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

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doc/src/sgml/ref/explain.sgml
PostgreSQL documentation
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<refentry id="sql-explain">
 <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>

 <refmeta>
  <refentrytitle>EXPLAIN</refentrytitle>
  <manvolnum>7</manvolnum>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>

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

 <refsynopsisdiv>
<synopsis>
EXPLAIN [ ( <replaceable class="parameter">option</replaceable> [, ...] ) ] <replaceable class="parameter">statement</replaceable>
EXPLAIN [ ANALYZE ] [ VERBOSE ] <replaceable class="parameter">statement</replaceable>

<phrase>where <replaceable class="parameter">option</replaceable> can be one of:</phrase>

    ANALYZE [ <replaceable class="parameter">boolean</replaceable> ]
    VERBOSE [ <replaceable class="parameter">boolean</replaceable> ]
    COSTS [ <replaceable class="parameter">boolean</replaceable> ]
    SETTINGS [ <replaceable class="parameter">boolean</replaceable> ]
    GENERIC_PLAN [ <replaceable class="parameter">boolean</replaceable> ]
    BUFFERS [ <replaceable class="parameter">boolean</replaceable> ]
    WAL [ <replaceable class="parameter">boolean</replaceable> ]
    TIMING [ <replaceable class="parameter">boolean</replaceable> ]
    SUMMARY [ <replaceable class="parameter">boolean</replaceable> ]
    FORMAT { TEXT | XML | JSON | YAML }
</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 cost units that are arbitrary, but conventionally
   mean disk page fetches).  Actually two numbers
   are shown: the start-up cost before the first row can be returned, and
   the total cost to return all the rows.  For most queries the total cost
   is what matters, but in contexts such as a subquery in <literal>EXISTS</literal>, the planner
   will choose the smallest start-up cost instead of the smallest total cost
   (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.  Then actual run time statistics are added to
   the display, including the total elapsed time expended within each plan
   node (in milliseconds) and the total number of rows it actually returned.
   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>, <command>MERGE</command>,
    <command>CREATE TABLE AS</command>,
    or <command>EXECUTE</command> statement
    without letting the command affect your data, use this approach:
<programlisting>
BEGIN;
EXPLAIN ANALYZE ...;
ROLLBACK;
</programlisting>
   </para>
  </important>

  <para>
   Only the <literal>ANALYZE</literal> and <literal>VERBOSE</literal> options
   can be specified, and only in that order, without surrounding the option
   list in parentheses.  Prior to <productname>PostgreSQL</productname> 9.0,
   the unparenthesized syntax was the only one supported.  It is expected that
   all new options will be supported only in the parenthesized syntax.
  </para>
 </refsect1>

