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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>F.29. pg_stat_statements</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="pgrowlocks.html" title="F.28. pgrowlocks" /><link rel="next" href="pgstattuple.html" title="F.30. pgstattuple" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">F.29. pg_stat_statements</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="pgrowlocks.html" title="F.28. pgrowlocks">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="contrib.html" title="Appendix F. Additional Supplied Modules">Up</a></td><th width="60%" align="center">Appendix F. Additional Supplied Modules</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 11.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="pgstattuple.html" title="F.30. pgstattuple">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="PGSTATSTATEMENTS"><div class="titlepage"><div><div><h2 class="title" style="clear: both">F.29. pg_stat_statements</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="pgstatstatements.html#id-1.11.7.38.6">F.29.1. The <code class="structname">pg_stat_statements</code> View</a></span></dt><dt><span class="sect2"><a href="pgstatstatements.html#id-1.11.7.38.7">F.29.2. Functions</a></span></dt><dt><span class="sect2"><a href="pgstatstatements.html#id-1.11.7.38.8">F.29.3. Configuration Parameters</a></span></dt><dt><span class="sect2"><a href="pgstatstatements.html#id-1.11.7.38.9">F.29.4. Sample Output</a></span></dt><dt><span class="sect2"><a href="pgstatstatements.html#id-1.11.7.38.10">F.29.5. Authors</a></span></dt></dl></div><a id="id-1.11.7.38.2" class="indexterm"></a><p>
The <code class="filename">pg_stat_statements</code> module provides a means for
tracking execution statistics of all SQL statements executed by a server.
</p><p>
The module must be loaded by adding <code class="literal">pg_stat_statements</code> to
<a class="xref" href="runtime-config-client.html#GUC-SHARED-PRELOAD-LIBRARIES">shared_preload_libraries</a> in
<code class="filename">postgresql.conf</code>, because it requires additional shared memory.
This means that a server restart is needed to add or remove the module.
</p><p>
When <code class="filename">pg_stat_statements</code> is loaded, it tracks
statistics across all databases of the server. To access and manipulate
these statistics, the module provides a view, <code class="structname">pg_stat_statements</code>,
and the utility functions <code class="function">pg_stat_statements_reset</code> and
<code class="function">pg_stat_statements</code>. These are not available globally but
can be enabled for a specific database with
<code class="command">CREATE EXTENSION pg_stat_statements</code>.
</p><div class="sect2" id="id-1.11.7.38.6"><div class="titlepage"><div><div><h3 class="title">F.29.1. The <code class="structname">pg_stat_statements</code> View</h3></div></div></div><p>
The statistics gathered by the module are made available via a
view named <code class="structname">pg_stat_statements</code>. This view
contains one row for each distinct database ID, user ID and query
ID (up to the maximum number of distinct statements that the module
can track). The columns of the view are shown in
<a class="xref" href="pgstatstatements.html#PGSTATSTATEMENTS-COLUMNS" title="Table F.21. pg_stat_statements Columns">Table F.21</a>.
</p><div class="table" id="PGSTATSTATEMENTS-COLUMNS"><p class="title"><strong>Table F.21. <code class="structname">pg_stat_statements</code> Columns</strong></p><div class="table-contents"><table class="table" summary="pg_stat_statements Columns" border="1"><colgroup><col /><col /><col /><col /></colgroup><thead><tr><th>Name</th><th>Type</th><th>References</th><th>Description</th></tr></thead><tbody><tr><td><code class="structfield">userid</code></td><td><code class="type">oid</code></td><td><code class="literal"><a class="link" href="catalog-pg-authid.html" title="52.8. pg_authid"><code class="structname">pg_authid</code></a>.oid</code></td><td>OID of user who executed the statement</td></tr><tr><td><code class="structfield">dbid</code></td><td><code class="type">oid</code></td><td><code class="literal"><a class="link" href="catalog-pg-database.html" title="52.15. pg_database"><code class="structname">pg_database</code></a>.oid</code></td><td>OID of database in which the statement was executed</td></tr><tr><td><code class="structfield">queryid</code></td><td><code class="type">bigint</code></td><td> </td><td>Internal hash code, computed from the statement's parse tree</td></tr><tr><td><code class="structfield">query</code></td><td><code class="type">text</code></td><td> </td><td>Text of a representative statement</td></tr><tr><td><code class="structfield">calls</code></td><td><code class="type">bigint</code></td><td> </td><td>Number of times executed</td></tr><tr><td><code class="structfield">total_time</code></td><td><code class="type">double precision</code></td><td> </td><td>Total time spent in the statement, in milliseconds</td></tr><tr><td><code class="structfield">min_time</code></td><td><code class="type">double precision</code></td><td> </td><td>Minimum time spent in the statement, in milliseconds</td></tr><tr><td><code class="structfield">max_time</code></td><td><code class="type">double precision</code></td><td> </td><td>Maximum