postgresql/doc/src/sgml/auto-explain.sgml

<!-- doc/src/sgml/auto-explain.sgml -->

<sect1 id="auto-explain" xreflabel="auto_explain">
 <title>auto_explain</title>

 <indexterm zone="auto-explain">
  <primary>auto_explain</primary>
 </indexterm>

 <para>
  The <filename>auto_explain</filename> module provides a means for
  logging execution plans of slow statements automatically, without
  having to run <xref linkend="sql-explain">
  by hand.  This is especially helpful for tracking down un-optimized queries
  in large applications.
 </para>

 <para>
  The module provides no SQL-accessible functions.  To use it, simply
  load it into the server.  You can load it into an individual session:

<programlisting>
LOAD 'auto_explain';
</programlisting>

  (You must be superuser to do that.)  More typical usage is to preload
  it into some or all sessions by including <literal>auto_explain</> in
  <xref linkend="guc-session-preload-libraries"> or
  <xref linkend="guc-shared-preload-libraries"> in
  <filename>postgresql.conf</>.  Then you can track unexpectedly slow queries
  no matter when they happen.  Of course there is a price in overhead for
  that.
 </para>

 <sect2>
  <title>Configuration Parameters</title>

 <para>
  There are several configuration parameters that control the behavior of
  <filename>auto_explain</filename>.  Note that the default behavior is
  to do nothing, so you must set at least
  <varname>auto_explain.log_min_duration</varname> if you want any results.
 </para>

  <variablelist>
   <varlistentry>
    <term>
     <varname>auto_explain.log_min_duration</varname> (<type>integer</type>)
    </term>
    <indexterm>
     <primary><varname>auto_explain.log_min_duration</> configuration parameter</primary>
    </indexterm>
    <listitem>
     <para>
      <varname>auto_explain.log_min_duration</varname> is the minimum statement
      execution time, in milliseconds, that will cause the statement's plan to
      be logged. Setting this to zero logs all plans. Minus-one (the default)
      disables logging of plans.  For example, if you set it to
      <literal>250ms</literal> then all statements that run 250ms or longer
      will be logged. Only superusers can change this setting.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <varname>auto_explain.log_analyze</varname> (<type>boolean</type>)
    </term>
    <indexterm>
     <primary><varname>auto_explain.log_analyze</> configuration parameter</primary>
    </indexterm>
    <listitem>
     <para>
      <varname>auto_explain.log_analyze</varname> causes <command>EXPLAIN ANALYZE</>
      output, rather than just <command>EXPLAIN</> output, to be printed
      when an execution plan is logged. This parameter is off by default.
      Only superusers can change this setting.
     </para>
     <note>
      <para>
       When this parameter is on, per-plan-node timing occurs for all
       statements executed, whether or not they run long enough to actually
       get logged.  This can have an extremely negative impact on performance.
      </para>
     </note>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <varname>auto_explain.log_verbose</varname> (<type>boolean</type>)
    </term>
    <indexterm>
     <primary><varname>auto_explain.log_verbose</> configuration parameter</primary>
    </indexterm>
    <listitem>
     <para>
      <varname>auto_explain.log_verbose</varname> causes <command>EXPLAIN VERBOSE</>
      output, rather than just <command>EXPLAIN</> output, to be printed
      when an execution plan is logged. This parameter is off by default.
      Only superusers can change this setting.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <varname>auto_explain.log_buffers</varname> (<type>boolean</type>)
    </term>
    <indexterm>
     <primary><varname>auto_explain.log_buffers</> configuration parameter</primary>
    </indexterm>
    <listitem>
     <para>
      <varname>auto_explain.log_buffers</varname> causes <command>EXPLAIN
      (ANALYZE, BUFFERS)</> output, rather than just <command>EXPLAIN</>
      output, to be printed when an execution plan is logged. This parameter is
      off by default. Only superusers can change this setting. This
      parameter has no effect unless <varname>auto_explain.log_analyze</>
      parameter is set.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <varname>auto_explain.log_format</varname> (<type>enum</type>)
    </term>
    <indexterm>
     <primary><varname>auto_explain.log_format</> configuration parameter</primary>
    </indexterm>
    <listitem>
     <para>
      <varname>auto_explain.log_format</varname> selects the
      <command>EXPLAIN</> output format to be used.
      The allowed values are <literal>text</literal>, <literal>xml</literal>,
      <literal>json</literal>, and <literal>yaml</literal>.  The default is text.
      Only superusers can change this setting.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <varname>auto_explain.log_timing</varname> (<type>boolean</type>)
    </term>
    <indexterm>
     <primary><varname>auto_explain.log_timing</> configuration parameter</primary>
    </indexterm>
    <listitem>
     <para>
      <varname>auto_explain.log_timing</varname> causes <command>EXPLAIN
      (ANALYZE, TIMING off)</> output, rather than just <command>EXPLAIN (ANALYZE)</>
      output. The overhead of repeatedly reading the system clock can slow down the
      query significantly on some systems, so it may be useful to set this
      parameter to off when only actual row counts, and not exact times, are needed.
      This parameter is only effective when <varname>auto_explain.log_analyze</varname>
      is also enabled.  This parameter is on by default.
      Only superusers can change this setting.
     </para>
    </listitem>
   </varlistentry>
   
   <varlistentry>
    <term>
     <varname>auto_explain.log_nested_statements</varname> (<type>boolean</type>)
    </term>
    <indexterm>
     <primary><varname>auto_explain.log_nested_statements</> configuration parameter</primary>
    </indexterm>
    <listitem>
     <para>
      <varname>auto_explain.log_nested_statements</varname> causes nested
      statements (statements executed inside a function) to be considered
      for logging.  When it is off, only top-level query plans are logged. This
      parameter is off by default. Only superusers can change this setting.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>

  <para>
   These parameters must be set in <filename>postgresql.conf</>.
   Typical usage might be:
  </para>

<programlisting>
# postgresql.conf
session_preload_libraries = 'auto_explain'

auto_explain.log_min_duration = '3s'
</programlisting>
 </sect2>

 <sect2>
  <title>Example</title>

<programlisting>
postgres=# LOAD 'auto_explain';
postgres=# SET auto_explain.log_min_duration = 0;
postgres=# SELECT count(*)
           FROM pg_class, pg_index
           WHERE oid = indrelid AND indisunique;
</programlisting>

  <para>
   This might produce log output such as:
  </para>

<screen><![CDATA[
LOG:  duration: 3.651 ms  plan:
  Query Text: SELECT count(*)
              FROM pg_class, pg_index
              WHERE oid = indrelid AND indisunique;
  Aggregate  (cost=16.79..16.80 rows=1 width=0) (actual time=3.626..3.627 rows=1 loops=1)
    ->  Hash Join  (cost=4.17..16.55 rows=92 width=0) (actual time=3.349..3.594 rows=92 loops=1)
          Hash Cond: (pg_class.oid = pg_index.indrelid)
          ->  Seq Scan on pg_class  (cost=0.00..9.55 rows=255 width=4) (actual time=0.016..0.140 rows=255 loops=1)
          ->  Hash  (cost=3.02..3.02 rows=92 width=4) (actual time=3.238..3.238 rows=92 loops=1)
                Buckets: 1024  Batches: 1  Memory Usage: 4kB
                ->  Seq Scan on pg_index  (cost=0.00..3.02 rows=92 width=4) (actual time=0.008..3.187 rows=92 loops=1)
                      Filter: indisunique
]]></screen>
 </sect2>

 <sect2>
  <title>Author</title>

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

</sect1>