701 lines
26 KiB
Plaintext
701 lines
26 KiB
Plaintext
<!-- doc/src/sgml/pgstatstatements.sgml -->
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<sect1 id="pgstatstatements" xreflabel="pg_stat_statements">
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<title>pg_stat_statements</title>
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<indexterm zone="pgstatstatements">
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<primary>pg_stat_statements</primary>
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</indexterm>
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<para>
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The <filename>pg_stat_statements</filename> module provides a means for
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tracking planning and execution statistics of all SQL statements executed by
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a server.
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</para>
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<para>
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The module must be loaded by adding <literal>pg_stat_statements</literal> to
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<xref linkend="guc-shared-preload-libraries"/> in
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<filename>postgresql.conf</filename>, because it requires additional shared memory.
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This means that a server restart is needed to add or remove the module.
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</para>
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<para>
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When <filename>pg_stat_statements</filename> is loaded, it tracks
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statistics across all databases of the server. To access and manipulate
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these statistics, the module provides a view, <structname>pg_stat_statements</structname>,
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and the utility functions <function>pg_stat_statements_reset</function> and
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<function>pg_stat_statements</function>. These are not available globally but
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can be enabled for a specific database with
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<command>CREATE EXTENSION pg_stat_statements</command>.
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</para>
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<sect2>
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<title>The <structname>pg_stat_statements</structname> View</title>
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<para>
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The statistics gathered by the module are made available via a
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view named <structname>pg_stat_statements</structname>. This view
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contains one row for each distinct database ID, user ID and query
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ID (up to the maximum number of distinct statements that the module
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can track). The columns of the view are shown in
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<xref linkend="pgstatstatements-columns"/>.
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</para>
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<table id="pgstatstatements-columns">
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<title><structname>pg_stat_statements</structname> Columns</title>
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<tgroup cols="4">
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<thead>
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<row>
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<entry>Name</entry>
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<entry>Type</entry>
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<entry>References</entry>
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<entry>Description</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry><structfield>userid</structfield></entry>
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<entry><type>oid</type></entry>
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<entry><literal><link linkend="catalog-pg-authid"><structname>pg_authid</structname></link>.oid</literal></entry>
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<entry>OID of user who executed the statement</entry>
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</row>
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<row>
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<entry><structfield>dbid</structfield></entry>
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<entry><type>oid</type></entry>
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<entry><literal><link linkend="catalog-pg-database"><structname>pg_database</structname></link>.oid</literal></entry>
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<entry>OID of database in which the statement was executed</entry>
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</row>
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<row>
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<entry><structfield>queryid</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry></entry>
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<entry>Internal hash code, computed from the statement's parse tree</entry>
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</row>
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<row>
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<entry><structfield>query</structfield></entry>
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<entry><type>text</type></entry>
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<entry></entry>
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<entry>Text of a representative statement</entry>
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</row>
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<row>
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<entry><structfield>plans</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry></entry>
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<entry>Number of times the statement was planned</entry>
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</row>
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<row>
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<entry><structfield>total_plan_time</structfield></entry>
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<entry><type>double precision</type></entry>
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<entry></entry>
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<entry>Total time spent planning the statement, in milliseconds</entry>
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</row>
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<row>
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<entry><structfield>min_plan_time</structfield></entry>
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<entry><type>double precision</type></entry>
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<entry></entry>
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<entry>Minimum time spent planning the statement, in milliseconds</entry>
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</row>
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<row>
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<entry><structfield>max_plan_time</structfield></entry>
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<entry><type>double precision</type></entry>
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<entry></entry>
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<entry>Maximum time spent planning the statement, in milliseconds</entry>
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</row>
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<row>
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<entry><structfield>mean_plan_time</structfield></entry>
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<entry><type>double precision</type></entry>
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<entry></entry>
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<entry>Mean time spent planning the statement, in milliseconds</entry>
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</row>
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<row>
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<entry><structfield>stddev_plan_time</structfield></entry>
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<entry><type>double precision</type></entry>
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<entry></entry>
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<entry>Population standard deviation of time spent planning the statement, in milliseconds</entry>
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</row>
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<row>
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<entry><structfield>calls</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry></entry>
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<entry>Number of times the statement was executed</entry>
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</row>
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<row>
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<entry><structfield>total_exec_time</structfield></entry>
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<entry><type>double precision</type></entry>
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<entry></entry>
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<entry>Total time spent executing the statement, in milliseconds</entry>
