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<!--
$PostgreSQL: pgsql/doc/src/sgml/ref/analyze.sgml,v 1.26 2009/08/02 22:14:51 tgl Exp $
PostgreSQL documentation
-->
<refentry id="SQL-ANALYZE">
<refmeta>
<refentrytitle id="sql-analyze-title">ANALYZE</refentrytitle>
<manvolnum>7</manvolnum>
<refmiscinfo>SQL - Language Statements</refmiscinfo>
</refmeta>
<refnamediv>
<refname>ANALYZE</refname>
<refpurpose>collect statistics about a database</refpurpose>
</refnamediv>
<indexterm zone="sql-analyze">
<primary>ANALYZE</primary>
</indexterm>
<refsynopsisdiv>
<synopsis>
ANALYZE [ VERBOSE ] [ <replaceable class="PARAMETER">table</replaceable> [ ( <replaceable class="PARAMETER">column</replaceable> [, ...] ) ] ]
</synopsis>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para>
<command>ANALYZE</command> collects statistics about the contents
of tables in the database, and stores the results in the <link
linkend="catalog-pg-statistic"><structname>pg_statistic</></>
system catalog. Subsequently, the query planner uses these
statistics to help determine the most efficient execution plans for
queries.
</para>
<para>
With no parameter, <command>ANALYZE</command> examines every table in the
current database. With a parameter, <command>ANALYZE</command> examines
only that table. It is further possible to give a list of column names,
in which case only the statistics for those columns are collected.
</para>
</refsect1>
<refsect1>
<title>Parameters</title>
<variablelist>
<varlistentry>
<term><literal>VERBOSE</literal></term>
<listitem>
<para>
Enables display of progress messages.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="PARAMETER">table</replaceable></term>
<listitem>
<para>
The name (possibly schema-qualified) of a specific table to
analyze. Defaults to all tables in the current database.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="PARAMETER">column</replaceable></term>
<listitem>
<para>
The name of a specific column to analyze. Defaults to all columns.
</para>
</listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1>
<title>Outputs</title>
<para>
When <literal>VERBOSE</> is specified, <command>ANALYZE</> emits
progress messages to indicate which table is currently being
processed. Various statistics about the tables are printed as well.
</para>
</refsect1>
<refsect1>
<title>Notes</title>
<para>
In the default <productname>PostgreSQL</productname> configuration,
<xref linkend="autovacuum" endterm="autovacuum-title">
takes care of automatic analyzing of tables when they are first loaded
with data, and as they change throughout regular operation.
When autovacuum is disabled,
it is a good idea to run <command>ANALYZE</command> periodically, or
just after making major changes in the contents of a table. Accurate
statistics will help the planner to choose the most appropriate query
plan, and thereby improve the speed of query processing. A common
strategy is to run <xref linkend="sql-vacuum" endterm="sql-vacuum-title">
and <command>ANALYZE</command> once a day during a low-usage time of day.
</para>
<para>
<command>ANALYZE</command>
requires only a read lock on the target table, so it can run in
parallel with other activity on the table.
</para>
<para>
The statistics collected by <command>ANALYZE</command> usually
include a list of some of the most common values in each column and
a histogram showing the approximate data distribution in each
column. One or both of these can be omitted if
<command>ANALYZE</command> deems them uninteresting (for example,
in a unique-key column, there are no common values) or if the
column data type does not support the appropriate operators. There
is more information about the statistics in <xref
linkend="maintenance">.
</para>
<para>
For large tables, <command>ANALYZE</command> takes a random sample
of the table contents, rather than examining every row. This
allows even very large tables to be analyzed in a small amount of
time. Note, however, that the statistics are only approximate, and
will change slightly each time <command>ANALYZE</command> is run,
even if the actual table contents did not change. This might result
in small changes in the planner's estimated costs shown by
<xref linkend="sql-explain" endterm="sql-explain-title">.
In rare situations, this non-determinism will cause the planner's
choices of query plans to change after <command>ANALYZE</command> is run.
To avoid this, raise the amount of statistics collected by
<command>ANALYZE</command>, as described below.
</para>
<para>
The extent of analysis can be controlled by adjusting the
<xref linkend="guc-default-statistics-target"> configuration variable, or
on a column-by-column basis by setting the per-column statistics
target with <command>ALTER TABLE ... ALTER COLUMN ... SET
STATISTICS</command> (see <xref linkend="sql-altertable"
endterm="sql-altertable-title">). The target value sets the
maximum number of entries in the most-common-value list and the
maximum number of bins in the histogram. The default target value
is 100, but this can be adjusted up or down to trade off accuracy of
planner estimates against the time taken for
<command>ANALYZE</command> and the amount of space occupied in
<literal>pg_statistic</literal>. In particular, setting the
statistics target to zero disables collection of statistics for
that column. It might be useful to do that for columns that are
never used as part of the <literal>WHERE</>, <literal>GROUP BY</>,
or <literal>ORDER BY</> clauses of queries, since the planner will
have no use for statistics on such columns.
</para>
<para>
The largest statistics target among the columns being analyzed determines
the number of table rows sampled to prepare the statistics. Increasing
the target causes a proportional increase in the time and space needed
to do <command>ANALYZE</command>.
</para>
<para>
One of the values estimated by <command>ANALYZE</command> is the number of
distinct values that appear in each column. Because only a subset of the
rows are examined, this estimate can sometimes be quite inaccurate, even
with the largest possible statistics target. If this inaccuracy leads to
bad query plans, a more accurate value can be determined manually and then
installed with
<command>ALTER TABLE ... ALTER COLUMN ... SET STATISTICS DISTINCT</>
(see <xref linkend="sql-altertable" endterm="sql-altertable-title">).
</para>
</refsect1>
<refsect1>
<title>Compatibility</title>
<para>
There is no <command>ANALYZE</command> statement in the SQL standard.
</para>
</refsect1>
<refsect1>
<title>See Also</title>
<simplelist type="inline">
<member><xref linkend="sql-vacuum" endterm="sql-vacuum-title"></member>
<member><xref linkend="app-vacuumdb" endterm="app-vacuumdb-title"></member>
<member><xref linkend="runtime-config-resource-vacuum-cost" endterm="runtime-config-resource-vacuum-cost-title"></member>
<member><xref linkend="autovacuum" endterm="autovacuum-title"></member>
</simplelist>
</refsect1>
</refentry>
|
