_stat_all_tables</structname>.<structfield>idx_tup_fetch</structfield>
count for the table, but it does not affect
<structname>pg_stat_all_indexes</structname>.<structfield>idx_tup_fetch</structfield>.
The optimizer also accesses indexes to check for supplied constants
whose values are outside the recorded range of the optimizer statistics
because the optimizer statistics might be stale.
</para>
<note>
<para>
The <structfield>idx_tup_read</structfield> and <structfield>idx_tup_fetch</structfield> counts
can be different even without any use of bitmap scans,
because <structfield>idx_tup_read</structfield> counts
index entries retrieved from the index while <structfield>idx_tup_fetch</structfield>
counts live rows fetched from the table. The latter will be less if any
dead or not-yet-committed rows are fetched using the index, or if any
heap fetches are avoided by means of an index-only scan.
</para>
</note>
<table id="pg-statio-all-tables-view" xreflabel="pg_statio_all_tables">
<title><structname>pg_statio_all_tables</structname> View</title>
<tgroup cols="3">
<thead>
<row>
<entry>Column</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><structfield>relid</structfield></entry>
<entry><type>oid</type></entry>
<entry>OID of a table</entry>
</row>
<row>
<entry><structfield>schemaname</structfield></entry>
<entry><type>name</type></entry>
<entry>Name of the schema that this table is in</entry>
</row>
<row>
<entry><structfield>relname</structfield></entry>
<entry><type>name</type></entry>
<entry>Name of this table</entry>
</row>
<row>
<entry><structfield>heap_blks_read</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of disk blocks read from this table</entry>
</row>
<row>
<entry><structfield>heap_blks_hit</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of buffer hits in this table</entry>
</row>
<row>
<entry><structfield>idx_blks_read</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of disk blocks read from all indexes on this table</entry>
</row>
<row>
<entry><structfield>idx_blks_hit</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of buffer hits in all indexes on this table</entry>
</row>
<row>
<entry><structfield>toast_blks_read</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of disk blocks read from this table's TOAST table (if any)</entry>
</row>
<row>
<entry><structfield>toast_blks_hit</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of buffer hits in this table's TOAST table (if any)</entry>
</row>
<row>
<entry><structfield>tidx_blks_read</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of disk blocks read from this table's TOAST table indexes (if any)</entry>
</row>
<row>
<entry><structfield>tidx_blks_hit</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of buffer hits in this table's TOAST table indexes (if any)</entry>
</row>
</tbody>
</tgroup>
</table>
<para>
The <structname>pg_statio_all_tables</structname> view will contain
one row for each table in the current database (including TOAST
tables), showing statistics about I/O on that specific table. The
<structname>pg_statio_user_tables</structname> and
<structname>pg_statio_sys_tables</structname> views
contain the same information,
but filtered to only show user and system tables respectively.
</para>
<table id="pg-statio-all-indexes-view" xreflabel="pg_statio_all_indexes">
<title><structname>pg_statio_all_indexes</structname> View</title>
<tgroup cols="3">
<thead>
<row>
<entry>Column</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><structfield>relid</structfield></entry>
<entry><type>oid</type></entry>
<entry>OID of the table for this index</entry>
</row>
<row>
<entry><structfield>indexrelid</structfield></entry>
<entry><type>oid</type></entry>
<entry>OID of this index</entry>
</row>
<row>
<entry><structfield>schemaname</structfield></entry>
<entry><type>name</type></entry>
<entry>Name of the schema this index is in</entry>
</row>
<row>
<entry><structfield>relname</structfield></entry>
<entry><type>name</type></entry>
<entry>Name of the table for this index</entry>
</row>
<row>
<entry><structfield>indexrelname</structfield></entry>
<entry><type>name</type></entry>
<entry>Name of this index</entry>
</row>
<row>
<entry><structfield>idx_blks_read</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of disk blocks read from this index</entry>
</row>
<row>
<entry><structfield>idx_blks_hit</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of buffer hits in this index</entry>
</row>
</tbody>
</tgroup>
</table>
<para>
The <structname>pg_statio_all_indexes</structname> view will contain
one row for each index in the current database,
showing statistics about I/O on that specific index. The
<structname>pg_statio_user_indexes</structname> and
<structname>pg_statio_sys_indexes</structname> views
contain the same information,
but filtered to only show user and system indexes respectively.
</para>
<table id="pg-statio-all-sequences-view" xreflabel="pg_statio_all_sequences">
<title><structname>pg_statio_all_sequences</structname> View</title>
<tgroup cols="3">
<thead>
<row>
<entry>Column</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><structfield>relid</structfield></entry>
<entry><type>oid</type></entry>
<entry>OID of a sequence</entry>
</row>
<row>
<entry><structfield>schemaname</structfield></entry>
<entry><type>name</type></entry>
<entry>Name of the schema this sequence is in</entry>
</row>
<row>
<entry><structfield>relname</structfield></entry>
<entry><type>name</type></entry>
<entry>Name of this sequence</entry>
</row>
<row>
<entry><structfield>blks_read</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of disk blocks read from this sequence</entry>
</row>
<row>
<entry><structfield>blks_hit</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of buffer hits in this sequence</entry>
</row>
</tbody>
</tgroup>
</table>
<para>
The <structname>pg_statio_all_sequences</structname> view will contain
one row for each sequence in the current database,
showing statistics about I/O on that specific sequence.
</para>
<table id="pg-stat-user-functions-view" xreflabel="pg_stat_user_functions">
<title><structname>pg_stat_user_functions</structname> View</title>
<tgroup cols="3">
<thead>
<row>
<entry>Column</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><structfield>funcid</structfield></entry>
<entry><type>oid</type></entry>
<entry>OID of a function</entry>
</row>
<row>
<entry><structfield>schemaname</structfield></entry>
<entry><type>name</type></entry>
<entry>Name of the schema this function is in</entry>
</row>
<row>
<entry><structfield>funcname</structfield></entry>
<entry><type>name</type></entry>
<entry>Name of this function</entry>
</row>
<row>
<entry><structfield>calls</structfield></entry>
<entry><type>bigint</type></entry>
<entry>Number of times this function has been called</entry>
</row>
<row>
<entry><structfield>total_time</structfield></entry>
<entry><type>double precision</type></entry>
<entry>Total time spent in this function and all other functions
called by it, in milliseconds</entry>
</row>
<row>
<entry><structfield>self_time</structfield></entry>
<entry><type>double precision</type></entry>
<entry>Total time spent in this function itself, not including
other functions called by it, in milliseconds</entry>
</row>
</tbody>
</tgroup>
</table>
<para>
The <structname>pg_stat_user_functions</structname> view will contain
one row for each tracked function, showing statistics about executions of
that function. The <xref linkend="guc-track-functions"/> parameter
controls exactly which functions are tracked.
</para>
</sect2>
<sect2 id="monitoring-stats-functions">
<title>Statistics Functions</title>
<para>
Other ways of looking at the statistics can be set up by writing
queries that use the same underlying statistics access functions used by
the standard views shown above. For details such as the functions' names,
consult the definitions of the standard views. (For example, in
<application>psql</application> you could issue <literal>\d+ pg_stat_activity</literal>.)
The access functions for per-database statistics take a database OID as an
argument to identify which database to report on.
The per-table and per-index functions take a table or index OID.
The functions for per-function statistics take a function OID.
Note that only tables, indexes, and functions in the current database
can be seen with these functions.
</para>
<para>
Additional functions related to statistics collection are listed in <xref
linkend="monitoring-stats-funcs-table"/>.