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</row>
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<row>
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<entry><structfield>min_exec_time</structfield></entry>
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<entry><type>double precision</type></entry>
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<entry></entry>
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<entry>Minimum time spent executing the statement, in milliseconds</entry>
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</row>
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<row>
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<entry><structfield>max_exec_time</structfield></entry>
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<entry><type>double precision</type></entry>
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<entry></entry>
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<entry>Maximum time spent executing the statement, in milliseconds</entry>
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</row>
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<row>
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<entry><structfield>mean_exec_time</structfield></entry>
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<entry><type>double precision</type></entry>
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<entry></entry>
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<entry>Mean time spent executing the statement, in milliseconds</entry>
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</row>
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<row>
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<entry><structfield>stddev_exec_time</structfield></entry>
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<entry><type>double precision</type></entry>
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<entry></entry>
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<entry>Population standard deviation of time spent executing the statement, in milliseconds</entry>
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</row>
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<row>
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<entry><structfield>rows</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry></entry>
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<entry>Total number of rows retrieved or affected by the statement</entry>
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</row>
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<row>
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<entry><structfield>shared_blks_hit</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry></entry>
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<entry>Total number of shared block cache hits by the statement</entry>
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</row>
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<row>
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<entry><structfield>shared_blks_read</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry></entry>
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<entry>Total number of shared blocks read by the statement</entry>
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</row>
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<row>
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<entry><structfield>shared_blks_dirtied</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry></entry>
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<entry>Total number of shared blocks dirtied by the statement</entry>
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</row>
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<row>
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<entry><structfield>shared_blks_written</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry></entry>
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<entry>Total number of shared blocks written by the statement</entry>
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</row>
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<row>
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<entry><structfield>local_blks_hit</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry></entry>
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<entry>Total number of local block cache hits by the statement</entry>
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</row>
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<row>
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<entry><structfield>local_blks_read</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry></entry>
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<entry>Total number of local blocks read by the statement</entry>
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</row>
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<row>
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<entry><structfield>local_blks_dirtied</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry></entry>
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<entry>Total number of local blocks dirtied by the statement</entry>
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</row>
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<row>
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<entry><structfield>local_blks_written</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry></entry>
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<entry>Total number of local blocks written by the statement</entry>
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</row>
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<row>
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<entry><structfield>temp_blks_read</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry></entry>
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<entry>Total number of temp blocks read by the statement</entry>
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</row>
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<row>
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<entry><structfield>temp_blks_written</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry></entry>
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<entry>Total number of temp blocks written by the statement</entry>
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</row>
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<row>
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<entry><structfield>blk_read_time</structfield></entry>
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<entry><type>double precision</type></entry>
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<entry></entry>
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<entry>
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Total time the statement spent reading blocks, in milliseconds
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(if <xref linkend="guc-track-io-timing"/> is enabled, otherwise zero)
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</entry>
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</row>
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<row>
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<entry><structfield>blk_write_time</structfield></entry>
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<entry><type>double precision</type></entry>
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<entry></entry>
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<entry>
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Total time the statement spent writing blocks, in milliseconds
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(if <xref linkend="guc-track-io-timing"/> is enabled, otherwise zero)
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</entry>
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</row>
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<row>
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<entry><structfield>wal_bytes</structfield></entry>
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<entry><type>numeric</type></entry>
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<entry></entry>
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<entry>
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Total amount of WAL bytes generated by the statement
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</entry>
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</row>
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<row>
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<entry><structfield>wal_records</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry></entry>
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<entry>
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Total number of WAL records generated by the statement
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</entry>
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</row>
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<row>
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<entry><structfield>wal_fpi</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry></entry>
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<entry>
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Total number of WAL full page images generated by the statement
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</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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<para>
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For security reasons, only superusers and members of the
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<literal>pg_read_all_stats</literal> role are allowed to see the SQL text and
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<structfield>queryid</structfield> of queries executed by other users.
