path: root/doc/src/sgml/ref/explain.sgml

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$PostgreSQL: pgsql/doc/src/sgml/ref/explain.sgml,v 1.41 2007/11/28 15:42:31 petere Exp $
PostgreSQL documentation
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<refentry id="SQL-EXPLAIN">
 <refmeta>
  <refentrytitle id="SQL-EXPLAIN-TITLE">EXPLAIN</refentrytitle>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>

 <refnamediv>
  <refname>EXPLAIN</refname>
  <refpurpose>show the execution plan of a statement</refpurpose>
 </refnamediv>

 <indexterm zone="sql-explain">
  <primary>EXPLAIN</primary>
 </indexterm>

 <indexterm zone="sql-explain">
  <primary>prepared statements</primary>
  <secondary>showing the query plan</secondary>
 </indexterm>

 <indexterm zone="sql-explain">
  <primary>cursor</primary>
  <secondary>showing the query plan</secondary>
 </indexterm>

 <refsynopsisdiv>
<synopsis>
EXPLAIN [ ANALYZE ] [ VERBOSE ] <replaceable class="parameter">statement</replaceable>
</synopsis>
 </refsynopsisdiv>

 <refsect1>
  <title>Description</title>

  <para>
   This command displays the execution plan that the
   <productname>PostgreSQL</productname> planner generates for the
   supplied statement.  The execution plan shows how the table(s)
   referenced by the statement will be scanned &mdash; by plain sequential scan,
   index scan, etc. &mdash; and if multiple tables are referenced, what join
   algorithms will be used to bring together the required rows from
   each input table.
  </para>

  <para>
   The most critical part of the display is the estimated statement execution
   cost, which is the planner's guess at how long it will take to run the
   statement (measured in units of disk page fetches).  Actually two numbers
   are shown: the start-up time before the first row can be returned, and
   the total time to return all the rows.  For most queries the total time
   is what matters, but in contexts such as a subquery in <literal>EXISTS</literal>, the planner
   will choose the smallest start-up time instead of the smallest total time
   (since the executor will stop after getting one row, anyway).
   Also, if you limit the number of rows to return with a <literal>LIMIT</literal> clause,
   the planner makes an appropriate interpolation between the endpoint
   costs to estimate which plan is really the cheapest.
  </para>

  <para>
   The <literal>ANALYZE</literal> option causes the statement to be actually executed, not only
   planned.  The total elapsed time expended within each plan node (in
   milliseconds) and total number of rows it actually returned are added to
   the display.  This is useful for seeing whether the planner's estimates
   are close to reality.
  </para>

  <important>
   <para>
    Keep in mind that the statement is actually executed when
    the <literal>ANALYZE</literal> option is used.  Although
    <command>EXPLAIN</command> will discard any output that a
    <command>SELECT</command> would return, other side effects of the
    statement will happen as usual.  If you wish to use
    <command>EXPLAIN ANALYZE</command> on an
    <command>INSERT</command>, <command>UPDATE</command>,
    <command>DELETE</command>, or <command>EXECUTE</command> statement
    without letting the command affect your data, use this approach:
<programlisting>
BEGIN;
EXPLAIN ANALYZE ...;
ROLLBACK;
</programlisting>
   </para>
  </important>
 </refsect1>

 <refsect1>
  <title>Parameters</title>

  <variablelist>
   <varlistentry>
    <term><literal>ANALYZE</literal></term>
    <listitem>
     <para>
      Carry out the command and show the actual run times.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>VERBOSE</literal></term>
    <listitem>
     <para>
      Show the full internal representation of the plan tree, rather
      than just a summary.  Usually this option is only useful for
      specialized debugging purposes.  The
      <literal>VERBOSE</literal> output is either pretty-printed or
      not, depending on the setting of the <xref
      linkend="guc-explain-pretty-print"> configuration parameter.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">statement</replaceable></term>
    <listitem>
     <para>
      Any <command>SELECT</>, <command>INSERT</>, <command>UPDATE</>,
      <command>DELETE</>, <command>VALUES</>, <command>EXECUTE</>, or
      <command>DECLARE</> statement, whose execution plan you wish to see.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>
 </refsect1>

 <refsect1>
  <title>Notes</title>

  <para>
   There is only sparse documentation on the optimizer's use of cost
   information in <productname>PostgreSQL</productname>.  Refer to
   <xref linkend="using-explain"> for more information.
  </para>