</para>
<table id="monitoring-stats-funcs-table">
<title>Additional Statistics Functions</title>
<tgroup cols="3">
<thead>
<row>
<entry>Function</entry>
<entry>Return Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<!-- See also the entry for this in func.sgml -->
<entry><literal><function>pg_backend_pid()</function></literal></entry>
<entry><type>integer</type></entry>
<entry>
Process ID of the server process handling the current session
</entry>
</row>
<row>
<entry><literal><function>pg_stat_get_activity</function>(<type>integer</type>)</literal><indexterm><primary>pg_stat_get_activity</primary></indexterm></entry>
<entry><type>setof record</type></entry>
<entry>
Returns a record of information about the backend with the specified PID, or
one record for each active backend in the system if <symbol>NULL</symbol> is
specified. The fields returned are a subset of those in the
<structname>pg_stat_activity</structname> view.
</entry>
</row>
<row>
<entry><literal><function>pg_stat_get_snapshot_timestamp()</function></literal><indexterm><primary>pg_stat_get_snapshot_timestamp</primary></indexterm></entry>
<entry><type>timestamp with time zone</type></entry>
<entry>
Returns the timestamp of the current statistics snapshot
</entry>
</row>
<row>
<entry><literal><function>pg_stat_clear_snapshot()</function></literal><indexterm><primary>pg_stat_clear_snapshot</primary></indexterm></entry>
<entry><type>void</type></entry>
<entry>
Discard the current statistics snapshot
</entry>
</row>
<row>
<entry><literal><function>pg_stat_reset()</function></literal><indexterm><primary>pg_stat_reset</primary></indexterm></entry>
<entry><type>void</type></entry>
<entry>
Reset all statistics counters for the current database to zero
(requires superuser privileges by default, but EXECUTE for this
function can be granted to others.)
</entry>
</row>
<row>
<entry><literal><function>pg_stat_reset_shared</function>(text)</literal><indexterm><primary>pg_stat_reset_shared</primary></indexterm></entry>
<entry><type>void</type></entry>
<entry>
Reset some cluster-wide statistics counters to zero, depending on the
argument (requires superuser privileges by default, but EXECUTE for
this function can be granted to others).
Calling <literal>pg_stat_reset_shared('bgwriter')</literal> will zero all the
counters shown in the <structname>pg_stat_bgwriter</structname> view.
Calling <literal>pg_stat_reset_shared('archiver')</literal> will zero all the
counters shown in the <structname>pg_stat_archiver</structname> view.
</entry>
</row>
<row>
<entry><literal><function>pg_stat_reset_single_table_counters</function>(oid)</literal><indexterm><primary>pg_stat_reset_single_table_counters</primary></indexterm></entry>
<entry><type>void</type></entry>
<entry>
Reset statistics for a single table or index in the current database to
zero (requires superuser privileges by default, but EXECUTE for this
function can be granted to others)
</entry>
</row>
<row>
<entry><literal><function>pg_stat_reset_single_function_counters</function>(oid)</literal><indexterm><primary>pg_stat_reset_single_function_counters</primary></indexterm></entry>
<entry><type>void</type></entry>
<entry>
Reset statistics for a single function in the current database to
zero (requires superuser privileges by default, but EXECUTE for this
function can be granted to others)
</entry>
</row>
</tbody>
</tgroup>
</table>
<para>
<function>pg_stat_get_activity</function>, the underlying function of
the <structname>pg_stat_activity</structname> view, returns a set of records
containing all the available information about each backend process.
Sometimes it may be more convenient to obtain just a subset of this
information. In such cases, an older set of per-backend statistics
access functions can be used; these are shown in <xref
linkend="monitoring-stats-backend-funcs-table"/>.
These access functions use a backend ID number, which ranges from one
to the number of currently active backends.
The function <function>pg_stat_get_backend_idset</function> provides a
convenient way to generate one row for each active backend for
invoking these functions. For example, to show the <acronym>PID</acronym>s and
current queries of all backends:
<programlisting>
SELECT pg_stat_get_backend_pid(s.backendid) AS pid,
pg_stat_get_backend_activity(s.backendid) AS query
FROM (SELECT pg_stat_get_backend_idset() AS backendid) AS s;
</programlisting>
</para>
<table id="monitoring-stats-backend-funcs-table">
<title>Per-Backend Statistics Functions</title>
<tgroup cols="3">
<thead>
<row>
<entry>Function</entry>
<entry>Return Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><literal><function>pg_stat_get_backend_idset()</function></literal></entry>
<entry><type>setof integer</type></entry>
<entry>Set of currently active backend ID numbers (from 1 to the
number of active backends)</entry>
</row>
<row>
<entry><literal><function>pg_stat_get_backend_activity(integer)</function></literal></entry>
<entry><type>text</type></entry>
<entry>Text of this backend's most recent query</entry>
</row>
<row>
<entry><literal><function>pg_stat_get_backend_activity_start(integer)</function></literal></entry>
<entry><type>timestamp with time zone</type></entry>
<entry>Time when the most recent query was started</entry>
</row>
<row>
<entry><literal><function>pg_stat_get_backend_client_addr(integer)</function></literal></entry>
<entry><type>inet</type></entry>
<entry>IP address of the client connected to this backend</entry>
</row>
<row>
<entry><literal><function>pg_stat_get_backend_client_port(integer)</function></literal></entry>
<entry><type>integer</type></entry>
<entry>TCP port number that the client is using for communication</entry>
</row>
<row>
<entry><literal><function>pg_stat_get_backend_dbid(integer)</function></literal></entry>
<entry><type>oid</type></entry>
<entry>OID of the database this backend is connected to</entry>
</row>
<row>
<entry><literal><function>pg_stat_get_backend_pid(integer)</function></literal></entry>
<entry><type>integer</type></entry>
<entry>Process ID of this backend</entry>
</row>
<row>
<entry><literal><function>pg_stat_get_backend_start(integer)</function></literal></entry>
<entry><type>timestamp with time zone</type></entry>
<entry>Time when this process was started</entry>
</row>
<row>
<entry><literal><function>pg_stat_get_backend_userid(integer)</function></literal></entry>
<entry><type>oid</type></entry>
<entry>OID of the user logged into this backend</entry>
</row>
<row>
<entry><literal><function>pg_stat_get_backend_wait_event_type(integer)</function></literal></entry>
<entry><type>text</type></entry>
<entry>Wait event type name if backend is currently waiting, otherwise NULL.
See <xref linkend="wait-event-table"/> for details.
</entry>
</row>
<row>
<entry><literal><function>pg_stat_get_backend_wait_event(integer)</function></literal></entry>
<entry><type>text</type></entry>
<entry>Wait event name if backend is currently waiting, otherwise NULL.
See <xref linkend="wait-event-table"/> for details.
</entry>
</row>
<row>
<entry><literal><function>pg_stat_get_backend_xact_start(integer)</function></literal></entry>
<entry><type>timestamp with time zone</type></entry>
<entry>Time when the current transaction was started</entry>
</row>
</tbody>
</tgroup>
</table>
</sect2>
</sect1>
<sect1 id="monitoring-locks">
<title>Viewing Locks</title>
<indexterm zone="monitoring-locks">
<primary>lock</primary>
<secondary>monitoring</secondary>
</indexterm>
<para>
Another useful tool for monitoring database activity is the
<structname>pg_locks</structname> system table. It allows the
database administrator to view information about the outstanding
locks in the lock manager. For example, this capability can be used
to:
<itemizedlist>
<listitem>
<para>
View all the locks currently outstanding, all the locks on
relations in a particular database, all the locks on a
particular relation, or all the locks held by a particular
<productname>PostgreSQL</productname> session.