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Other users can see the statistics, however, if the view has been installed
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in their database.
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</para>
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<para>
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Plannable queries (that is, <command>SELECT</command>, <command>INSERT</command>,
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<command>UPDATE</command>, and <command>DELETE</command>) are combined into a single
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<structname>pg_stat_statements</structname> entry whenever they have identical query
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structures according to an internal hash calculation. Typically, two
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queries will be considered the same for this purpose if they are
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semantically equivalent except for the values of literal constants
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appearing in the query. Utility commands (that is, all other commands)
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are compared strictly on the basis of their textual query strings, however.
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</para>
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<para>
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When a constant's value has been ignored for purposes of matching the query
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to other queries, the constant is replaced by a parameter symbol, such
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as <literal>$1</literal>, in the <structname>pg_stat_statements</structname>
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display.
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The rest of the query text is that of the first query that had the
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particular <structfield>queryid</structfield> hash value associated with the
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<structname>pg_stat_statements</structname> entry.
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</para>
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<para>
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In some cases, queries with visibly different texts might get merged into a
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single <structname>pg_stat_statements</structname> entry. Normally this will happen
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only for semantically equivalent queries, but there is a small chance of
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hash collisions causing unrelated queries to be merged into one entry.
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(This cannot happen for queries belonging to different users or databases,
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however.)
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</para>
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<para>
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Since the <structfield>queryid</structfield> hash value is computed on the
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post-parse-analysis representation of the queries, the opposite is
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also possible: queries with identical texts might appear as
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separate entries, if they have different meanings as a result of
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factors such as different <varname>search_path</varname> settings.
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</para>
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<para>
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Consumers of <structname>pg_stat_statements</structname> may wish to use
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<structfield>queryid</structfield> (perhaps in combination with
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<structfield>dbid</structfield> and <structfield>userid</structfield>) as a more stable
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and reliable identifier for each entry than its query text.
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However, it is important to understand that there are only limited
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guarantees around the stability of the <structfield>queryid</structfield> hash
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value. Since the identifier is derived from the
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post-parse-analysis tree, its value is a function of, among other
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things, the internal object identifiers appearing in this representation.
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This has some counterintuitive implications. For example,
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<filename>pg_stat_statements</filename> will consider two apparently-identical
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queries to be distinct, if they reference a table that was dropped
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and recreated between the executions of the two queries.
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The hashing process is also sensitive to differences in
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machine architecture and other facets of the platform.
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Furthermore, it is not safe to assume that <structfield>queryid</structfield>
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will be stable across major versions of <productname>PostgreSQL</productname>.
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</para>
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<para>
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As a rule of thumb, <structfield>queryid</structfield> values can be assumed to be
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stable and comparable only so long as the underlying server version and
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catalog metadata details stay exactly the same. Two servers
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participating in replication based on physical WAL replay can be expected
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to have identical <structfield>queryid</structfield> values for the same query.
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However, logical replication schemes do not promise to keep replicas
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identical in all relevant details, so <structfield>queryid</structfield> will
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not be a useful identifier for accumulating costs across a set of logical
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replicas. If in doubt, direct testing is recommended.
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</para>
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<para>
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The parameter symbols used to replace constants in
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representative query texts start from the next number after the
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highest <literal>$</literal><replaceable>n</replaceable> parameter in the original query
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text, or <literal>$1</literal> if there was none. It's worth noting that in
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some cases there may be hidden parameter symbols that affect this
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numbering. For example, <application>PL/pgSQL</application> uses hidden parameter
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symbols to insert values of function local variables into queries, so that
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a <application>PL/pgSQL</application> statement like <literal>SELECT i + 1 INTO j</literal>
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would have representative text like <literal>SELECT i + $2</literal>.
