pgstatstatements.sgml

<!-- doc/src/sgml/pgstatstatements.sgml -->

<sect1 id="pgstatstatements" xreflabel="pg_stat_statements">
 <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>

 <para>
   When <filename>pg_stat_statements</filename> is loaded, it tracks
   statistics across all databases of the server.  To access and manipulate
   these statistics, the module provides a view, <structname>pg_stat_statements</>,
   and the utility functions <function>pg_stat_statements_reset</> and
   <function>pg_stat_statements</>.  These are not available globally but
   can be enabled for a specific database with
   <command>CREATE EXTENSION pg_stat_statements</>.
 </para>

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

  <para>
   The statistics gathered by the module are made available via a
   view named <structname>pg_stat_statements</>.  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
   <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>queryid</structfield></entry>
      <entry><type>bigint</type></entry>
      <entry></entry>
      <entry>Internal hash code, computed from the statement's parse tree</entry>
     </row>

     <row>
      <entry><structfield>query</structfield></entry>
      <entry><type>text</type></entry>
      <entry></entry>
      <entry>Text of a representative statement</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 milliseconds</entry>
     </row>

     <row>
      <entry><structfield>min_time</structfield></entry>
      <entry><type>double precision</type></entry>
      <entry></entry>
      <entry>Minimum time spent in the statement, in milliseconds</entry>
     </row>

     <row>
      <entry><structfield>max_time</structfield></entry>
      <entry><type>double precision</type></entry>
      <entry></entry>
      <entry>Maximum time spent in the statement, in milliseconds</entry>
     </row>

     <row>
      <entry><structfield>mean_time</structfield></entry>
      <entry><type>double precision</type></entry>
      <entry></entry>
      <entry>Mean time spent in the statement, in milliseconds</entry>
     </row>

     <row>
      <entry><structfield>stddev_time</structfield></entry>
      <entry><type>double precision</type></entry>
      <entry></entry>
      <entry>Population standard deviation of time spent in the statement, in milliseconds</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>

     <row>
      <entry><structfield>shared_blks_hit</structfield></entry>
      <entry><type>bigint</type></entry>
      <entry></entry>
      <entry>Total number of shared block cache hits by the statement</entry>
     </row>

     <row>
      <entry><structfield>shared_blks_read</structfield></entry>
      <entry><type>bigint</type></entry>
      <entry></entry>
      <entry>Total number of shared blocks read by the statement</entry>
     </row>

     <row>
      <entry><structfield>shared_blks_dirtied</structfield></entry>
      <entry><type>bigint</type></entry>
      <entry></entry>
      <entry>Total number of shared blocks dirtied by the statement</entry>
     </row>

     <row>
      <entry><structfield>shared_blks_written</structfield></entry>
      <entry><type>bigint</type></entry>
      <entry></entry>
      <entry>Total number of shared blocks written by the statement</entry>
     </row>

     <row>
      <entry><structfield>local_blks_hit</structfield></entry>
      <entry><type>bigint</type></entry>
      <entry></entry>
      <entry>Total number of local block cache hits by the statement</entry>
     </row>

     <row>
      <entry><structfield>local_blks_read</structfield></entry>
      <entry><type>bigint</type></entry>
      <entry></entry>
      <entry>Total number of local blocks read by the statement</entry>
     </row>

     <row>
      <entry><structfield>local_blks_dirtied</structfield></entry>
      <entry><type>bigint</type></entry>
      <entry></entry>
      <entry>Total number of local blocks dirtied by the statement</entry>
     </row>

     <row>
      <entry><structfield>local_blks_written</structfield></entry>
      <entry><type>bigint</type></entry>
      <entry></entry>
      <entry>Total number of local blocks written by the statement</entry>
     </row>

     <row>
      <entry><structfield>temp_blks_read</structfield></entry>
      <entry><type>bigint</type></entry>
      <entry></entry>
      <entry>Total number of temp blocks read by the statement</entry>
     </row>

     <row>
      <entry><structfield>temp_blks_written</structfield></entry>
      <entry><type>bigint</type></entry>
      <entry></entry>
      <entry>Total number of temp blocks written by the statement</entry>
     </row>

     <row>
      <entry><structfield>blk_read_time</structfield></entry>
      <entry><type>double precision</type></entry>
      <entry></entry>
      <entry>
        Total time the statement spent reading blocks, in milliseconds
        (if <xref linkend="guc-track-io-timing"> is enabled, otherwise zero)
      </entry>
     </row>