</para>
</listitem>
<listitem>
<para>
Determine the relation in the current database with the most
ungranted locks (which might be a source of contention among
database clients).
</para>
</listitem>
<listitem>
<para>
Determine the effect of lock contention on overall database
performance, as well as the extent to which contention varies
with overall database traffic.
</para>
</listitem>
</itemizedlist>
Details of the <structname>pg_locks</structname> view appear in
<xref linkend="view-pg-locks"/>.
For more information on locking and managing concurrency with
<productname>PostgreSQL</productname>, refer to <xref linkend="mvcc"/>.
</para>
</sect1>
<sect1 id="progress-reporting">
<title>Progress Reporting</title>
<para>
<productname>PostgreSQL</productname> has the ability to report the progress of
certain commands during command execution. Currently, the only commands
which support progress reporting are <command>CREATE INDEX</command>,
<command>VACUUM</command> and
<command>CLUSTER</command>. This may be expanded in the future.
</para>
<sect2 id="create-index-progress-reporting">
<title>CREATE INDEX Progress Reporting</title>
<para>
Whenever <command>CREATE INDEX</command> or <command>REINDEX</command> is running, the
<structname>pg_stat_progress_create_index</structname> view will contain
one row for each backend that is currently creating indexes. The tables
below describe the information that will be reported and provide information
about how to interpret it.
</para>
<table id="pg-stat-progress-create-index-view" xreflabel="pg_stat_progress_create_index">
<title><structname>pg_stat_progress_create_index</structname> View</title>
<tgroup cols="3">
<thead>
<row>
<entry>Column</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><structfield>pid</structfield></entry>
<entry><type>integer</type></entry>
<entry>Process ID of backend.</entry>
</row>
<row>
<entry><structfield>datid</structfield></entry>
<entry><type>oid</type></entry>
<entry>OID of the database to which this backend is connected.</entry>
</row>
<row>
<entry><structfield>datname</structfield></entry>
<entry><type>name</type></entry>
<entry>Name of the database to which this backend is connected.</entry>
</row>
<row>
<entry><structfield>relid</structfield></entry>
<entry><type>oid</type></entry>
<entry>OID of the table on which the index is being created.</entry>
</row>
<row>
<entry><structfield>index_relid</structfield></entry>
<entry><type>oid</type></entry>
<entry>OID of the index being created or reindexed. During a
non-concurrent <command>CREATE INDEX</command>, this is 0.</entry>
</row>
<row>
<entry><structfield>command</structfield></entry>
<entry><type>text</type></entry>
<entry>
The command that is running: <literal>CREATE INDEX</literal>,
<literal>CREATE INDEX CONCURRENTLY</literal>,
<literal>REINDEX</literal>, or <literal>REINDEX CONCURRENTLY</literal>.
</entry>
</row>
<row>
<entry><structfield>phase</structfield></entry>
<entry><type>text</type></entry>
<entry>
Current processing phase of index creation. See <xref linkend='create-index-phases'/>.
</entry>
</row>
<row>
<entry><structfield>lockers_total</structfield></entry>
<entry><type>bigint</type></entry>
<entry>
Total number of lockers to wait for, when applicable.
</entry>
</row>
<row>
<entry><structfield>lockers_done</structfield></entry>
<entry><type>bigint</type></entry>
<entry>
Number of lockers already waited for.
</entry>
</row>
<row>
<entry><structfield>current_locker_pid</structfield></entry>
<entry><type>bigint</type></entry>
<entry>
Process ID of the locker currently being waited for.
</entry>
</row>
<row>
<entry><structfield>blocks_total</structfield></entry>
<entry><type>bigint</type></entry>
<entry>
Total number of blocks to be processed in the current phase.
</entry>
</row>
<row>
<entry><structfield>blocks_done</structfield></entry>
<entry><type>bigint</type></entry>
<entry>
Number of blocks already processed in the current phase.
</entry>
</row>
<row>
<entry><structfield>tuples_total</structfield></entry>
<entry><type>bigint</type></entry>
<entry>
Total number of tuples to be processed in the current phase.
</entry>
</row>
<row>
<entry><structfield>tuples_done</structfield></entry>
<entry><type>bigint</type></entry>
<entry>
Number of tuples already processed in the current phase.
</entry>
</row>
<row>
<entry><structfield>partitions_total</structfield></entry>
<entry><type>bigint</type></entry>
<entry>
When creating an index on a partitioned table, this column is set to
the total number of partitions on which the index is to be created.
</entry>
</row>
<row>
<entry><structfield>partitions_done</structfield></entry>
<entry><type>bigint</type></entry>
<entry>
When creating an index on a partitioned table, this column is set to
the number of partitions on which the index has been completed.
</entry>
</row>
</tbody>
</tgroup>
</table>
<table id="create-index-phases">
<title>CREATE INDEX Phases</title>
<tgroup cols="2">
<thead>
<row>
<entry>Phase</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><literal>initializing</literal></entry>
<entry>
<command>CREATE INDEX</command> or <command>REINDEX</command> is preparing to create the index. This
phase is expected to be very brief.
</entry>
</row>
<row>
<entry><literal>waiting for writers before build</literal></entry>
<entry>
<command>CREATE INDEX CONCURRENTLY</command> or <command>REINDEX CONCURRENTLY</command> is waiting for transactions
with write locks that can potentially see the table to finish.
This phase is skipped when not in concurrent mode.
Columns <structname>lockers_total</structname>, <structname>lockers_done</structname>
and <structname>current_locker_pid</structname> contain the progress
information for this phase.
</entry>
</row>
<row>
<entry><literal>building index</literal></entry>
<entry>
The index is being built by the access method-specific code. In this phase,
access methods that support progress reporting fill in their own progress data,
and the subphase is indicated in this column. Typically,
<structname>blocks_total</structname> and <structname>blocks_done</structname>
will contain progress data, as well as potentially
<structname>tuples_total</structname> and <structname>tuples_done</structname>.
</entry>
</row>
<row>
<entry><literal>waiting for writers before validation</literal></entry>
<entry>
<command>CREATE INDEX CONCURRENTLY</command> or <command>REINDEX CONCURRENTLY</command> is waiting for transactions
with write locks that can potentially write into the table to finish.
This phase is skipped when not in concurrent mode.
Columns <structname>lockers_total</structname>, <structname>lockers_done</structname>
and <structname>current_locker_pid</structname> contain the progress
information for this phase.
</entry>
</row>
<row>
<entry><literal>index validation: scanning index</literal></entry>
<entry>
<command>CREATE INDEX CONCURRENTLY</command> is scanning the index searching
for tuples that need to be validated.
This phase is skipped when not in concurrent mode.
Columns <structname>blocks_total</structname> (set to the total size of the index)
and <structname>blocks_done</structname> contain the progress information for this phase.
</entry>
</row>
<row>
<entry><literal>index validation: sorting tuples</literal></entry>
<entry>
<command>CREATE INDEX CONCURRENTLY</command> is sorting the output of the
index scanning phase.
</entry>
</row>
<row>
<entry><literal>index validation: scanning table</literal></entry>
<entry>
<command>CREATE INDEX CONCURRENTLY</command> is scanning the table
to validate the index tuples collected in the previous two phases.
This phase is skipped when not in concurrent mode.
Columns <structname>blocks_total</structname> (set to the total size of the table)
and <structname>blocks_done</structname> contain the progress information for this phase.
</entry>
</row>
<row>
<entry><literal>waiting for old snapshots</literal></entry>
<entry>
<command>CREATE INDEX CONCURRENTLY</command> or <command>REINDEX CONCURRENTLY</command> is waiting for transactions
that can potentially see the table to release their snapshots. This
phase is skipped when not in concurrent mode.