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</para>
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<para>
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The representative query texts are kept in an external disk file, and do
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not consume shared memory. Therefore, even very lengthy query texts can
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be stored successfully. However, if many long query texts are
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accumulated, the external file might grow unmanageably large. As a
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recovery method if that happens, <filename>pg_stat_statements</filename> may
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choose to discard the query texts, whereupon all existing entries in
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the <structname>pg_stat_statements</structname> view will show
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null <structfield>query</structfield> fields, though the statistics associated with
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each <structfield>queryid</structfield> are preserved. If this happens, consider
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reducing <varname>pg_stat_statements.max</varname> to prevent
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recurrences.
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</para>
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<para>
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<structfield>plans</structfield> and <structfield>calls</structfield> aren't
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always expected to match because planning and execution statistics are
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updated at their respective end phase, and only for successful operations.
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For example, if a statement is successfully planned but fails during
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the execution phase, only its planning statistics will be updated.
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If planning is skipped because a cached plan is used, only its execution
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statistics will be updated.
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</para>
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</sect2>
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<sect2>
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<title>Functions</title>
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<variablelist>
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<varlistentry>
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<term>
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<function>pg_stat_statements_reset(userid Oid, dbid Oid, queryid bigint) returns void</function>
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<indexterm>
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<primary>pg_stat_statements_reset</primary>
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</indexterm>
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</term>
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<listitem>
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<para>
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<function>pg_stat_statements_reset</function> discards statistics
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gathered so far by <filename>pg_stat_statements</filename> corresponding
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to the specified <structfield>userid</structfield>, <structfield>dbid</structfield>
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and <structfield>queryid</structfield>. If any of the parameters are not
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specified, the default value <literal>0</literal>(invalid) is used for
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each of them and the statistics that match with other parameters will be
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reset. If no parameter is specified or all the specified parameters are
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<literal>0</literal>(invalid), it will discard all statistics. By
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default, this function can only be executed by superusers. Access may be
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granted to others using <command>GRANT</command>.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<function>pg_stat_statements(showtext boolean) returns setof record</function>
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<indexterm>
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<primary>pg_stat_statements</primary>
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<secondary>function</secondary>
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</indexterm>
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</term>
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<listitem>
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<para>
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The <structname>pg_stat_statements</structname> view is defined in
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terms of a function also named <function>pg_stat_statements</function>.
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It is possible for clients to call
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the <function>pg_stat_statements</function> function directly, and by
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specifying <literal>showtext := false</literal> have query text be
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omitted (that is, the <literal>OUT</literal> argument that corresponds
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to the view's <structfield>query</structfield> column will return nulls). This
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feature is intended to support external tools that might wish to avoid
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the overhead of repeatedly retrieving query texts of indeterminate
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length. Such tools can instead cache the first query text observed
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for each entry themselves, since that is
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all <filename>pg_stat_statements</filename> itself does, and then retrieve
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query texts only as needed. Since the server stores query texts in a
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file, this approach may reduce physical I/O for repeated examination
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of the <structname>pg_stat_statements</structname> data.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</sect2>
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<sect2>
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<title>Configuration Parameters</title>
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<variablelist>
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<varlistentry>
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<term>
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<varname>pg_stat_statements.max</varname> (<type>integer</type>)
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</term>
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<listitem>
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<para>
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<varname>pg_stat_statements.max</varname> is the maximum number of
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statements tracked by the module (i.e., the maximum number of rows
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in the <structname>pg_stat_statements</structname> view). If more distinct
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statements than that are observed, information about the least-executed
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statements is discarded.
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The default value is 5000.
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This parameter can only be set at server start.
