path: root/doc/src/sgml/pgstatstatements.sgml

<!-- $PostgreSQL: pgsql/doc/src/sgml/pgstatstatements.sgml,v 1.2 2009/05/18 11:08:24 petere Exp $ -->

<sect1 id="pgstatstatements">
 <title>pg_stat_statements</title>

 <indexterm zone="pgstatstatements">
  <primary>pg_stat_statements</primary>
 </indexterm>

 <para>
  The <filename>pg_stat_statements</filename> module provides a means for
  tracking execution statistics of all SQL statements executed by a server.
 </para>

 <para>
  The module must be loaded by adding <literal>pg_stat_statements</> to
  <xref linkend="guc-shared-preload-libraries"> in
  <filename>postgresql.conf</>, because it requires additional shared memory.
  This means that a server restart is needed to add or remove the module.
 </para>

 <sect2>
  <title>The <structname>pg_stat_statements</structname> view</title>

  <para>
   The statistics gathered by the module are made available via a system view
   named <structname>pg_stat_statements</>.  This view contains one row for
   each distinct query text, database ID, and user ID (up to the maximum
   number of distinct statements that the module can track).  The columns
   of the view are shown in <xref linkend="pgstatstatements-columns">.
  </para>

  <table id="pgstatstatements-columns">
   <title><structname>pg_stat_statements</> columns</title>

   <tgroup cols="4">
    <thead>
     <row>
      <entry>Name</entry>
      <entry>Type</entry>
      <entry>References</entry>
      <entry>Description</entry>
     </row>
    </thead>
    <tbody>
     <row>
      <entry><structfield>userid</structfield></entry>
      <entry><type>oid</type></entry>
      <entry><literal><link linkend="catalog-pg-authid"><structname>pg_authid</structname></link>.oid</literal></entry>
      <entry>OID of user who executed the statement</entry>
     </row>

     <row>
      <entry><structfield>dbid</structfield></entry>
      <entry><type>oid</type></entry>
      <entry><literal><link linkend="catalog-pg-database"><structname>pg_database</structname></link>.oid</literal></entry>
      <entry>OID of database in which the statement was executed</entry>
     </row>

    <row>
      <entry><structfield>query</structfield></entry>
      <entry><type>text</type></entry>
      <entry></entry>
      <entry>Text of the statement (up to <xref linkend="guc-track-activity-query-size"> bytes)</entry>
     </row>

     <row>
      <entry><structfield>calls</structfield></entry>
      <entry><type>bigint</type></entry>
      <entry></entry>
      <entry>Number of times executed</entry>
     </row>

     <row>
      <entry><structfield>total_time</structfield></entry>
      <entry><type>double precision</type></entry>
      <entry></entry>
      <entry>Total time spent in the statement, in seconds</entry>
     </row>

     <row>
      <entry><structfield>rows</structfield></entry>
      <entry><type>bigint</type></entry>
      <entry></entry>
      <entry>Total number of rows retrieved or affected by the statement</entry>
     </row>

    </tbody>
   </tgroup>
  </table>

  <para>
   This view, and the function <function>pg_stat_statements_reset</>,
   are available only in databases they have been specifically installed into
   by running the <filename>pg_stat_statements.sql</> install script.
   However, statistics are tracked across all databases of the server
   whenever the <filename>pg_stat_statements</filename> module is loaded
   into the server, regardless of presence of the view.
  </para>

  <para>
   For security reasons, non-superusers are not allowed to see the text of
   queries executed by other users.  They can see the statistics, however,
   if the view has been installed in their database.
  </para>

  <para>
   Note that statements are considered the same if they have the same text,
   regardless of the values of any out-of-line parameters used in the
   statement.  Using out-of-line parameters will help to group statements
   together and may make the statistics more useful.
  </para>
 </sect2>

 <sect2>
  <title>Functions</title>

  <variablelist>
   <varlistentry>
    <term>
     <function>pg_stat_statements_reset() returns void</function>
    </term>

    <listitem>
     <para>
      <function>pg_stat_statements_reset</function> discards all statistics
      gathered so far by <filename>pg_stat_statements</>.
      By default, this function can only be executed by superusers.
     </para>
    </listitem>
   </varlistentry>

  </variablelist>
 </sect2>

 <sect2>
  <title>Configuration parameters</title>

  <variablelist>
   <varlistentry>
    <term>
     <varname>pg_stat_statements.max</varname> (<type>integer</type>)
    </term>

    <listitem>
     <para>
      <varname>pg_stat_statements.max</varname> is the maximum number of
      statements tracked by the module (i.e., the maximum number of rows
      in the <structname>pg_stat_statements</> view).  If more distinct
      statements than that are observed, information about the least-executed
      statements is discarded.
      The default value is 1000.
      This parameter can only be set at server start.
     </para>
    </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</> to track top-level statements (those issued
      directly by clients), <literal>all</> to also track nested statements
      (such as statements invoked within functions), or <literal>none</> to
      disable.
      The default value is <literal>top</>.
      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</> then statistics are not saved at
      shutdown nor reloaded at server start.
      The default value is <literal>on</>.
      This parameter can only be set in the <filename>postgresql.conf</>
      file or on the server command line.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>

  <para>
   The module requires additional shared memory amounting to about
   <varname>pg_stat_statements.max</varname> <literal>*</>
   <xref linkend="guc-track-activity-query-size"> bytes.  Note that this
   memory is consumed whenever the module is loaded, even if
   <varname>pg_stat_statements.track</> is set to <literal>none</>.
  </para>

  <para>
   In order to set any of these parameters in your
   <filename>postgresql.conf</> file,
   you will need to add <literal>pg_stat_statements</> to
   <xref linkend="guc-custom-variable-classes">.  Typical usage might be:
  </para>

  <programlisting>
# postgresql.conf
shared_preload_libraries = 'pg_stat_statements'

custom_variable_classes = 'pg_stat_statements'
pg_stat_statements.max = 10000
pg_stat_statements.track = all
  </programlisting>
 </sect2>

 <sect2>
  <title>Sample output</title>

  <programlisting>
$ pgbench -i bench

postgres=# SELECT pg_stat_statements_reset();

$ pgbench -c10 -t300 -M prepared bench

postgres=# \x
postgres=# SELECT * FROM pg_stat_statements ORDER BY total_time DESC LIMIT 3;
-[ RECORD 1 ]------------------------------------------------------------
userid     | 10
dbid       | 63781
query      | UPDATE branches SET bbalance = bbalance + $1 WHERE bid = $2;
calls      | 3000
total_time | 20.716706
rows       | 3000
-[ RECORD 2 ]------------------------------------------------------------
userid     | 10
dbid       | 63781
query      | UPDATE tellers SET tbalance = tbalance + $1 WHERE tid = $2;
calls      | 3000
total_time | 17.1107649999999
rows       | 3000
-[ RECORD 3 ]------------------------------------------------------------
userid     | 10
dbid       | 63781
query      | UPDATE accounts SET abalance = abalance + $1 WHERE aid = $2;
calls      | 3000
total_time | 0.645601
rows       | 3000
  </programlisting>
 </sect2>

 <sect2>
  <title>Author</title>

  <para>
   Takahiro Itagaki <email>itagaki.takahiro@oss.ntt.co.jp</email>
  </para>
 </sect2>

</sect1>