Columns <structname>lockers_total</structname>, <structname>lockers_done</structname>
and <structname>current_locker_pid</structname> contain the progress
information for this phase.
</entry>
</row>
<row>
<entry><literal>waiting for readers before marking dead</literal></entry>
<entry>
<command>REINDEX CONCURRENTLY</command> is waiting for transactions
with read locks on the table to finish, before marking the old index dead.
This phase is skipped when not in concurrent mode.
Columns <structname>lockers_total</structname>, <structname>lockers_done</structname>
and <structname>current_locker_pid</structname> contain the progress
information for this phase.
</entry>
</row>
<row>
<entry><literal>waiting for readers before dropping</literal></entry>
<entry>
<command>REINDEX CONCURRENTLY</command> is waiting for transactions
with read locks on the table to finish, before dropping the old index.
This phase is skipped when not in concurrent mode.
Columns <structname>lockers_total</structname>, <structname>lockers_done</structname>
and <structname>current_locker_pid</structname> contain the progress
information for this phase.
</entry>
</row>
</tbody>
</tgroup>
</table>
</sect2>
<sect2 id="vacuum-progress-reporting">
<title>VACUUM Progress Reporting</title>
<para>
Whenever <command>VACUUM</command> is running, the
<structname>pg_stat_progress_vacuum</structname> view will contain
one row for each backend (including autovacuum worker processes) that is
currently vacuuming. The tables below describe the information
that will be reported and provide information about how to interpret it.
Progress for <command>VACUUM FULL</command> commands is reported via
<structname>pg_stat_progress_cluster</structname>
because both <command>VACUUM FULL</command> and <command>CLUSTER</command>
rewrite the table, while regular <command>VACUUM</command> only modifies it
in place. See <xref linkend='cluster-progress-reporting'/>.
</para>
<table id="pg-stat-progress-vacuum-view" xreflabel="pg_stat_progress_vacuum">
<title><structname>pg_stat_progress_vacuum</structname> View</title>
<tgroup cols="3">
<thead>
<row>
<entry>Column</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><structfield>pid</structfield></entry>
<entry><type>integer</type></entry>
<entry>Process ID of backend.</entry>
</row>
<row>
<entry><structfield>datid</structfield></entry>
<entry><type>oid</type></entry>
<entry>OID of the database to which this backend is connected.</entry>
</row>
<row>
<entry><structfield>datname</structfield></entry>
<entry><type>name</type></entry>
<entry>Name of the database to which this backend is connected.</entry>
</row>
<row>
<entry><structfield>relid</structfield></entry>
<entry><type>oid</type></entry>
<entry>OID of the table being vacuumed.</entry>
</row>
<row>
<entry><structfield>phase</structfield></entry>
<entry><type>text</type></entry>
<entry>
Current processing phase of vacuum. See <xref linkend='vacuum-phases'/>.
</entry>
</row>
<row>
<entry><structfield>heap_blks_total</structfield></entry>
<entry><type>bigint</type></entry>
<entry>
Total number of heap blocks in the table. This number is reported
as of the beginning of the scan; blocks added later will not be (and
need not be) visited by this <command>VACUUM</command>.
</entry>
</row>
<row>
<entry><structfield>heap_blks_scanned</structfield></entry>
<entry><type>bigint</type></entry>
<entry>
Number of heap blocks scanned. Because the
<link linkend="storage-vm">visibility map</link> is used to optimize scans,
some blocks will be skipped without inspection; skipped blocks are
included in this total, so that this number will eventually become
equal to <structfield>heap_blks_total</structfield> when the vacuum is complete.
This counter only advances when the phase is <literal>scanning heap</literal>.
</entry>
</row>
<row>
<entry><structfield>heap_blks_vacuumed</structfield></entry>
<entry><type>bigint</type></entry>
<entry>
Number of heap blocks vacuumed. Unless the table has no indexes, this
counter only advances when the phase is <literal>vacuuming heap</literal>.
Blocks that contain no dead tuples are skipped, so the counter may
sometimes skip forward in large increments.
</entry>
</row>
<row>
<entry><structfield>index_vacuum_count</structfield></entry>
<entry><type>bigint</type></entry>
<entry>
Number of completed index vacuum cycles.
</entry>
</row>
<row>
<entry><structfield>max_dead_tuples</structfield></entry>
<entry><type>bigint</type></entry>
<entry>
Number of dead tuples that we can store before needing to perform
an index vacuum cycle, based on
<xref linkend="guc-maintenance-work-mem"/>.
</entry>
</row>
<row>
<entry><structfield>num_dead_tuples</structfield></entry>
<entry><type>bigint</type></entry>
<entry>
Number of dead tuples collected since the last index vacuum cycle.
</entry>
</row>
</tbody>
</tgroup>
</table>
<table id="vacuum-phases">
<title>VACUUM Phases</title>
<tgroup cols="2">
<thead>
<row>
<entry>Phase</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><literal>initializing</literal></entry>
<entry>
<command>VACUUM</command> is preparing to begin scanning the heap. This
phase is expected to be very brief.
</entry>
</row>
<row>
<entry><literal>scanning heap</literal></entry>
<entry>
<command>VACUUM</command> is currently scanning the heap. It will prune and
defragment each page if required, and possibly perform freezing
activity. The <structfield>heap_blks_scanned</structfield> column can be used
to monitor the progress of the scan.
</entry>
</row>
<row>
<entry><literal>vacuuming indexes</literal></entry>
<entry>
<command>VACUUM</command> is currently vacuuming the indexes. If a table has
any indexes, this will happen at least once per vacuum, after the heap
has been completely scanned. It may happen multiple times per vacuum
if <xref linkend="guc-maintenance-work-mem"/> is insufficient to
store the number of dead tuples found.
</entry>
</row>
<row>
<entry><literal>vacuuming heap</literal></entry>
<entry>
<command>VACUUM</command> is currently vacuuming the heap. Vacuuming the heap
is distinct from scanning the heap, and occurs after each instance of
vacuuming indexes. If <structfield>heap_blks_scanned</structfield> is less than
<structfield>heap_blks_total</structfield>, the system will return to scanning
the heap after this phase is completed; otherwise, it will begin
cleaning up indexes after this phase is completed.
</entry>
</row>
<row>
<entry><literal>cleaning up indexes</literal></entry>
<entry>
<command>VACUUM</command> is currently cleaning up indexes. This occurs after
the heap has been completely scanned and all vacuuming of the indexes
and the heap has been completed.
</entry>
</row>
<row>
<entry><literal>truncating heap</literal></entry>
<entry>
<command>VACUUM</command> is currently truncating the heap so as to return
empty pages at the end of the relation to the operating system. This
occurs after cleaning up indexes.
</entry>
</row>
<row>
<entry><literal>performing final cleanup</literal></entry>
<entry>
<command>VACUUM</command> is performing final cleanup. During this phase,
<command>VACUUM</command> will vacuum the free space map, update statistics
in <literal>pg_class</literal>, and report statistics to the statistics
collector. When this phase is completed, <command>VACUUM</command> will end.
</entry>
</row>
</tbody>
</tgroup>
</table>
</sect2>
<sect2 id="cluster-progress-reporting">
<title>CLUSTER Progress Reporting</title>
<para>
Whenever <command>CLUSTER</command> or <command>VACUUM FULL</command> is
running, the <structname>pg_stat_progress_cluster</structname> view will
contain a row for each backend that is currently running either command.
The tables below describe the information that will be reported and
provide information about how to interpret it.