time spent in the statement, in milliseconds</td></tr><tr><td><code class="structfield">mean_time</code></td><td><code class="type">double precision</code></td><td> </td><td>Mean time spent in the statement, in milliseconds</td></tr><tr><td><code class="structfield">stddev_time</code></td><td><code class="type">double precision</code></td><td> </td><td>Population standard deviation of time spent in the statement, in milliseconds</td></tr><tr><td><code class="structfield">rows</code></td><td><code class="type">bigint</code></td><td> </td><td>Total number of rows retrieved or affected by the statement</td></tr><tr><td><code class="structfield">shared_blks_hit</code></td><td><code class="type">bigint</code></td><td> </td><td>Total number of shared block cache hits by the statement</td></tr><tr><td><code class="structfield">shared_blks_read</code></td><td><code class="type">bigint</code></td><td> </td><td>Total number of shared blocks read by the statement</td></tr><tr><td><code class="structfield">shared_blks_dirtied</code></td><td><code class="type">bigint</code></td><td> </td><td>Total number of shared blocks dirtied by the statement</td></tr><tr><td><code class="structfield">shared_blks_written</code></td><td><code class="type">bigint</code></td><td> </td><td>Total number of shared blocks written by the statement</td></tr><tr><td><code class="structfield">local_blks_hit</code></td><td><code class="type">bigint</code></td><td> </td><td>Total number of local block cache hits by the statement</td></tr><tr><td><code class="structfield">local_blks_read</code></td><td><code class="type">bigint</code></td><td> </td><td>Total number of local blocks read by the statement</td></tr><tr><td><code class="structfield">local_blks_dirtied</code></td><td><code class="type">bigint</code></td><td> </td><td>Total number of local blocks dirtied by the statement</td></tr><tr><td><code class="structfield">local_blks_written</code></td><td><code class="type">bigint</code></td><td> </td><td>Total number of local blocks written by the statement</td></tr><tr><td><code class="structfield">temp_blks_read</code></td><td><code class="type">bigint</code></td><td> </td><td>Total number of temp blocks read by the statement</td></tr><tr><td><code class="structfield">temp_blks_written</code></td><td><code class="type">bigint</code></td><td> </td><td>Total number of temp blocks written by the statement</td></tr><tr><td><code class="structfield">blk_read_time</code></td><td><code class="type">double precision</code></td><td> </td><td>
Total time the statement spent reading blocks, in milliseconds
(if <a class="xref" href="runtime-config-statistics.html#GUC-TRACK-IO-TIMING">track_io_timing</a> is enabled, otherwise zero)
</td></tr><tr><td><code class="structfield">blk_write_time</code></td><td><code class="type">double precision</code></td><td> </td><td>
Total time the statement spent writing blocks, in milliseconds
(if <a class="xref" href="runtime-config-statistics.html#GUC-TRACK-IO-TIMING">track_io_timing</a> is enabled, otherwise zero)
</td></tr></tbody></table></div></div><br class="table-break" /><p>
For security reasons, only superusers and members of the
<code class="literal">pg_read_all_stats</code> role are allowed to see the SQL text and
<code class="structfield">queryid</code> of queries executed by other users.
Other users can see the statistics, however, if the view has been installed
in their database.
</p><p>
Plannable queries (that is, <code class="command">SELECT</code>, <code class="command">INSERT</code>,
<code class="command">UPDATE</code>, and <code class="command">DELETE</code>) are combined into a single
<code class="structname">pg_stat_statements</code> entry whenever they have identical query
structures according to an internal hash calculation. Typically, two
queries will be considered the same for this purpose if they are
semantically equivalent except for the values of literal constants
appearing in the query. Utility commands (that is, all other commands)
are compared strictly on the basis of their textual query strings, however.
</p><p>
When a constant's value has been ignored for purposes of matching the query
to other queries, the constant is replaced by a parameter symbol, such
as <code class="literal">$1</code>, in the <code class="structname">pg_stat_statements</code>
display.
The rest of the query text is that of the first query that had the
particular <code class="structfield">queryid</code> hash value associated with the
<code class="structname">pg_stat_statements</code> entry.
</p><p>
In some cases, queries with visibly different texts might get merged into a
single <code class="structname">pg_stat_statements</code> entry. Normally this will happen
only for semantically equivalent queries, but there is a small chance of
hash collisions causing unrelated queries to be merged into one entry.
(This cannot happen for queries belonging to different users or databases,
however.)
</p><p>
Since the <code class="structfield">queryid</code> hash value is computed on the
post-parse-analysis representation of the queries, the opposite is
also possible: queries with identical texts might appear as
separate entries, if they have different meanings as a result of
factors such as different <code class="varname">search_path</code> settings.
