mirror of https://github.com/postgres/postgres
You can not select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
264 lines
8.6 KiB
264 lines
8.6 KiB
<!--
|
|
$Header: /cvsroot/pgsql/doc/src/sgml/ref/explain.sgml,v 1.26 2003/04/26 23:56:51 petere Exp $
|
|
PostgreSQL documentation
|
|
-->
|
|
|
|
<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>
|
|
|
|
<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 statenebt. The execution plan shows how the table(s)
|
|
referenced by the statement will be scanned---by plain sequential scan,
|
|
index scan, etc.---and if multiple tables are referenced, what join
|
|
algorithms will be used to bring together the required row 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>Parameter</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
|
|
debugging <productname>PostgreSQL</productname>. The
|
|
<literal>VERBOSE</literal> output is either pretty-printed or
|
|
not, depending on the setting of the
|
|
<varname>explain_pretty_print</varname> 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>EXECUTE</>, or <command>DECLARE</>
|
|
statement, whose execution plan you wish to see.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
</variablelist>
|
|
</refsect1>
|
|
|
|
<refsect1>
|
|
<title>Diagnostics</title>
|
|
|
|
<para>
|
|
<command>EXPLAIN</command> prints the execution plan of the
|
|
specified statement from the <productname>PostgreSQL</productname>
|
|
planner.
|
|
</para>
|
|
</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 may be chosen.
|
|
</para>
|
|
|
|
<para>
|
|
Prior to <productname>PostgreSQL</productname> 7.3, the plan was
|
|
emitted in the form of a <literal>NOTICE</literal> message. Now it
|
|
appears as a query result (formatted like a table with a single
|
|
text column).
|
|
</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 < 10;
|
|
|
|
QUERY PLAN
|
|
---------------------------------------------------------------------
|
|
Aggregate (cost=23.93..23.93 rows=1 width=4)
|
|
-> Index Scan using fi on foo (cost=0.00..23.92 rows=6 width=4)
|
|
Index Cond: (i < 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 > $1 AND id < $2
|
|
GROUP BY foo;
|
|
|
|
EXPLAIN ANALYZE EXECUTE query(100, 200);
|
|
|
|
QUERY PLAN
|
|
-------------------------------------------------------------------------------------------------------------------------
|
|
HashAggregate (cost=39.53..39.53 rows=1 width=8) (actual time=0.66..0.67 rows=7 loops=1)
|
|
-> Index Scan using test_pkey on test (cost=0.00..32.97 rows=1311 width=8) (actual time=0.05..0.39 rows=99 loops=1)
|
|
Index Cond: ((id > $1) AND (id < $2))
|
|
Total runtime: 0.85 msec
|
|
(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, may 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>
|
|
</refentry>
|
|
|
|
<!-- Keep this comment at the end of the file
|
|
Local variables:
|
|
mode: sgml
|
|
sgml-omittag:nil
|
|
sgml-shorttag:t
|
|
sgml-minimize-attributes:nil
|
|
sgml-always-quote-attributes:t
|
|
sgml-indent-step:1
|
|
sgml-indent-data:t
|
|
sgml-parent-document:nil
|
|
sgml-default-dtd-file:"../reference.ced"
|
|
sgml-exposed-tags:nil
|
|
sgml-local-catalogs:"/usr/lib/sgml/catalog"
|
|
sgml-local-ecat-files:nil
|
|
End:
|
|
-->
|
|
|