path: root/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</literal> in
  <xref linkend="guc-session-preload-libraries"/> or
  <xref linkend="guc-shared-preload-libraries"/> in
  <filename>postgresql.conf</filename>.  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>)
     <indexterm>
      <primary><varname>auto_explain.log_min_duration</varname> configuration parameter</primary>
     </indexterm>
    </term>
    <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 <literal>0</literal> logs all plans.
      <literal>-1</literal> (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>)
     <indexterm>
      <primary><varname>auto_explain.log_analyze</varname> configuration parameter</primary>
     </indexterm>
    </term>
    <listitem>
     <para>
      <varname>auto_explain.log_analyze</varname> causes <command>EXPLAIN ANALYZE</command>
      output, rather than just <command>EXPLAIN</command> 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.
       Turning off <varname>auto_explain.log_timing</varname> ameliorates the
       performance cost, at the price of obtaining less information.
      </para>
     </note>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <varname>auto_explain.log_buffers</varname> (<type>boolean</type>)
     <indexterm>
      <primary><varname>auto_explain.log_buffers</varname> configuration parameter</primary>
     </indexterm>
    </term>
    <listitem>
     <para>
      <varname>auto_explain.log_buffers</varname> controls whether buffer
      usage statistics are printed when an execution plan is logged; it's
      equivalent to the <literal>BUFFERS</literal> option of <command>EXPLAIN</command>.
      This parameter has no effect
      unless <varname>auto_explain.log_analyze</varname> is enabled.
      This parameter is off by default.
      Only superusers can change this setting.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <varname>auto_explain.log_wal</varname> (<type>boolean</type>)
     <indexterm>
      <primary><varname>auto_explain.log_wal</varname> configuration parameter</primary>
     </indexterm>
    </term>
    <listitem>
     <para>
      <varname>auto_explain.log_wal</varname> controls whether WAL
      usage statistics are printed when an execution plan is logged; it's
      equivalent to the <literal>WAL</literal> option of <command>EXPLAIN</command>.
      This parameter has no effect
      unless <varname>auto_explain.log_analyze</varname> is enabled.
      This parameter is off by default.
      Only superusers can change this setting.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <varname>auto_explain.log_timing</varname> (<type>boolean</type>)
     <indexterm>
      <primary><varname>auto_explain.log_timing</varname> configuration parameter</primary>
     </indexterm>
    </term>
    <listitem>
     <para>
      <varname>auto_explain.log_timing</varname> controls whether per-node
      timing information is printed when an execution plan is logged; it's
      equivalent to the <literal>TIMING</literal> option of <command>EXPLAIN</command>.
      The overhead of repeatedly reading the system clock can slow down
      queries 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 has no effect
      unless <varname>auto_explain.log_analyze</varname> is enabled.
      This parameter is on by default.
      Only superusers can change this setting.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <varname>auto_explain.log_triggers</varname> (<type>boolean</type>)
     <indexterm>
      <primary><varname>auto_explain.log_triggers</varname> configuration parameter</primary>
     </indexterm>
    </term>
    <listitem>
     <para>
      <varname>auto_explain.log_triggers</varname> causes trigger
      execution statistics to be included when an execution plan is logged.
      This parameter has no effect
      unless <varname>auto_explain.log_analyze</varname> is enabled.
      This parameter is off by default.
      Only superusers can change this setting.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <varname>auto_explain.log_verbose</varname> (<type>boolean</type>)
     <indexterm>
      <primary><varname>auto_explain.log_verbose</varname> configuration parameter</primary>
     </indexterm>
    </term>
    <listitem>
     <para>
      <varname>auto_explain.log_verbose</varname> controls whether verbose
      details are printed when an execution plan is logged; it's
      equivalent to the <literal>VERBOSE</literal> option of <command>EXPLAIN</command>.
      This parameter is off by default.
      Only superusers can change this setting.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <varname>auto_explain.log_settings</varname> (<type>boolean</type>)
     <indexterm>
      <primary><varname>auto_explain.log_settings</varname> configuration parameter</primary>
     </indexterm>
    </term>
    <listitem>
     <para>
      <varname>auto_explain.log_settings</varname> controls whether information
      about modified configuration options is printed when an execution plan is logged.
      Only options affecting query planning with value different from the built-in
      default value are included in the output.  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>)
     <indexterm>
      <primary><varname>auto_explain.log_format</varname> configuration parameter</primary>
     </indexterm>
    </term>
    <listitem>
     <para>
      <varname>auto_explain.log_format</varname> selects the
      <command>EXPLAIN</command> 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_level</varname> (<type>enum</type>)
     <indexterm>
      <primary><varname>auto_explain.log_level</varname> configuration parameter</primary>
     </indexterm>
    </term>
    <listitem>
     <para>
      <varname>auto_explain.log_level</varname> selects the log level at which
      auto_explain will log the query plan.
      Valid values are <literal>DEBUG5</literal>, <literal>DEBUG4</literal>,
      <literal>DEBUG3</literal>, <literal>DEBUG2</literal>,
      <literal>DEBUG1</literal>, <literal>INFO</literal>,
      <literal>NOTICE</literal>, <literal>WARNING</literal>,
      and <literal>LOG</literal>. The default is <literal>LOG</literal>.
      Only superusers can change this setting.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <varname>auto_explain.log_nested_statements</varname> (<type>boolean</type>)
     <indexterm>
      <primary><varname>auto_explain.log_nested_statements</varname> configuration parameter</primary>
     </indexterm>
    </term>
    <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>

   <varlistentry>
    <term>
     <varname>auto_explain.sample_rate</varname> (<type>real</type>)
     <indexterm>
      <primary><varname>auto_explain.sample_rate</varname> configuration parameter</primary>
     </indexterm>
    </term>
    <listitem>
     <para>
      <varname>auto_explain.sample_rate</varname> causes auto_explain to only
      explain a fraction of the statements in each session.  The default is 1,
      meaning explain all the queries.  In case of nested statements, either all
      will be explained or none. Only superusers can change this setting.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>

  <para>
   In ordinary usage, these parameters are set
   in <filename>postgresql.conf</filename>, although superusers can alter them
   on-the-fly within their own sessions.
   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=# SET auto_explain.log_analyze = true;
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>