</p><p>
Consumers of <code class="structname">pg_stat_statements</code> may wish to use
<code class="structfield">queryid</code> (perhaps in combination with
<code class="structfield">dbid</code> and <code class="structfield">userid</code>) as a more stable
and reliable identifier for each entry than its query text.
However, it is important to understand that there are only limited
guarantees around the stability of the <code class="structfield">queryid</code> hash
value. Since the identifier is derived from the
post-parse-analysis tree, its value is a function of, among other
things, the internal object identifiers appearing in this representation.
This has some counterintuitive implications. For example,
<code class="filename">pg_stat_statements</code> will consider two apparently-identical
queries to be distinct, if they reference a table that was dropped
and recreated between the executions of the two queries.
The hashing process is also sensitive to differences in
machine architecture and other facets of the platform.
Furthermore, it is not safe to assume that <code class="structfield">queryid</code>
will be stable across major versions of <span class="productname">PostgreSQL</span>.
</p><p>
As a rule of thumb, <code class="structfield">queryid</code> values can be assumed to be
stable and comparable only so long as the underlying server version and
catalog metadata details stay exactly the same. Two servers
participating in replication based on physical WAL replay can be expected
to have identical <code class="structfield">queryid</code> values for the same query.
However, logical replication schemes do not promise to keep replicas
identical in all relevant details, so <code class="structfield">queryid</code> will
not be a useful identifier for accumulating costs across a set of logical
replicas. If in doubt, direct testing is recommended.
</p><p>
The parameter symbols used to replace constants in
representative query texts start from the next number after the
highest <code class="literal">$</code><em class="replaceable"><code>n</code></em> parameter in the original query
text, or <code class="literal">$1</code> if there was none. It's worth noting that in
some cases there may be hidden parameter symbols that affect this
numbering. For example, <span class="application">PL/pgSQL</span> uses hidden parameter
symbols to insert values of function local variables into queries, so that
a <span class="application">PL/pgSQL</span> statement like <code class="literal">SELECT i + 1 INTO j</code>
would have representative text like <code class="literal">SELECT i + $2</code>.
</p><p>
The representative query texts are kept in an external disk file, and do
not consume shared memory. Therefore, even very lengthy query texts can
be stored successfully. However, if many long query texts are
accumulated, the external file might grow unmanageably large. As a
recovery method if that happens, <code class="filename">pg_stat_statements</code> may
choose to discard the query texts, whereupon all existing entries in
the <code class="structname">pg_stat_statements</code> view will show
null <code class="structfield">query</code> fields, though the statistics associated with
each <code class="structfield">queryid</code> are preserved. If this happens, consider
reducing <code class="varname">pg_stat_statements.max</code> to prevent
recurrences.
</p></div><div class="sect2" id="id-1.11.7.38.7"><div class="titlepage"><div><div><h3 class="title">F.29.2. Functions</h3></div></div></div><div class="variablelist"><dl class="variablelist"><dt><span class="term">
<code class="function">pg_stat_statements_reset() returns void</code>
<a id="id-1.11.7.38.7.2.1.1.2" class="indexterm"></a>
</span></dt><dd><p>
<code class="function">pg_stat_statements_reset</code> discards all statistics
gathered so far by <code class="filename">pg_stat_statements</code>.
By default, this function can only be executed by superusers.
</p></dd><dt><span class="term">
<code class="function">pg_stat_statements(showtext boolean) returns setof record</code>
<a id="id-1.11.7.38.7.2.2.1.2" class="indexterm"></a>
</span></dt><dd><p>
The <code class="structname">pg_stat_statements</code> view is defined in
terms of a function also named <code class="function">pg_stat_statements</code>.
It is possible for clients to call
the <code class="function">pg_stat_statements</code> function directly, and by
specifying <code class="literal">showtext := false</code> have query text be
omitted (that is, the <code class="literal">OUT</code> argument that corresponds
to the view's <code class="structfield">query</code> column will return nulls). This
feature is intended to support external tools that might wish to avoid
the overhead of repeatedly retrieving query texts of indeterminate
length. Such tools can instead cache the first query text observed
for each entry themselves, since that is
all <code class="filename">pg_stat_statements</code> itself does, and then retrieve
query texts only as needed. Since the server stores query texts in a
file, this approach may reduce physical I/O for repeated examination
of the <code class="structname">pg_stat_statements</code> data.
</p></dd></dl></div></div><div class="sect2" id="id-1.11.7.38.8"><div class="titlepage"><div><div><h3 class="title">F.29.3. Configuration Parameters</h3></div></div></div><div class="variablelist"><dl class="variablelist"><dt><span class="term">
<code class="varname">pg_stat_statements.max</code> (<code class="type">integer</code>)
</span></dt><dd><p>
<code class="varname">pg_stat_statements.max</code> is the maximum number of
statements tracked by the module (i.e., the maximum number of rows
in the <code class="structname">pg_stat_statements</code> view). If more distinct
statements than that are observed, information about the least-executed
statements is discarded.