</para>
<table id="pg-stat-progress-cluster-view" xreflabel="pg_stat_progress_cluster">
<title><structname>pg_stat_progress_cluster</structname> View</title>
<tgroup cols="3">
<thead>
<row>
<entry>Column</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><structfield>pid</structfield></entry>
<entry><type>integer</type></entry>
<entry>Process ID of backend.</entry>
</row>
<row>
<entry><structfield>datid</structfield></entry>
<entry><type>oid</type></entry>
<entry>OID of the database to which this backend is connected.</entry>
</row>
<row>
<entry><structfield>datname</structfield></entry>
<entry><type>name</type></entry>
<entry>Name of the database to which this backend is connected.</entry>
</row>
<row>
<entry><structfield>relid</structfield></entry>
<entry><type>oid</type></entry>
<entry>OID of the table being clustered.</entry>
</row>
<row>
<entry><structfield>command</structfield></entry>
<entry><type>text</type></entry>
<entry>
The command that is running. Either <literal>CLUSTER</literal> or <literal>VACUUM FULL</literal>.
</entry>
</row>
<row>
<entry><structfield>phase</structfield></entry>
<entry><type>text</type></entry>
<entry>
Current processing phase. See <xref linkend='cluster-phases' />.
</entry>
</row>
<row>
<entry><structfield>cluster_index_relid</structfield></entry>
<entry><type>oid</type></entry>
<entry>
If the table is being scanned using an index, this is the OID of the
index being used; otherwise, it is zero.
</entry>
</row>
<row>
<entry><structfield>heap_tuples_scanned</structfield></entry>
<entry><type>bigint</type></entry>
<entry>
Number of heap tuples scanned.
This counter only advances when the phase is
<literal>seq scanning heap</literal>,
<literal>index scanning heap</literal>
or <literal>writing new heap</literal>.
</entry>
</row>
<row>
<entry><structfield>heap_tuples_written</structfield></entry>
<entry><type>bigint</type></entry>
<entry>
Number of heap tuples written.
This counter only advances when the phase is
<literal>seq scanning heap</literal>,
<literal>index scanning heap</literal>
or <literal>writing new heap</literal>.
</entry>
</row>
<row>
<entry><structfield>heap_blks_total</structfield></entry>
<entry><type>bigint</type></entry>
<entry>
Total number of heap blocks in the table. This number is reported
as of the beginning of <literal>seq scanning heap</literal>.
</entry>
</row>
<row>
<entry><structfield>heap_blks_scanned</structfield></entry>
<entry><type>bigint</type></entry>
<entry>
Number of heap blocks scanned. This counter only advances when the
phase is <literal>seq scanning heap</literal>.
</entry>
</row>
<row>
<entry><structfield>index_rebuild_count</structfield></entry>
<entry><type>bigint</type></entry>
<entry>
Number of indexes rebuilt. This counter only advances when the phase
is <literal>rebuilding index</literal>.
</entry>
</row>
</tbody>
</tgroup>
</table>
<table id="cluster-phases">
<title>CLUSTER and VACUUM FULL Phases</title>
<tgroup cols="2">
<thead>
<row>
<entry>Phase</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><literal>initializing</literal></entry>
<entry>
The command is preparing to begin scanning the heap. This phase is
expected to be very brief.
</entry>
</row>
<row>
<entry><literal>seq scanning heap</literal></entry>
<entry>
The command is currently scanning the table using a sequential scan.
</entry>
</row>
<row>
<entry><literal>index scanning heap</literal></entry>
<entry>
<command>CLUSTER</command> is currently scanning the table using an index scan.
</entry>
</row>
<row>
<entry><literal>sorting tuples</literal></entry>
<entry>
<command>CLUSTER</command> is currently sorting tuples.
</entry>
</row>
<row>
<entry><literal>writing new heap</literal></entry>
<entry>
<command>CLUSTER</command> is currently writing the new heap.
</entry>
</row>
<row>
<entry><literal>swapping relation files</literal></entry>
<entry>
The command is currently swapping newly-built files into place.
</entry>
</row>
<row>
<entry><literal>rebuilding index</literal></entry>
<entry>
The command is currently rebuilding an index.
</entry>
</row>
<row>
<entry><literal>performing final cleanup</literal></entry>
<entry>
The command is performing final cleanup. When this phase is
completed, <command>CLUSTER</command>
or <command>VACUUM FULL</command> will end.
</entry>
</row>
</tbody>
</tgroup>
</table>
</sect2>
</sect1>
<sect1 id="dynamic-trace">
<title>Dynamic Tracing</title>
<indexterm zone="dynamic-trace">
<primary>DTrace</primary>
</indexterm>
<para>
<productname>PostgreSQL</productname> provides facilities to support
dynamic tracing of the database server. This allows an external
utility to be called at specific points in the code and thereby trace
execution.
</para>
<para>
A number of probes or trace points are already inserted into the source
code. These probes are intended to be used by database developers and
administrators. By default the probes are not compiled into
<productname>PostgreSQL</productname>; the user needs to explicitly tell
the configure script to make the probes available.
</para>
<para>
Currently, the
<ulink url="https://en.wikipedia.org/wiki/DTrace">DTrace</ulink>
utility is supported, which, at the time of this writing, is available
on Solaris, macOS, FreeBSD, NetBSD, and Oracle Linux. The
<ulink url="http://sourceware.org/systemtap/">SystemTap</ulink> project
for Linux provides a DTrace equivalent and can also be used. Supporting other dynamic
tracing utilities is theoretically possible by changing the definitions for
the macros in <filename>src/include/utils/probes.h</filename>.
</para>
<sect2 id="compiling-for-trace">
<title>Compiling for Dynamic Tracing</title>
<para>
By default, probes are not available, so you will need to
explicitly tell the configure script to make the probes available
in <productname>PostgreSQL</productname>. To include DTrace support
specify <option>--enable-dtrace</option> to configure. See <xref
linkend="install-procedure"/> for further information.
</para>
</sect2>
<sect2 id="trace-points">
<title>Built-in Probes</title>
<para>
A number of standard probes are provided in the source code,
as shown in <xref linkend="dtrace-probe-point-table"/>;
<xref linkend="typedefs-table"/>
shows the types used in the probes. More probes can certainly be
added to enhance <productname>PostgreSQL</productname>'s observability.
</para>
<table id="dtrace-probe-point-table">
<title>Built-in DTrace Probes</title>
<tgroup cols="3">
<thead>
<row>
<entry>Name</entry>
<entry>Parameters</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><literal>transaction-start</literal></entry>
<entry><literal>(LocalTransactionId)</literal></entry>
<entry>Probe that fires at the start of a new transaction.
arg0 is the transaction ID.</entry>
</row>
<row>
<entry><literal>transaction-commit</literal></entry>
<entry><literal>(LocalTransactionId)</literal></entry>
<entry>Probe that fires when a transaction completes successfully.
arg0 is the transaction ID.</entry>
</row>
<row>
<entry><literal>transaction-abort</literal></entry>
<entry><literal>(LocalTransactionId)</literal></entry>
<entry>Probe that fires when a transaction completes unsuccessfully.
arg0 is the transaction ID.</entry>
</row>
<row>
<entry><literal>query-start</literal></entry>
<entry><literal>(const char *)</literal></entry>
<entry>Probe that fires when the processing of a query is started.
arg0 is the query string.</entry>
</row>
<row>
<entry><literal>query-done</literal></entry>
<entry><literal>(const char *)</literal></entry>
<entry>Probe that fires when the processing of a query is complete.
