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

<!-- $PostgreSQL: pgsql/doc/src/sgml/auto-explain.sgml,v 1.4 2009/08/10 05:46:50 tgl Exp $ -->

<sect1 id="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" endterm="sql-explain-title">
  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 all sessions by including <literal>auto_explain</> in
  <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 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_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>,
      and <literal>json</literal>.  The default is text.
      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>
   In order to set these parameters in your <filename>postgresql.conf</> file,
   you will need to add <literal>auto_explain</> to
   <xref linkend="guc-custom-variable-classes">.  Typical usage might be:
  </para>

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

custom_variable_classes = '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>

  <programlisting><![CDATA[
  LOG:  duration: 0.986 ms  plan:
          Aggregate  (cost=14.90..14.91 rows=1 width=0)
            ->  Hash Join  (cost=3.91..14.70 rows=81 width=0)
                  Hash Cond: (pg_class.oid = pg_index.indrelid)
                  ->  Seq Scan on pg_class  (cost=0.00..8.27 rows=227 width=4)
                  ->  Hash  (cost=2.90..2.90 rows=81 width=4)
                        ->  Seq Scan on pg_index  (cost=0.00..2.90 rows=81 width=4)
                              Filter: indisunique
  STATEMENT:  SELECT count(*)
            FROM pg_class, pg_index
           WHERE oid = indrelid AND indisunique;
]]>
  </programlisting>
 </sect2>

 <sect2>
  <title>Author</title>

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

</sect1>