The default value is 5000.
This parameter can only be set at server start.
</p></dd><dt><span class="term">
<code class="varname">pg_stat_statements.track</code> (<code class="type">enum</code>)
</span></dt><dd><p>
<code class="varname">pg_stat_statements.track</code> controls which statements
are counted by the module.
Specify <code class="literal">top</code> to track top-level statements (those issued
directly by clients), <code class="literal">all</code> to also track nested statements
(such as statements invoked within functions), or <code class="literal">none</code> to
disable statement statistics collection.
The default value is <code class="literal">top</code>.
Only superusers can change this setting.
</p></dd><dt><span class="term">
<code class="varname">pg_stat_statements.track_utility</code> (<code class="type">boolean</code>)
</span></dt><dd><p>
<code class="varname">pg_stat_statements.track_utility</code> controls whether
utility commands are tracked by the module. Utility commands are
all those other than <code class="command">SELECT</code>, <code class="command">INSERT</code>,
<code class="command">UPDATE</code> and <code class="command">DELETE</code>.
The default value is <code class="literal">on</code>.
Only superusers can change this setting.
</p></dd><dt><span class="term">
<code class="varname">pg_stat_statements.save</code> (<code class="type">boolean</code>)
</span></dt><dd><p>
<code class="varname">pg_stat_statements.save</code> specifies whether to
save statement statistics across server shutdowns.
If it is <code class="literal">off</code> then statistics are not saved at
shutdown nor reloaded at server start.
The default value is <code class="literal">on</code>.
This parameter can only be set in the <code class="filename">postgresql.conf</code>
file or on the server command line.
</p></dd></dl></div><p>
The module requires additional shared memory proportional to
<code class="varname">pg_stat_statements.max</code>. Note that this
memory is consumed whenever the module is loaded, even if
<code class="varname">pg_stat_statements.track</code> is set to <code class="literal">none</code>.
</p><p>
These parameters must be set in <code class="filename">postgresql.conf</code>.
Typical usage might be:
</p><pre class="programlisting">
# postgresql.conf
shared_preload_libraries = 'pg_stat_statements'
pg_stat_statements.max = 10000
pg_stat_statements.track = all
</pre><p>
</p></div><div class="sect2" id="id-1.11.7.38.9"><div class="titlepage"><div><div><h3 class="title">F.29.4. Sample Output</h3></div></div></div><pre class="screen">
bench=# SELECT pg_stat_statements_reset();
$ pgbench -i bench
$ pgbench -c10 -t300 bench
bench=# \x
bench=# SELECT query, calls, total_time, rows, 100.0 * shared_blks_hit /
nullif(shared_blks_hit + shared_blks_read, 0) AS hit_percent
FROM pg_stat_statements ORDER BY total_time DESC LIMIT 5;
-[ RECORD 1 ]---------------------------------------------------------------------
query | UPDATE pgbench_branches SET bbalance = bbalance + $1 WHERE bid = $2;
calls | 3000
total_time | 9609.00100000002
rows | 2836
hit_percent | 99.9778970000200936
-[ RECORD 2 ]---------------------------------------------------------------------
query | UPDATE pgbench_tellers SET tbalance = tbalance + $1 WHERE tid = $2;
calls | 3000
total_time | 8015.156
rows | 2990
hit_percent | 99.9731126579631345
-[ RECORD 3 ]---------------------------------------------------------------------
query | copy pgbench_accounts from stdin
calls | 1
total_time | 310.624
rows | 100000
hit_percent | 0.30395136778115501520
-[ RECORD 4 ]---------------------------------------------------------------------
query | UPDATE pgbench_accounts SET abalance = abalance + $1 WHERE aid = $2;
calls | 3000
total_time | 271.741999999997
rows | 3000
hit_percent | 93.7968855088209426
-[ RECORD 5 ]---------------------------------------------------------------------
query | alter table pgbench_accounts add primary key (aid)
calls | 1
total_time | 81.42
rows | 0
hit_percent | 34.4947735191637631
</pre></div><div class="sect2" id="id-1.11.7.38.10"><div class="titlepage"><div><div><h3 class="title">F.29.5. Authors</h3></div></div></div><p>
Takahiro Itagaki <code class="email"><<a class="email" href="mailto:itagaki.takahiro@oss.ntt.co.jp">itagaki.takahiro@oss.ntt.co.jp</a>></code>.
Query normalization added by Peter Geoghegan <code class="email"><<a class="email" href="mailto:peter@2ndquadrant.com">peter@2ndquadrant.com</a>></code>.
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