arg0 is the query string.</entry>
</row>
<row>
<entry><literal>query-parse-start</literal></entry>
<entry><literal>(const char *)</literal></entry>
<entry>Probe that fires when the parsing of a query is started.
arg0 is the query string.</entry>
</row>
<row>
<entry><literal>query-parse-done</literal></entry>
<entry><literal>(const char *)</literal></entry>
<entry>Probe that fires when the parsing of a query is complete.
arg0 is the query string.</entry>
</row>
<row>
<entry><literal>query-rewrite-start</literal></entry>
<entry><literal>(const char *)</literal></entry>
<entry>Probe that fires when the rewriting of a query is started.
arg0 is the query string.</entry>
</row>
<row>
<entry><literal>query-rewrite-done</literal></entry>
<entry><literal>(const char *)</literal></entry>
<entry>Probe that fires when the rewriting of a query is complete.
arg0 is the query string.</entry>
</row>
<row>
<entry><literal>query-plan-start</literal></entry>
<entry><literal>()</literal></entry>
<entry>Probe that fires when the planning of a query is started.</entry>
</row>
<row>
<entry><literal>query-plan-done</literal></entry>
<entry><literal>()</literal></entry>
<entry>Probe that fires when the planning of a query is complete.</entry>
</row>
<row>
<entry><literal>query-execute-start</literal></entry>
<entry><literal>()</literal></entry>
<entry>Probe that fires when the execution of a query is started.</entry>
</row>
<row>
<entry><literal>query-execute-done</literal></entry>
<entry><literal>()</literal></entry>
<entry>Probe that fires when the execution of a query is complete.</entry>
</row>
<row>
<entry><literal>statement-status</literal></entry>
<entry><literal>(const char *)</literal></entry>
<entry>Probe that fires anytime the server process updates its
<structname>pg_stat_activity</structname>.<structfield>status</structfield>.
arg0 is the new status string.</entry>
</row>
<row>
<entry><literal>checkpoint-start</literal></entry>
<entry><literal>(int)</literal></entry>
<entry>Probe that fires when a checkpoint is started.
arg0 holds the bitwise flags used to distinguish different checkpoint
types, such as shutdown, immediate or force.</entry>
</row>
<row>
<entry><literal>checkpoint-done</literal></entry>
<entry><literal>(int, int, int, int, int)</literal></entry>
<entry>Probe that fires when a checkpoint is complete.
(The probes listed next fire in sequence during checkpoint processing.)
arg0 is the number of buffers written. arg1 is the total number of
buffers. arg2, arg3 and arg4 contain the number of WAL files added,
removed and recycled respectively.</entry>
</row>
<row>
<entry><literal>clog-checkpoint-start</literal></entry>
<entry><literal>(bool)</literal></entry>
<entry>Probe that fires when the CLOG portion of a checkpoint is started.
arg0 is true for normal checkpoint, false for shutdown
checkpoint.</entry>
</row>
<row>
<entry><literal>clog-checkpoint-done</literal></entry>
<entry><literal>(bool)</literal></entry>
<entry>Probe that fires when the CLOG portion of a checkpoint is
complete. arg0 has the same meaning as for <literal>clog-checkpoint-start</literal>.</entry>
</row>
<row>
<entry><literal>subtrans-checkpoint-start</literal></entry>
<entry><literal>(bool)</literal></entry>
<entry>Probe that fires when the SUBTRANS portion of a checkpoint is
started.
arg0 is true for normal checkpoint, false for shutdown
checkpoint.</entry>
</row>
<row>
<entry><literal>subtrans-checkpoint-done</literal></entry>
<entry><literal>(bool)</literal></entry>
<entry>Probe that fires when the SUBTRANS portion of a checkpoint is
complete. arg0 has the same meaning as for
<literal>subtrans-checkpoint-start</literal>.</entry>
</row>
<row>
<entry><literal>multixact-checkpoint-start</literal></entry>
<entry><literal>(bool)</literal></entry>
<entry>Probe that fires when the MultiXact portion of a checkpoint is
started.
arg0 is true for normal checkpoint, false for shutdown
checkpoint.</entry>
</row>
<row>
<entry><literal>multixact-checkpoint-done</literal></entry>
<entry><literal>(bool)</literal></entry>
<entry>Probe that fires when the MultiXact portion of a checkpoint is
complete. arg0 has the same meaning as for
<literal>multixact-checkpoint-start</literal>.</entry>
</row>
<row>
<entry><literal>buffer-checkpoint-start</literal></entry>
<entry><literal>(int)</literal></entry>
<entry>Probe that fires when the buffer-writing portion of a checkpoint
is started.
arg0 holds the bitwise flags used to distinguish different checkpoint
types, such as shutdown, immediate or force.</entry>
</row>
<row>
<entry><literal>buffer-sync-start</literal></entry>
<entry><literal>(int, int)</literal></entry>
<entry>Probe that fires when we begin to write dirty buffers during
checkpoint (after identifying which buffers must be written).
arg0 is the total number of buffers.
arg1 is the number that are currently dirty and need to be written.</entry>
</row>
<row>
<entry><literal>buffer-sync-written</literal></entry>
<entry><literal>(int)</literal></entry>
<entry>Probe that fires after each buffer is written during checkpoint.
arg0 is the ID number of the buffer.</entry>
</row>
<row>
<entry><literal>buffer-sync-done</literal></entry>
<entry><literal>(int, int, int)</literal></entry>
<entry>Probe that fires when all dirty buffers have been written.
arg0 is the total number of buffers.
arg1 is the number of buffers actually written by the checkpoint process.
arg2 is the number that were expected to be written (arg1 of
<literal>buffer-sync-start</literal>); any difference reflects other processes flushing
buffers during the checkpoint.</entry>
</row>
<row>
<entry><literal>buffer-checkpoint-sync-start</literal></entry>
<entry><literal>()</literal></entry>
<entry>Probe that fires after dirty buffers have been written to the
kernel, and before starting to issue fsync requests.</entry>
</row>
<row>
<entry><literal>buffer-checkpoint-done</literal></entry>
<entry><literal>()</literal></entry>
<entry>Probe that fires when syncing of buffers to disk is
complete.</entry>
</row>
<row>
<entry><literal>twophase-checkpoint-start</literal></entry>
<entry><literal>()</literal></entry>
<entry>Probe that fires when the two-phase portion of a checkpoint is
started.</entry>
</row>
<row>
<entry><literal>twophase-checkpoint-done</literal></entry>
<entry><literal>()</literal></entry>
<entry>Probe that fires when the two-phase portion of a checkpoint is
complete.</entry>
</row>
<row>
<entry><literal>buffer-read-start</literal></entry>
<entry><literal>(ForkNumber, BlockNumber, Oid, Oid, Oid, int, bool)</literal></entry>
<entry>Probe that fires when a buffer read is started.
arg0 and arg1 contain the fork and block numbers of the page (but
arg1 will be -1 if this is a relation extension request).
arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs
identifying the relation.
arg5 is the ID of the backend which created the temporary relation for a
local buffer, or <symbol>InvalidBackendId</symbol> (-1) for a shared buffer.
arg6 is true for a relation extension request, false for normal
read.</entry>
</row>
<row>
<entry><literal>buffer-read-done</literal></entry>
<entry><literal>(ForkNumber, BlockNumber, Oid, Oid, Oid, int, bool, bool)</literal></entry>
<entry>Probe that fires when a buffer read is complete.
arg0 and arg1 contain the fork and block numbers of the page (if this
is a relation extension request, arg1 now contains the block number
of the newly added block).
arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs
identifying the relation.
arg5 is the ID of the backend which created the temporary relation for a
local buffer, or <symbol>InvalidBackendId</symbol> (-1) for a shared buffer.
arg6 is true for a relation extension request, false for normal
read.
arg7 is true if the buffer was found in the pool, false if not.</entry>
</row>
<row>
<entry><literal>buffer-flush-start</literal></entry>
<entry><literal>(ForkNumber, BlockNumber, Oid, Oid, Oid)</literal></entry>
<entry>Probe that fires before issuing any write request for a shared
buffer.
arg0 and arg1 contain the fork and block numbers of the page.
arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs
identifying the relation.</entry>
</row>
<row>
<entry><literal>buffer-flush-done</literal></entry>
<entry><literal>(ForkNumber, BlockNumber, Oid, Oid, Oid)</literal></entry>
<entry>Probe that fires when a write request is complete. (Note
that this just reflects the time to pass the data to the kernel;
it's typically not actually been written to disk yet.)
