explain.sgml

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$PostgreSQL: pgsql/doc/src/sgml/ref/explain.sgml,v 1.39 2007/01/31 23:26:04 momjian 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
    <literal>ANALYZE</literal> 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 <command>ANALYZE</command> 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 tables exceeds
   <varname>geqo_threshold</> causing genetic query optimization to be 
   used, the execution plan is likely to change each time the statement
   is executed.
  </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>
   And 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>