<?xml version="1.0" encoding="UTF-8" standalone="no"?> <!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>. </p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="pgrowlocks.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="contrib.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="pgstattuple.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">F.28. pgrowlocks </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> F.30. pgstattuple</td></tr></table></div></body></html>