The arguments are the same as for <literal>buffer-flush-start</literal>.</entry>
</row>
<row>
<entry><literal>buffer-write-dirty-start</literal></entry>
<entry><literal>(ForkNumber, BlockNumber, Oid, Oid, Oid)</literal></entry>
<entry>Probe that fires when a server process begins to write a dirty
buffer. (If this happens often, it implies that
<xref linkend="guc-shared-buffers"/> is too
small or the background writer control parameters need adjustment.)
arg0 and arg1 contain the fork and block numbers of the page.
arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs
identifying the relation.</entry>
</row>
<row>
<entry><literal>buffer-write-dirty-done</literal></entry>
<entry><literal>(ForkNumber, BlockNumber, Oid, Oid, Oid)</literal></entry>
<entry>Probe that fires when a dirty-buffer write is complete.
The arguments are the same as for <literal>buffer-write-dirty-start</literal>.</entry>
</row>
<row>
<entry><literal>wal-buffer-write-dirty-start</literal></entry>
<entry><literal>()</literal></entry>
<entry>Probe that fires when a server process begins to write a
dirty WAL buffer because no more WAL buffer space is available.
(If this happens often, it implies that
<xref linkend="guc-wal-buffers"/> is too small.)</entry>
</row>
<row>
<entry><literal>wal-buffer-write-dirty-done</literal></entry>
<entry><literal>()</literal></entry>
<entry>Probe that fires when a dirty WAL buffer write is complete.</entry>
</row>
<row>
<entry><literal>wal-insert</literal></entry>
<entry><literal>(unsigned char, unsigned char)</literal></entry>
<entry>Probe that fires when a WAL record is inserted.
arg0 is the resource manager (rmid) for the record.
arg1 contains the info flags.</entry>
</row>
<row>
<entry><literal>wal-switch</literal></entry>
<entry><literal>()</literal></entry>
<entry>Probe that fires when a WAL segment switch is requested.</entry>
</row>
<row>
<entry><literal>smgr-md-read-start</literal></entry>
<entry><literal>(ForkNumber, BlockNumber, Oid, Oid, Oid, int)</literal></entry>
<entry>Probe that fires when beginning to read a block from a relation.
arg0 and arg1 contain the fork and block numbers of the page.
arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs
identifying the relation.
arg5 is the ID of the backend which created the temporary relation for a
local buffer, or <symbol>InvalidBackendId</symbol> (-1) for a shared buffer.</entry>
</row>
<row>
<entry><literal>smgr-md-read-done</literal></entry>
<entry><literal>(ForkNumber, BlockNumber, Oid, Oid, Oid, int, int, int)</literal></entry>
<entry>Probe that fires when a block read is complete.
arg0 and arg1 contain the fork and block numbers of the page.
arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs
identifying the relation.
arg5 is the ID of the backend which created the temporary relation for a
local buffer, or <symbol>InvalidBackendId</symbol> (-1) for a shared buffer.
arg6 is the number of bytes actually read, while arg7 is the number
requested (if these are different it indicates trouble).</entry>
</row>
<row>
<entry><literal>smgr-md-write-start</literal></entry>
<entry><literal>(ForkNumber, BlockNumber, Oid, Oid, Oid, int)</literal></entry>
<entry>Probe that fires when beginning to write a block to a relation.
arg0 and arg1 contain the fork and block numbers of the page.
arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs
identifying the relation.
arg5 is the ID of the backend which created the temporary relation for a
local buffer, or <symbol>InvalidBackendId</symbol> (-1) for a shared buffer.</entry>
</row>
<row>
<entry><literal>smgr-md-write-done</literal></entry>
<entry><literal>(ForkNumber, BlockNumber, Oid, Oid, Oid, int, int, int)</literal></entry>
<entry>Probe that fires when a block write is complete.
arg0 and arg1 contain the fork and block numbers of the page.
arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs
identifying the relation.
arg5 is the ID of the backend which created the temporary relation for a
local buffer, or <symbol>InvalidBackendId</symbol> (-1) for a shared buffer.
arg6 is the number of bytes actually written, while arg7 is the number
requested (if these are different it indicates trouble).</entry>
</row>
<row>
<entry><literal>sort-start</literal></entry>
<entry><literal>(int, bool, int, int, bool, int)</literal></entry>
<entry>Probe that fires when a sort operation is started.
arg0 indicates heap, index or datum sort.
arg1 is true for unique-value enforcement.
arg2 is the number of key columns.
arg3 is the number of kilobytes of work memory allowed.
arg4 is true if random access to the sort result is required.
arg5 indicates serial when <literal>0</literal>, parallel worker when
<literal>1</literal>, or parallel leader when <literal>2</literal>.</entry>
</row>
<row>
<entry><literal>sort-done</literal></entry>
<entry><literal>(bool, long)</literal></entry>
<entry>Probe that fires when a sort is complete.
arg0 is true for external sort, false for internal sort.
arg1 is the number of disk blocks used for an external sort,
or kilobytes of memory used for an internal sort.</entry>
</row>
<row>
<entry><literal>lwlock-acquire</literal></entry>
<entry><literal>(char *, LWLockMode)</literal></entry>
<entry>Probe that fires when an LWLock has been acquired.
arg0 is the LWLock's tranche.
arg1 is the requested lock mode, either exclusive or shared.</entry>
</row>
<row>
<entry><literal>lwlock-release</literal></entry>
<entry><literal>(char *)</literal></entry>
<entry>Probe that fires when an LWLock has been released (but note
that any released waiters have not yet been awakened).
arg0 is the LWLock's tranche.</entry>
</row>
<row>
<entry><literal>lwlock-wait-start</literal></entry>
<entry><literal>(char *, LWLockMode)</literal></entry>
<entry>Probe that fires when an LWLock was not immediately available and
a server process has begun to wait for the lock to become available.
arg0 is the LWLock's tranche.
arg1 is the requested lock mode, either exclusive or shared.</entry>
</row>
<row>
<entry><literal>lwlock-wait-done</literal></entry>
<entry><literal>(char *, LWLockMode)</literal></entry>
<entry>Probe that fires when a server process has been released from its
wait for an LWLock (it does not actually have the lock yet).
arg0 is the LWLock's tranche.
arg1 is the requested lock mode, either exclusive or shared.</entry>
</row>
<row>
<entry><literal>lwlock-condacquire</literal></entry>
<entry><literal>(char *, LWLockMode)</literal></entry>
<entry>Probe that fires when an LWLock was successfully acquired when the
caller specified no waiting.
arg0 is the LWLock's tranche.
arg1 is the requested lock mode, either exclusive or shared.</entry>
</row>
<row>
<entry><literal>lwlock-condacquire-fail</literal></entry>
<entry><literal>(char *, LWLockMode)</literal></entry>
<entry>Probe that fires when an LWLock was not successfully acquired when
the caller specified no waiting.
arg0 is the LWLock's tranche.
arg1 is the requested lock mode, either exclusive or shared.</entry>
</row>
<row>
<entry><literal>lock-wait-start</literal></entry>
<entry><literal>(unsigned int, unsigned int, unsigned int, unsigned int, unsigned int, LOCKMODE)</literal></entry>
<entry>Probe that fires when a request for a heavyweight lock (lmgr lock)
has begun to wait because the lock is not available.
arg0 through arg3 are the tag fields identifying the object being
locked. arg4 indicates the type of object being locked.
arg5 indicates the lock type being requested.</entry>
</row>
<row>
<entry><literal>lock-wait-done</literal></entry>
<entry><literal>(unsigned int, unsigned int, unsigned int, unsigned int, unsigned int, LOCKMODE)</literal></entry>
<entry>Probe that fires when a request for a heavyweight lock (lmgr lock)
has finished waiting (i.e., has acquired the lock).
