<!-- doc/src/sgml/auto-explain.sgml -->
<sect1 id="auto-explain" xreflabel="auto_explain">
<title>auto_explain &mdash; log execution plans of slow queries</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 id="auto-explain-configuration-parameters">
<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 id="auto-explain-configuration-parameters-log-min-duration">
<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 id="auto-explain-configuration-parameters-log-parameter-max-length">
<term>
<varname>auto_explain.log_parameter_max_length</varname> (<type>integer</type>)
<indexterm>
<primary><varname>auto_explain.log_parameter_max_length</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
<varname>auto_explain.log_parameter_max_length</varname> controls the
logging of query parameter values. A value of <literal>-1</literal> (the
default) logs the parameter values in full. <literal>0</literal> disables
logging of parameter values. A value greater than zero truncates each
parameter value to that many bytes. Only superusers can change this
setting.
</para>
</listitem>
</varlistentry>
<varlistentry id="auto-explain-configuration-parameters-log-analyze">
<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 id="auto-explain-configuration-parameters-log-buffers">
<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 id="auto-explain-configuration-parameters-log-wal">
<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 id="auto-explain-configuration-parameters-log-timing">
<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 id="auto-explain-configuration-parameters-log-triggers">
<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 id="auto-explain-configuration-parameters-log-verbose">
<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 id="auto-explain-configuration-parameters-log-settings">
<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 id="auto-explain-configuration-parameters-log-format">
<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 id="auto-explain-configuration-parameters-log-level">
<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 id="auto-explain-configuration-parameters-log-nested-statements">
<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 id="auto-explain-configuration-parameters-sample-rate">
<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 id="auto-explain-example">
<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=1.00 loops=1)
-> Hash Join (cost=4.17..16.55 rows=92 width=0) (actual time=3.349..3.594 rows=92.00 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=255.00 loops=1)
-> Hash (cost=3.02..3.02 rows=92 width=4) (actual time=3.238..3.238 rows=92.00 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=92.00 loops=1)
Filter: indisunique
]]></screen>
</sect2>
<sect2 id="auto-explain-author">
<title>Author</title>
<para>
Takahiro Itagaki <email>itagaki.takahiro@oss.ntt.co.jp</email>
</para>
</sect2>
</sect1>