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ac86237603
This fixes a couple of grammar mistakes, typos and inconsistencies in the documentation. Particularly, the configuration parsing allows only "kB" to mean kilobyte but there were references in the docs to "KB". Some instances of the latter are still in the code comments. Some parameter values were mentioned with "Minus-one", and using directly "-1" with proper markups is more helpful to the reader. Some of these have been pointed out by Justin, and some others are things I bumped into. Author: Justin Pryzby, Michael Paquier Reviewed-by: Tom Lane Discussion: https://postgr.es/m/20190330224333.GQ5815@telsasoft.com
321 lines
11 KiB
Plaintext
321 lines
11 KiB
Plaintext
<!-- doc/src/sgml/auto-explain.sgml -->
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<sect1 id="auto-explain" xreflabel="auto_explain">
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<title>auto_explain</title>
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<indexterm zone="auto-explain">
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<primary>auto_explain</primary>
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</indexterm>
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<para>
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The <filename>auto_explain</filename> module provides a means for
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logging execution plans of slow statements automatically, without
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having to run <xref linkend="sql-explain"/>
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by hand. This is especially helpful for tracking down un-optimized queries
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in large applications.
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</para>
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<para>
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The module provides no SQL-accessible functions. To use it, simply
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load it into the server. You can load it into an individual session:
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<programlisting>
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LOAD 'auto_explain';
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</programlisting>
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(You must be superuser to do that.) More typical usage is to preload
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it into some or all sessions by including <literal>auto_explain</literal> in
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<xref linkend="guc-session-preload-libraries"/> or
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<xref linkend="guc-shared-preload-libraries"/> in
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<filename>postgresql.conf</filename>. Then you can track unexpectedly slow queries
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no matter when they happen. Of course there is a price in overhead for
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that.
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</para>
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<sect2>
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<title>Configuration Parameters</title>
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<para>
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There are several configuration parameters that control the behavior of
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<filename>auto_explain</filename>. Note that the default behavior is
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to do nothing, so you must set at least
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<varname>auto_explain.log_min_duration</varname> if you want any results.
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</para>
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<variablelist>
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<varlistentry>
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<term>
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<varname>auto_explain.log_min_duration</varname> (<type>integer</type>)
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<indexterm>
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<primary><varname>auto_explain.log_min_duration</varname> configuration parameter</primary>
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</indexterm>
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</term>
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<listitem>
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<para>
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<varname>auto_explain.log_min_duration</varname> is the minimum statement
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execution time, in milliseconds, that will cause the statement's plan to
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be logged. Setting this to <literal>0</literal> logs all plans.
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<literal>-1</literal> (the default) disables logging of plans. For
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example, if you set it to <literal>250ms</literal> then all statements
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that run 250ms or longer will be logged. Only superusers can change this
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setting.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<varname>auto_explain.log_analyze</varname> (<type>boolean</type>)
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<indexterm>
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<primary><varname>auto_explain.log_analyze</varname> configuration parameter</primary>
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</indexterm>
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</term>
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<listitem>
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<para>
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<varname>auto_explain.log_analyze</varname> causes <command>EXPLAIN ANALYZE</command>
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output, rather than just <command>EXPLAIN</command> output, to be printed
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when an execution plan is logged. This parameter is off by default.
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Only superusers can change this setting.
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</para>
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<note>
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<para>
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When this parameter is on, per-plan-node timing occurs for all
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statements executed, whether or not they run long enough to actually
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get logged. This can have an extremely negative impact on performance.
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Turning off <varname>auto_explain.log_timing</varname> ameliorates the
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performance cost, at the price of obtaining less information.
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</para>
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</note>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<varname>auto_explain.log_buffers</varname> (<type>boolean</type>)
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<indexterm>
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<primary><varname>auto_explain.log_buffers</varname> configuration parameter</primary>
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</indexterm>
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</term>
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<listitem>
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<para>
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<varname>auto_explain.log_buffers</varname> controls whether buffer
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usage statistics are printed when an execution plan is logged; it's
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equivalent to the <literal>BUFFERS</literal> option of <command>EXPLAIN</command>.
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This parameter has no effect
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unless <varname>auto_explain.log_analyze</varname> is enabled.
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This parameter is off by default.
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Only superusers can change this setting.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<varname>auto_explain.log_timing</varname> (<type>boolean</type>)
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<indexterm>
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<primary><varname>auto_explain.log_timing</varname> configuration parameter</primary>
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</indexterm>
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</term>
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<listitem>
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<para>
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<varname>auto_explain.log_timing</varname> controls whether per-node
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timing information is printed when an execution plan is logged; it's
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equivalent to the <literal>TIMING</literal> option of <command>EXPLAIN</command>.
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The overhead of repeatedly reading the system clock can slow down
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queries significantly on some systems, so it may be useful to set this
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parameter to off when only actual row counts, and not exact times, are
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needed.
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This parameter has no effect
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unless <varname>auto_explain.log_analyze</varname> is enabled.
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This parameter is on by default.
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Only superusers can change this setting.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<varname>auto_explain.log_triggers</varname> (<type>boolean</type>)
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<indexterm>
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<primary><varname>auto_explain.log_triggers</varname> configuration parameter</primary>
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</indexterm>
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</term>
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<listitem>
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<para>
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<varname>auto_explain.log_triggers</varname> causes trigger
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execution statistics to be included when an execution plan is logged.
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This parameter has no effect
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unless <varname>auto_explain.log_analyze</varname> is enabled.