 <refsect1>
  <title>Parameters</title>

  <variablelist>
   <varlistentry>
    <term><literal>ANALYZE</literal></term>
    <listitem>
     <para>
      Carry out the command and show actual run times and other statistics.
      This parameter defaults to <literal>FALSE</literal>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>VERBOSE</literal></term>
    <listitem>
     <para>
      Display additional information regarding the plan.  Specifically, include
      the output column list for each node in the plan tree, schema-qualify
      table and function names, always label variables in expressions with
      their range table alias, and always print the name of each trigger for
      which statistics are displayed.  The query identifier will also be
      displayed if one has been computed, see <xref
      linkend="guc-compute-query-id"/> for more details.  This parameter
      defaults to <literal>FALSE</literal>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>COSTS</literal></term>
    <listitem>
     <para>
      Include information on the estimated startup and total cost of each
      plan node, as well as the estimated number of rows and the estimated
      width of each row.
      This parameter defaults to <literal>TRUE</literal>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>SETTINGS</literal></term>
    <listitem>
     <para>
      Include information on configuration parameters.  Specifically, include
      options affecting query planning with value different from the built-in
      default value.  This parameter defaults to <literal>FALSE</literal>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>GENERIC_PLAN</literal></term>
    <listitem>
     <para>
      Allow the statement to contain parameter placeholders like
      <literal>$1</literal>, and generate a generic plan that does not
      depend on the values of those parameters.
      See <link linkend="sql-prepare"><command>PREPARE</command></link>
      for details about generic plans and the types of statement that
      support parameters.
      This parameter cannot be used together with <literal>ANALYZE</literal>.
      It defaults to <literal>FALSE</literal>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>BUFFERS</literal></term>
    <listitem>
     <para>
      Include information on buffer usage. Specifically, include the number of
      shared blocks hit, read, dirtied, and written, the number of local blocks
      hit, read, dirtied, and written, the number of temp blocks read and
      written, and the time spent reading and writing data file blocks and
      temporary file blocks (in milliseconds) if
      <xref linkend="guc-track-io-timing"/> is enabled.  A
      <emphasis>hit</emphasis> means that a read was avoided because the block
      was found already in cache when needed.
      Shared blocks contain data from regular tables and indexes;
      local blocks contain data from temporary tables and indexes;
      while temporary blocks contain short-term working data used in sorts,
      hashes, Materialize plan nodes, and similar cases.
      The number of blocks <emphasis>dirtied</emphasis> indicates the number of
      previously unmodified blocks that were changed by this query; while the
      number of blocks <emphasis>written</emphasis> indicates the number of
      previously-dirtied blocks evicted from cache by this backend during
      query processing.
      The number of blocks shown for an
      upper-level node includes those used by all its child nodes.  In text
      format, only non-zero values are printed.  This parameter defaults to
      <literal>FALSE</literal>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>WAL</literal></term>
    <listitem>
     <para>
      Include information on WAL record generation. Specifically, include the
      number of records, number of full page images (fpi) and the amount of WAL
      generated in bytes. In text format, only non-zero values are printed.
      This parameter may only be used when <literal>ANALYZE</literal> is also
      enabled.  It defaults to <literal>FALSE</literal>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>TIMING</literal></term>
    <listitem>
     <para>
      Include actual startup time and time spent in each node in the output.
      The overhead of repeatedly reading the system clock can slow down the
      query significantly on some systems, so it may be useful to set this
      parameter to <literal>FALSE</literal> when only actual row counts, and
      not exact times, are needed.  Run time of the entire statement is
      always measured, even when node-level timing is turned off with this
      option.
      This parameter may only be used when <literal>ANALYZE</literal> is also
      enabled.  It defaults to <literal>TRUE</literal>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>SUMMARY</literal></term>
    <listitem>
     <para>
      Include summary information (e.g., totaled timing information) after the
      query plan.  Summary information is included by default when
      <literal>ANALYZE</literal> is used but otherwise is not included by
      default, but can be enabled using this option.  Planning time in
      <command>EXPLAIN EXECUTE</command> includes the time required to fetch
      the plan from the cache and the time required for re-planning, if
      necessary.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>FORMAT</literal></term>
    <listitem>
     <para>
      Specify the output format, which can be TEXT, XML, JSON, or YAML.
      Non-text output contains the same information as the text output
      format, but is easier for programs to parse.  This parameter defaults to
      <literal>TEXT</literal>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">boolean</replaceable></term>
    <listitem>
     <para>
      Specifies whether the selected option should be turned on or off.
      You can write <literal>TRUE</literal>, <literal>ON</literal>, or
      <literal>1</literal> to enable the option, and <literal>FALSE</literal>,
      <literal>OFF</literal>, or <literal>0</literal> to disable it.  The
      <replaceable class="parameter">boolean</replaceable> value can also
      be omitted, in which case <literal>TRUE</literal> is assumed.
     </para>
    </listitem>
   </varlistentry>

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

 <refsect1>
  <title>Outputs</title>

   <para>
    The command's result is a textual description of the plan selected
    for the <replaceable class="parameter">statement</replaceable>,
    optionally annotated with execution statistics.
    <xref linkend="using-explain"/> describes the information provided.
   </para>
 </refsect1>

