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PostgreSQL documentation
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<refentry id="SQL-ANALYZE">
<refmeta>
<refentrytitle id="sql-analyze-title">ANALYZE</refentrytitle>
<refmiscinfo>SQL - Language Statements</refmiscinfo>
</refmeta>
<refnamediv>
<refname>
ANALYZE
</refname>
<refpurpose>
collect statistics about a database
</refpurpose>
</refnamediv>
<refsynopsisdiv>
<refsynopsisdivinfo>
<date>2001-05-04</date>
</refsynopsisdivinfo>
<synopsis>
ANALYZE [ VERBOSE ] [ <replaceable class="PARAMETER">table</replaceable> [ (<replaceable class="PARAMETER">column</replaceable> [, ...] ) ] ]
</synopsis>
<refsect2 id="R2-SQL-ANALYZE-1">
<refsect2info>
<date>2001-05-04</date>
</refsect2info>
<title>
Inputs
</title>
<para>
<variablelist>
<varlistentry>
<term>VERBOSE</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>
</para>
</refsect2>
<refsect2 id="R2-SQL-ANALYZE-2">
<refsect2info>
<date>2001-05-04</date>
</refsect2info>
<title>
Outputs
</title>
<para>
<variablelist>
<varlistentry>
<term><computeroutput>
<returnvalue>ANALYZE</returnvalue>
</computeroutput></term>
<listitem>
<para>
The command is complete.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</refsect2>
</refsynopsisdiv>
<refsect1 id="R1-SQL-ANALYZE-1">
<refsect1info>
<date>2001-05-04</date>
</refsect1info>
<title>
Description
</title>
<para>
<command>ANALYZE</command> collects statistics about the contents of
<productname>PostgreSQL</productname> tables, and stores the results in
the system table <literal>pg_statistic</literal>. Subsequently,
the query planner uses the 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 updated.
</para>
<refsect2 id="R2-SQL-ANALYZE-3">
<refsect2info>
<date>2001-05-04</date>
</refsect2info>
<title>
Notes
</title>
<para>
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>
Unlike <command>VACUUM FULL</command>,
<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>
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 may result in small changes in the planner's
estimated costs shown by <command>EXPLAIN</command>.
</para>
<para>
The collected statistics 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 may 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 the <citetitle>User's
Guide</citetitle>.
</para>
<para>
The extent of analysis can be controlled by adjusting 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 10, 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 may be useful to do that for columns that
are never used as part of the WHERE, GROUP BY, or 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>
</refsect2>
</refsect1>
<refsect1 id="R1-SQL-ANALYZE-3">
<title>
Compatibility
</title>
<refsect2 id="R2-SQL-ANALYZE-4">
<refsect2info>
<date>2001-05-04</date>
</refsect2info>
<title>
SQL92
</title>
<para>
There is no <command>ANALYZE</command> statement in <acronym>SQL92</acronym>.
</para>
</refsect2>
</refsect1>
</refentry>
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