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This parameter is off by default.
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Only superusers can change this setting.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<varname>auto_explain.log_verbose</varname> (<type>boolean</type>)
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<indexterm>
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<primary><varname>auto_explain.log_verbose</varname> configuration parameter</primary>
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</indexterm>
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</term>
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<listitem>
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<para>
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<varname>auto_explain.log_verbose</varname> controls whether verbose
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details are printed when an execution plan is logged; it's
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equivalent to the <literal>VERBOSE</literal> option of <command>EXPLAIN</command>.
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This parameter is off by default.
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Only superusers can change this setting.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<varname>auto_explain.log_settings</varname> (<type>boolean</type>)
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<indexterm>
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<primary><varname>auto_explain.log_settings</varname> configuration parameter</primary>
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</indexterm>
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</term>
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<listitem>
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<para>
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<varname>auto_explain.log_settings</varname> controls whether information
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about modified configuration options are printed when execution plan is logged.
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Only options affecting query planning with value different from the built-in
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default value are included in the output. This parameter is off by default.
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Only superusers can change this setting.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<varname>auto_explain.log_format</varname> (<type>enum</type>)
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<indexterm>
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<primary><varname>auto_explain.log_format</varname> configuration parameter</primary>
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</indexterm>
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</term>
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<listitem>
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<para>
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<varname>auto_explain.log_format</varname> selects the
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<command>EXPLAIN</command> output format to be used.
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The allowed values are <literal>text</literal>, <literal>xml</literal>,
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<literal>json</literal>, and <literal>yaml</literal>. The default is text.
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Only superusers can change this setting.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<varname>auto_explain.log_level</varname> (<type>enum</type>)
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<indexterm>
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<primary><varname>auto_explain.log_level</varname> configuration parameter</primary>
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</indexterm>
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</term>
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<listitem>
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<para>
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<varname>auto_explain.log_level</varname> selects the log level at which
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auto_explain will log the query plan.
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Valid values are <literal>DEBUG5</literal>, <literal>DEBUG4</literal>,
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<literal>DEBUG3</literal>, <literal>DEBUG2</literal>,
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<literal>DEBUG1</literal>, <literal>INFO</literal>,
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<literal>NOTICE</literal>, <literal>WARNING</literal>,
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and <literal>LOG</literal>. The default is <literal>LOG</literal>.
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Only superusers can change this setting.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<varname>auto_explain.log_nested_statements</varname> (<type>boolean</type>)
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<indexterm>
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<primary><varname>auto_explain.log_nested_statements</varname> configuration parameter</primary>
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</indexterm>
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</term>
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<listitem>
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<para>
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<varname>auto_explain.log_nested_statements</varname> causes nested
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statements (statements executed inside a function) to be considered
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for logging. When it is off, only top-level query plans are logged. This
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parameter is off by default. Only superusers can change this setting.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<varname>auto_explain.sample_rate</varname> (<type>real</type>)
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<indexterm>
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<primary><varname>auto_explain.sample_rate</varname> configuration parameter</primary>
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</indexterm>
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</term>
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<listitem>
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<para>
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<varname>auto_explain.sample_rate</varname> causes auto_explain to only
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explain a fraction of the statements in each session. The default is 1,
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meaning explain all the queries. In case of nested statements, either all
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will be explained or none. Only superusers can change this setting.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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<para>
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In ordinary usage, these parameters are set
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in <filename>postgresql.conf</filename>, although superusers can alter them
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on-the-fly within their own sessions.
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Typical usage might be:
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</para>
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<programlisting>
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# postgresql.conf
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session_preload_libraries = 'auto_explain'
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auto_explain.log_min_duration = '3s'
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</programlisting>
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</sect2>
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<sect2>
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<title>Example</title>
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<programlisting>
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postgres=# LOAD 'auto_explain';
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postgres=# SET auto_explain.log_min_duration = 0;
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postgres=# SET auto_explain.log_analyze = true;
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postgres=# SELECT count(*)
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FROM pg_class, pg_index
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WHERE oid = indrelid AND indisunique;
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</programlisting>
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<para>
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This might produce log output such as:
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</para>
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<screen><![CDATA[
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LOG: duration: 3.651 ms plan:
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Query Text: SELECT count(*)
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FROM pg_class, pg_index
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WHERE oid = indrelid AND indisunique;
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Aggregate (cost=16.79..16.80 rows=1 width=0) (actual time=3.626..3.627 rows=1 loops=1)
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-> Hash Join (cost=4.17..16.55 rows=92 width=0) (actual time=3.349..3.594 rows=92 loops=1)
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Hash Cond: (pg_class.oid = pg_index.indrelid)
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-> Seq Scan on pg_class (cost=0.00..9.55 rows=255 width=4) (actual time=0.016..0.140 rows=255 loops=1)
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-> Hash (cost=3.02..3.02 rows=92 width=4) (actual time=3.238..3.238 rows=92 loops=1)
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Buckets: 1024 Batches: 1 Memory Usage: 4kB
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-> Seq Scan on pg_index (cost=0.00..3.02 rows=92 width=4) (actual time=0.008..3.187 rows=92 loops=1)
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Filter: indisunique
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]]></screen>
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</sect2>
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<sect2>
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<title>Author</title>
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<para>
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Takahiro Itagaki <email>itagaki.takahiro@oss.ntt.co.jp</email>
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</para>
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</sect2>
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</sect1>
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