  <para>
   In order to allow the <productname>PostgreSQL</productname> query
   planner to make reasonably informed decisions when optimizing
   queries, the <xref linkend="sql-analyze" endterm="sql-analyze-title">
   statement should be run to record statistics about the distribution
   of data within the table. If you have not done this (or if the
   statistical distribution of the data in the table has changed
   significantly since the last time <command>ANALYZE</command> was
   run), the estimated costs are unlikely to conform to the real
   properties of the query, and consequently an inferior query plan
   might be chosen.
  </para>

  <para>
   Genetic query optimization (<acronym>GEQO</acronym>) randomly tests
   execution plans.  Therefore, when the number of join relations
   exceeds <xref linkend="guc-geqo-threshold"> causing genetic query
   optimization to be used, the execution plan is likely to change
   each time the statement is executed.
  </para>

  <para>
   In order to measure the run-time cost of each node in the execution
   plan, the current implementation of <command>EXPLAIN
   ANALYZE</command> can add considerable profiling overhead to query
   execution. As a result, running <command>EXPLAIN ANALYZE</command>
   on a query can sometimes take significantly longer than executing
   the query normally. The amount of overhead depends on the nature of
   the query.
  </para>
 </refsect1>

 <refsect1>
  <title>Examples</title>

  <para>
   To show the plan for a simple query on a table with a single
   <type>integer</type> column and 10000 rows:

<programlisting>
EXPLAIN SELECT * FROM foo;

                       QUERY PLAN
---------------------------------------------------------
 Seq Scan on foo  (cost=0.00..155.00 rows=10000 width=4)
(1 row)
</programlisting>
  </para>

  <para>
   If there is an index and we use a query with an indexable
   <literal>WHERE</literal> condition, <command>EXPLAIN</command>
   might show a different plan:

<programlisting>
EXPLAIN SELECT * FROM foo WHERE i = 4;

                         QUERY PLAN
--------------------------------------------------------------
 Index Scan using fi on foo  (cost=0.00..5.98 rows=1 width=4)
   Index Cond: (i = 4)
(2 rows)
</programlisting>
  </para>

  <para>
   Here is an example of a query plan for a query using an aggregate
   function:

<programlisting>
EXPLAIN SELECT sum(i) FROM foo WHERE i &lt; 10;

                             QUERY PLAN
---------------------------------------------------------------------
 Aggregate  (cost=23.93..23.93 rows=1 width=4)
   -&gt;  Index Scan using fi on foo  (cost=0.00..23.92 rows=6 width=4)
         Index Cond: (i &lt; 10)
(3 rows)
</programlisting>
  </para>

  <para>
   Here is an example of using <command>EXPLAIN EXECUTE</command> to
   display the execution plan for a prepared query:

<programlisting>
PREPARE query(int, int) AS SELECT sum(bar) FROM test
    WHERE id &gt; $1 AND id &lt; $2
    GROUP BY foo;

EXPLAIN ANALYZE EXECUTE query(100, 200);

                                                       QUERY PLAN                                                        
-------------------------------------------------------------------------------------------------------------------------
 HashAggregate  (cost=39.53..39.53 rows=1 width=8) (actual time=0.661..0.672 rows=7 loops=1)
   -&gt;  Index Scan using test_pkey on test  (cost=0.00..32.97 rows=1311 width=8) (actual time=0.050..0.395 rows=99 loops=1)
         Index Cond: ((id &gt; $1) AND (id &lt; $2))
 Total runtime: 0.851 ms
(4 rows)
</programlisting>
  </para>

  <para>
   Of course, the specific numbers shown here depend on the actual
   contents of the tables involved.  Also note that the numbers, and
   even the selected query strategy, might vary between
   <productname>PostgreSQL</productname> releases due to planner
   improvements. In addition, the <command>ANALYZE</command> command
   uses random sampling to estimate data statistics; therefore, it is
   possible for cost estimates to change after a fresh run of
   <command>ANALYZE</command>, even if the actual distribution of data
   in the table has not changed.
  </para>
 </refsect1>

 <refsect1>
  <title>Compatibility</title>

  <para>
   There is no <command>EXPLAIN</command> statement defined in the SQL standard.
  </para>
 </refsect1>

 <refsect1>
  <title>See Also</title>

  <simplelist type="inline">
   <member><xref linkend="sql-analyze" endterm="sql-analyze-title"></member>
  </simplelist>
 </refsect1>
</refentry>