The arguments are the same as for <literal>lock-wait-start</literal>.</entry>
</row>
<row>
<entry><literal>deadlock-found</literal></entry>
<entry><literal>()</literal></entry>
<entry>Probe that fires when a deadlock is found by the deadlock
detector.</entry>
</row>
</tbody>
</tgroup>
</table>
<table id="typedefs-table">
<title>Defined Types Used in Probe Parameters</title>
<tgroup cols="2">
<thead>
<row>
<entry>Type</entry>
<entry>Definition</entry>
</row>
</thead>
<tbody>
<row>
<entry><type>LocalTransactionId</type></entry>
<entry><type>unsigned int</type></entry>
</row>
<row>
<entry><type>LWLockMode</type></entry>
<entry><type>int</type></entry>
</row>
<row>
<entry><type>LOCKMODE</type></entry>
<entry><type>int</type></entry>
</row>
<row>
<entry><type>BlockNumber</type></entry>
<entry><type>unsigned int</type></entry>
</row>
<row>
<entry><type>Oid</type></entry>
<entry><type>unsigned int</type></entry>
</row>
<row>
<entry><type>ForkNumber</type></entry>
<entry><type>int</type></entry>
</row>
<row>
<entry><type>bool</type></entry>
<entry><type>unsigned char</type></entry>
</row>
</tbody>
</tgroup>
</table>
</sect2>
<sect2 id="using-trace-points">
<title>Using Probes</title>
<para>
The example below shows a DTrace script for analyzing transaction
counts in the system, as an alternative to snapshotting
<structname>pg_stat_database</structname> before and after a performance test:
<programlisting>
#!/usr/sbin/dtrace -qs
postgresql$1:::transaction-start
{
@start["Start"] = count();
self->ts = timestamp;
}
postgresql$1:::transaction-abort
{
@abort["Abort"] = count();
}
postgresql$1:::transaction-commit
/self->ts/
{
@commit["Commit"] = count();
@time["Total time (ns)"] = sum(timestamp - self->ts);
self->ts=0;
}
</programlisting>
When executed, the example D script gives output such as:
<screen>
# ./txn_count.d `pgrep -n postgres` or ./txn_count.d &lt;PID&gt;
^C
Start 71
Commit 70
Total time (ns) 2312105013
</screen>
</para>
<note>
<para>
SystemTap uses a different notation for trace scripts than DTrace does,
even though the underlying trace points are compatible. One point worth
noting is that at this writing, SystemTap scripts must reference probe
names using double underscores in place of hyphens. This is expected to
be fixed in future SystemTap releases.
</para>
</note>
<para>
You should remember that DTrace scripts need to be carefully written and
debugged, otherwise the trace information collected might
be meaningless. In most cases where problems are found it is the
instrumentation that is at fault, not the underlying system. When
discussing information found using dynamic tracing, be sure to enclose
the script used to allow that too to be checked and discussed.
</para>
</sect2>
<sect2 id="defining-trace-points">
<title>Defining New Probes</title>
<para>
New probes can be defined within the code wherever the developer
desires, though this will require a recompilation. Below are the steps
for inserting new probes:
</para>
<procedure>
<step>
<para>
Decide on probe names and data to be made available through the probes
</para>
</step>
<step>
<para>
Add the probe definitions to <filename>src/backend/utils/probes.d</filename>
</para>
</step>
<step>
<para>
Include <filename>pg_trace.h</filename> if it is not already present in the
module(s) containing the probe points, and insert
<literal>TRACE_POSTGRESQL</literal> probe macros at the desired locations
in the source code
</para>
</step>
<step>
<para>
Recompile and verify that the new probes are available
</para>
</step>
</procedure>
<formalpara>
<title>Example:</title>
<para>
Here is an example of how you would add a probe to trace all new
transactions by transaction ID.
</para>
</formalpara>
<procedure>
<step>
<para>
Decide that the probe will be named <literal>transaction-start</literal> and
requires a parameter of type <type>LocalTransactionId</type>
</para>
</step>
<step>
<para>
Add the probe definition to <filename>src/backend/utils/probes.d</filename>:
<programlisting>
probe transaction__start(LocalTransactionId);
</programlisting>
Note the use of the double underline in the probe name. In a DTrace
script using the probe, the double underline needs to be replaced with a
hyphen, so <literal>transaction-start</literal> is the name to document for
users.
</para>
</step>
<step>
<para>
At compile time, <literal>transaction__start</literal> is converted to a macro
called <literal>TRACE_POSTGRESQL_TRANSACTION_START</literal> (notice the
underscores are single here), which is available by including
<filename>pg_trace.h</filename>. Add the macro call to the appropriate location
in the source code. In this case, it looks like the following:
<programlisting>
TRACE_POSTGRESQL_TRANSACTION_START(vxid.localTransactionId);
</programlisting>
</para>
</step>
<step>
<para>
After recompiling and running the new binary, check that your newly added
probe is available by executing the following DTrace command. You
should see similar output:
<screen>
# dtrace -ln transaction-start
ID PROVIDER MODULE FUNCTION NAME
18705 postgresql49878 postgres StartTransactionCommand transaction-start
18755 postgresql49877 postgres StartTransactionCommand transaction-start
18805 postgresql49876 postgres StartTransactionCommand transaction-start
18855 postgresql49875 postgres StartTransactionCommand transaction-start
18986 postgresql49873 postgres StartTransactionCommand transaction-start
</screen>
</para>
</step>
</procedure>
<para>
There are a few things to be careful about when adding trace macros
to the C code:
<itemizedlist>
<listitem>
<para>
You should take care that the data types specified for a probe's
parameters match the data types of the variables used in the macro.
Otherwise, you will get compilation errors.
</para>
</listitem>
<listitem>
<para>
On most platforms, if <productname>PostgreSQL</productname> is
built with <option>--enable-dtrace</option>, the arguments to a trace
macro will be evaluated whenever control passes through the
macro, <emphasis>even if no tracing is being done</emphasis>. This is
usually not worth worrying about if you are just reporting the
values of a few local variables. But beware of putting expensive
function calls into the arguments. If you need to do that,
consider protecting the macro with a check to see if the trace
is actually enabled:
<programlisting>
if (TRACE_POSTGRESQL_TRANSACTION_START_ENABLED())
TRACE_POSTGRESQL_TRANSACTION_START(some_function(...));
</programlisting>
Each trace macro has a corresponding <literal>ENABLED</literal> macro.
</para>
</listitem>
</itemizedlist>
</para>
</sect2>
</sect1>
</chapter>
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