     <row>
      <entry><structfield>blk_write_time</structfield></entry>
      <entry><type>double precision</type></entry>
      <entry></entry>
      <entry>
        Total time the statement spent writing blocks, in milliseconds
        (if <xref linkend="guc-track-io-timing"> is enabled, otherwise zero)
      </entry>
     </row>

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

  <para>
   For security reasons, only superusers and members of the
   <literal>pg_read_all_stats</literal> role are allowed to see the SQL text and
   <structfield>queryid</structfield> of queries executed by other users.
   Other users can see the statistics, however, if the view has been installed
   in their database.
  </para>

  <para>
   Plannable queries (that is, <command>SELECT</>, <command>INSERT</>,
   <command>UPDATE</>, and <command>DELETE</>) are combined into a single
   <structname>pg_stat_statements</> 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.
  </para>

  <para>
   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 <literal>$1</literal>, in the <structname>pg_stat_statements</>
   display.
   The rest of the query text is that of the first query that had the
   particular <structfield>queryid</> hash value associated with the
   <structname>pg_stat_statements</> entry.
  </para>

  <para>
   In some cases, queries with visibly different texts might get merged into a
   single <structname>pg_stat_statements</> 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.)
  </para>

  <para>
   Since the <structfield>queryid</> 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 <varname>search_path</> settings.
  </para>

  <para>
   Consumers of <structname>pg_stat_statements</> may wish to use
   <structfield>queryid</> (perhaps in combination with
   <structfield>dbid</> and <structfield>userid</>) 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 <structfield>queryid</> 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,
   <filename>pg_stat_statements</> 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 <structfield>queryid</>
   will be stable across major versions of <productname>PostgreSQL</>.
  </para>

  <para>
   As a rule of thumb, <structfield>queryid</> 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 <structfield>queryid</> values for the same query.
   However, logical replication schemes do not promise to keep replicas
   identical in all relevant details, so <structfield>queryid</> will
   not be a useful identifier for accumulating costs across a set of logical
   replicas.  If in doubt, direct testing is recommended.
  </para>

  <para>
   The parameter symbols used to replace constants in
   representative query texts start from the next number after the
   highest <literal>$</><replaceable>n</> parameter in the original query
   text, or <literal>$1</> if there was none.  It's worth noting that in
   some cases there may be hidden parameter symbols that affect this
   numbering.  For example, <application>PL/pgSQL</> uses hidden parameter
   symbols to insert values of function local variables into queries, so that
   a <application>PL/pgSQL</> statement like <literal>SELECT i + 1 INTO j</>
   would have representative text like <literal>SELECT i + $2</>.
  </para>

  <para>
   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, <filename>pg_stat_statements</> may
   choose to discard the query texts, whereupon all existing entries in
   the <structname>pg_stat_statements</> view will show
   null <structfield>query</> fields, though the statistics associated with
   each <structfield>queryid</> are preserved.  If this happens, consider
   reducing <varname>pg_stat_statements.max</varname> to prevent
   recurrences.
  </para>
 </sect2>

 <sect2>
  <title>Functions</title>

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

   <varlistentry>
    <term>
     <function>pg_stat_statements(showtext boolean) returns setof record</function>
     <indexterm>
      <primary>pg_stat_statements</primary>
      <secondary>function</secondary>
     </indexterm>
    </term>

    <listitem>
     <para>
      The <structname>pg_stat_statements</structname> view is defined in
      terms of a function also named <function>pg_stat_statements</>.
      It is possible for clients to call
      the <function>pg_stat_statements</function> function directly, and by
      specifying <literal>showtext := false</literal> have query text be
      omitted (that is, the <literal>OUT</literal> argument that corresponds
      to the view's <structfield>query</> 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 <filename>pg_stat_statements</> 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 <structname>pg_stat_statements</structname> data.
     </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 5000.
      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 statement statistics collection.
      The default value is <literal>top</>.
      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>INSERT</>,
      <command>UPDATE</> and <command>DELETE</>.
      The default value is <literal>on</>.
      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 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</> is set to <literal>none</>.
  </para>

  <para>
   These parameters must be set in <filename>postgresql.conf</>.
   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_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
</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>