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</para>
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</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term>
|
|
<varname>pg_stat_statements.track</varname> (<type>enum</type>)
|
|
</term>
|
|
|
|
<listitem>
|
|
<para>
|
|
<varname>pg_stat_statements.track</varname> controls which statements
|
|
are counted by the module.
|
|
Specify <literal>top</literal> to track top-level statements (those issued
|
|
directly by clients), <literal>all</literal> to also track nested statements
|
|
(such as statements invoked within functions), or <literal>none</literal> to
|
|
disable statement statistics collection.
|
|
The default value is <literal>top</literal>.
|
|
Only superusers can change this setting.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term>
|
|
<varname>pg_stat_statements.track_utility</varname> (<type>boolean</type>)
|
|
</term>
|
|
|
|
<listitem>
|
|
<para>
|
|
<varname>pg_stat_statements.track_utility</varname> controls whether
|
|
utility commands are tracked by the module. Utility commands are
|
|
all those other than <command>SELECT</command>, <command>INSERT</command>,
|
|
<command>UPDATE</command> and <command>DELETE</command>.
|
|
The default value is <literal>on</literal>.
|
|
Only superusers can change this setting.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term>
|
|
<varname>pg_stat_statements.track_planning</varname> (<type>boolean</type>)
|
|
</term>
|
|
|
|
<listitem>
|
|
<para>
|
|
<varname>pg_stat_statements.track_planning</varname> controls whether
|
|
planning operations and duration are tracked by the module.
|
|
The default value is <literal>on</literal>.
|
|
Only superusers can change this setting.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term>
|
|
<varname>pg_stat_statements.save</varname> (<type>boolean</type>)
|
|
</term>
|
|
|
|
<listitem>
|
|
<para>
|
|
<varname>pg_stat_statements.save</varname> specifies whether to
|
|
save statement statistics across server shutdowns.
|
|
If it is <literal>off</literal> then statistics are not saved at
|
|
shutdown nor reloaded at server start.
|
|
The default value is <literal>on</literal>.
|
|
This parameter can only be set in the <filename>postgresql.conf</filename>
|
|
file or on the server command line.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
</variablelist>
|
|
|
|
<para>
|
|
The module requires additional shared memory proportional to
|
|
<varname>pg_stat_statements.max</varname>. Note that this
|
|
memory is consumed whenever the module is loaded, even if
|
|
<varname>pg_stat_statements.track</varname> is set to <literal>none</literal>.
|
|
</para>
|
|
|
|
<para>
|
|
These parameters must be set in <filename>postgresql.conf</filename>.
|
|
Typical usage might be:
|
|
|
|
<programlisting>
|
|
# postgresql.conf
|
|
shared_preload_libraries = 'pg_stat_statements'
|
|
|
|
pg_stat_statements.max = 10000
|
|
pg_stat_statements.track = all
|
|
</programlisting>
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2>
|
|
<title>Sample Output</title>
|
|
|
|
<screen>
|
|
bench=# SELECT pg_stat_statements_reset();
|
|
|
|
$ pgbench -i bench
|
|
$ pgbench -c10 -t300 bench
|
|
|
|
bench=# \x
|
|
bench=# SELECT query, calls, total_exec_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_exec_time DESC LIMIT 5;
|
|
-[ RECORD 1 ]---+--------------------------------------------------------------------
|
|
query | UPDATE pgbench_branches SET bbalance = bbalance + $1 WHERE bid = $2
|
|
calls | 3000
|
|
total_exec_time | 25565.855387
|
|
rows | 3000
|
|
hit_percent | 100.0000000000000000
|
|