 <refsect1>
  <title>Notes</title>

  <para>
   In order to allow the <productname>PostgreSQL</productname> query
   planner to make reasonably informed decisions when optimizing
   queries, the <link
   linkend="catalog-pg-statistic"><structname>pg_statistic</structname></link>
   data should be up-to-date for all tables used in the query.  Normally
   the <link linkend="autovacuum">autovacuum daemon</link> will take care
   of that automatically.  But if a table has recently had substantial
   changes in its contents, you might need to do a manual
   <link linkend="sql-analyze"><command>ANALYZE</command></link> rather than wait for autovacuum to catch up
   with the changes.
  </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> adds 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, as well as the platform being used.  The worst case occurs
   for plan nodes that in themselves require very little time per
   execution, and on machines that have relatively slow operating
   system calls for obtaining the time of day.
  </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>
  Here is the same query, with JSON output formatting:
<programlisting>
EXPLAIN (FORMAT JSON) SELECT * FROM foo;
           QUERY PLAN
--------------------------------
 [                             +
   {                           +
     "Plan": {                 +
       "Node Type": "Seq Scan",+
       "Relation Name": "foo", +
       "Alias": "foo",         +
       "Startup Cost": 0.00,   +
       "Total Cost": 155.00,   +
       "Plan Rows": 10000,     +
       "Plan 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 the same query, but in YAML format:
<programlisting>
EXPLAIN (FORMAT YAML) SELECT * FROM foo WHERE i='4';
          QUERY PLAN
-------------------------------
 - Plan:                      +
     Node Type: "Index Scan"  +
     Scan Direction: "Forward"+
     Index Name: "fi"         +
     Relation Name: "foo"     +
     Alias: "foo"             +
     Startup Cost: 0.00       +
     Total Cost: 5.98         +
     Plan Rows: 1             +
     Plan Width: 4            +
     Index Cond: "(i = 4)"
(1 row)
</programlisting>

    XML format is left as an exercise for the reader.
  </para>
  <para>
   Here is the same plan with cost estimates suppressed:

<programlisting>
EXPLAIN (COSTS FALSE) SELECT * FROM foo WHERE i = 4;

        QUERY PLAN
----------------------------
 Index Scan using fi on foo
   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
-------------------------------------------------------------------&zwsp;--
 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
-------------------------------------------------------------------&zwsp;------------------------------------------------------
 HashAggregate  (cost=10.77..10.87 rows=10 width=12) (actual time=0.043..0.044 rows=10 loops=1)
   Group Key: foo
   Batches: 1  Memory Usage: 24kB
   -&gt;  Index Scan using test_pkey on test  (cost=0.29..10.27 rows=99 width=8) (actual time=0.009..0.025 rows=99 loops=1)
         Index Cond: ((id &gt; 100) AND (id &lt; 200))
 Planning Time: 0.244 ms
 Execution Time: 0.073 ms
(7 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>

  <para>
   Notice that the previous example showed a <quote>custom</quote> plan
   for the specific parameter values given in <command>EXECUTE</command>.
   We might also wish to see the generic plan for a parameterized
   query, which can be done with <literal>GENERIC_PLAN</literal>:

<programlisting>
EXPLAIN (GENERIC_PLAN)
  SELECT sum(bar) FROM test
    WHERE id &gt; $1 AND id &lt; $2
    GROUP BY foo;

                                  QUERY PLAN
-------------------------------------------------------------------&zwsp;------------
 HashAggregate  (cost=26.79..26.89 rows=10 width=12)
   Group Key: foo
   -&gt;  Index Scan using test_pkey on test  (cost=0.29..24.29 rows=500 width=8)
         Index Cond: ((id &gt; $1) AND (id &lt; $2))
(4 rows)
</programlisting>

   In this case the parser correctly inferred that <literal>$1</literal>
   and <literal>$2</literal> should have the same data type
   as <literal>id</literal>, so the lack of parameter type information
   from <command>PREPARE</command> was not a problem.  In other cases
   it might be necessary to explicitly specify types for the parameter
   symbols, which can be done by casting them, for example:

<programlisting>
EXPLAIN (GENERIC_PLAN)
  SELECT sum(bar) FROM test
    WHERE id &gt; $1::integer AND id &lt; $2::integer
    GROUP BY foo;
</programlisting>
  </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"/></member>
  </simplelist>
 </refsect1>
</refentry>