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Add auto-explain contrib module for automatic logging of the plans of
slow-running queries. Takahiro Itagaki
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Tom Lane
176
doc/src/sgml/auto-explain.sgml
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176
doc/src/sgml/auto-explain.sgml
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<!-- $PostgreSQL: pgsql/doc/src/sgml/auto-explain.sgml,v 1.1 2008/11/19 02:59:28 tgl Exp $ -->
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<sect1 id="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" endterm="sql-explain-title">
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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 all sessions by including <literal>auto_explain</> in
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<xref linkend="guc-shared-preload-libraries"> in
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<filename>postgresql.conf</>. 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>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>explain.log_min_duration</varname> (<type>integer</type>)
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</term>
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<indexterm>
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<primary><varname>explain.log_min_duration</> configuration parameter</primary>
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</indexterm>
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<listitem>
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<para>
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<varname>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 zero logs all plans. Minus-one (the default)
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disables logging of plans. For example, if you set it to
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<literal>250ms</literal> then all statements that run 250ms or longer
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will be logged. 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>explain.log_analyze</varname> (<type>boolean</type>)
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</term>
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<indexterm>
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<primary><varname>explain.log_analyze</> configuration parameter</primary>
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</indexterm>
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<listitem>
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<para>
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<varname>explain.log_analyze</varname> causes <command>EXPLAIN ANALYZE</>
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output, rather than just <command>EXPLAIN</> 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 extremely negative impact on performance.
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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>explain.log_verbose</varname> (<type>boolean</type>)
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</term>
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<indexterm>
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<primary><varname>explain.log_verbose</> configuration parameter</primary>
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</indexterm>
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<listitem>
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<para>
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<varname>explain.log_verbose</varname> causes <command>EXPLAIN VERBOSE</>
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output, rather than just <command>EXPLAIN</> 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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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<varname>explain.log_nested_statements</varname> (<type>boolean</type>)
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</term>
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<indexterm>
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<primary><varname>explain.log_nested_statements</> configuration parameter</primary>
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</indexterm>
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<listitem>
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<para>
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<varname>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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</variablelist>
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<para>
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In order to set these parameters in your <filename>postgresql.conf</> file,
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you will need to add <literal>explain</> in
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<varname>custom_variable_classes</>. Typical usage might be:
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</para>
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<programlisting>
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# postgresql.conf
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shared_preload_libraries = 'auto_explain'
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custom_variable_classes = 'explain'
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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 explain.log_min_duration = 0;
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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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<programlisting>
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LOG: duration: 0.986 ms plan:
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Aggregate (cost=14.90..14.91 rows=1 width=0)
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-> Hash Join (cost=3.91..14.70 rows=81 width=0)
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Hash Cond: (pg_class.oid = pg_index.indrelid)
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-> Seq Scan on pg_class (cost=0.00..8.27 rows=227 width=4)
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-> Hash (cost=2.90..2.90 rows=81 width=4)
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-> Seq Scan on pg_index (cost=0.00..2.90 rows=81 width=4)
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Filter: indisunique
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STATEMENT: 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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</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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@ -1,4 +1,4 @@
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<!-- $PostgreSQL: pgsql/doc/src/sgml/contrib.sgml,v 1.9 2008/07/29 18:31:20 tgl Exp $ -->
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<!-- $PostgreSQL: pgsql/doc/src/sgml/contrib.sgml,v 1.10 2008/11/19 02:59:28 tgl Exp $ -->
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<appendix id="contrib">
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<title>Additional Supplied Modules</title>
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@ -79,6 +79,7 @@ psql -d dbname -f <replaceable>SHAREDIR</>/contrib/<replaceable>module</>.sql
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</para>
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&adminpack;
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&auto-explain;
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&btree-gist;
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&chkpass;
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&citext;
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@ -1,4 +1,4 @@
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<!-- $PostgreSQL: pgsql/doc/src/sgml/filelist.sgml,v 1.57 2008/07/29 18:31:20 tgl Exp $ -->
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<!-- $PostgreSQL: pgsql/doc/src/sgml/filelist.sgml,v 1.58 2008/11/19 02:59:28 tgl Exp $ -->
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<!entity history SYSTEM "history.sgml">
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<!entity info SYSTEM "info.sgml">
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@ -92,6 +92,7 @@
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<!-- contrib information -->
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<!entity contrib SYSTEM "contrib.sgml">
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<!entity adminpack SYSTEM "adminpack.sgml">
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<!entity auto-explain SYSTEM "auto-explain.sgml">
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<!entity btree-gist SYSTEM "btree-gist.sgml">
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<!entity chkpass SYSTEM "chkpass.sgml">
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<!entity citext SYSTEM "citext.sgml">
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