-[ RECORD 2 ]---+--------------------------------------------------------------------
|
|
query | UPDATE pgbench_tellers SET tbalance = tbalance + $1 WHERE tid = $2
|
|
calls | 3000
|
|
total_exec_time | 20756.669379
|
|
rows | 3000
|
|
hit_percent | 100.0000000000000000
|
|
-[ RECORD 3 ]---+--------------------------------------------------------------------
|
|
query | copy pgbench_accounts from stdin
|
|
calls | 1
|
|
total_exec_time | 291.865911
|
|
rows | 100000
|
|
hit_percent | 100.0000000000000000
|
|
-[ RECORD 4 ]---+--------------------------------------------------------------------
|
|
query | UPDATE pgbench_accounts SET abalance = abalance + $1 WHERE aid = $2
|
|
calls | 3000
|
|
total_exec_time | 271.232977
|
|
rows | 3000
|
|
hit_percent | 98.8454011741682975
|
|
-[ RECORD 5 ]---+--------------------------------------------------------------------
|
|
query | alter table pgbench_accounts add primary key (aid)
|
|
calls | 1
|
|
total_exec_time | 160.588563
|
|
rows | 0
|
|
hit_percent | 100.0000000000000000
|
|
|
|
|
|
bench=# SELECT pg_stat_statements_reset(0,0,s.queryid) FROM pg_stat_statements AS s
|
|
WHERE s.query = 'UPDATE pgbench_branches SET bbalance = bbalance + $1 WHERE bid = $2';
|
|
|
|
bench=# SELECT query, calls, total_exec_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_exec_time DESC LIMIT 5;
|
|
-[ RECORD 1 ]---+--------------------------------------------------------------------
|
|
query | UPDATE pgbench_tellers SET tbalance = tbalance + $1 WHERE tid = $2
|
|
calls | 3000
|
|
total_exec_time | 20756.669379
|
|
rows | 3000
|
|
hit_percent | 100.0000000000000000
|
|
-[ RECORD 2 ]---+--------------------------------------------------------------------
|
|
query | copy pgbench_accounts from stdin
|
|
calls | 1
|
|
total_exec_time | 291.865911
|
|
rows | 100000
|
|
hit_percent | 100.0000000000000000
|
|
-[ RECORD 3 ]---+--------------------------------------------------------------------
|
|
query | UPDATE pgbench_accounts SET abalance = abalance + $1 WHERE aid = $2
|
|
calls | 3000
|
|
total_exec_time | 271.232977
|
|
rows | 3000
|
|
hit_percent | 98.8454011741682975
|
|
-[ RECORD 4 ]---+--------------------------------------------------------------------
|
|
query | alter table pgbench_accounts add primary key (aid)
|
|
calls | 1
|
|
total_exec_time | 160.588563
|
|
rows | 0
|
|
hit_percent | 100.0000000000000000
|
|
-[ RECORD 5 ]---+--------------------------------------------------------------------
|
|
query | vacuum analyze pgbench_accounts
|
|
calls | 1
|
|
total_exec_time | 136.448116
|
|
rows | 0
|
|
hit_percent | 99.9201915403032721
|
|
|
|
bench=# SELECT pg_stat_statements_reset(0,0,0);
|
|
|
|
bench=# SELECT query, calls, total_exec_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_exec_time DESC LIMIT 5;
|
|
-[ RECORD 1 ]---+-----------------------------------------------------------------------------
|
|
query | SELECT pg_stat_statements_reset(0,0,0)
|
|
calls | 1
|
|
total_exec_time | 0.189497
|
|
rows | 1
|
|
hit_percent |
|
|
-[ RECORD 2 ]---+-----------------------------------------------------------------------------
|
|
query | SELECT query, calls, total_exec_time, rows, $1 * shared_blks_hit / +
|
|
| nullif(shared_blks_hit + shared_blks_read, $2) AS hit_percent+
|
|
| FROM pg_stat_statements ORDER BY total_exec_time DESC LIMIT $3
|
|
calls | 0
|
|
total_exec_time | 0
|
|
rows | 0
|
|
hit_percent |
|
|
|
|
</screen>
|
|
</sect2>
|
|
|
|
<sect2>
|
|
<title>Authors</title>
|
|
|
|
<para>
|
|
Takahiro Itagaki <email>itagaki.takahiro@oss.ntt.co.jp</email>.
|
|
Query normalization added by Peter Geoghegan <email>peter@2ndquadrant.com</email>.
|
|
</para>
|
|
</sect2>
|
|
|
|
</sect1>
|