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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>ANALYZE</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="sql-alterview.html" title="ALTER VIEW" /><link rel="next" href="sql-begin.html" title="BEGIN" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">ANALYZE</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="sql-alterview.html" title="ALTER VIEW">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="sql-commands.html" title="SQL Commands">Up</a></td><th width="60%" align="center">SQL Commands</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 11.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="sql-begin.html" title="BEGIN">Next</a></td></tr></table><hr></hr></div><div class="refentry" id="SQL-ANALYZE"><div class="titlepage"></div><a id="id-1.9.3.46.1" class="indexterm"></a><div class="refnamediv"><h2><span class="refentrytitle">ANALYZE</span></h2><p>ANALYZE — collect statistics about a database</p></div><div class="refsynopsisdiv"><h2>Synopsis</h2><pre class="synopsis">
ANALYZE [ ( <em class="replaceable"><code>option</code></em> [, ...] ) ] [ <em class="replaceable"><code>table_and_columns</code></em> [, ...] ]
ANALYZE [ VERBOSE ] [ <em class="replaceable"><code>table_and_columns</code></em> [, ...] ]
<span class="phrase">where <em class="replaceable"><code>option</code></em> can be one of:</span>
VERBOSE
<span class="phrase">and <em class="replaceable"><code>table_and_columns</code></em> is:</span>
<em class="replaceable"><code>table_name</code></em> [ ( <em class="replaceable"><code>column_name</code></em> [, ...] ) ]
</pre></div><div class="refsect1" id="id-1.9.3.46.5"><h2>Description</h2><p>
<code class="command">ANALYZE</code> collects statistics about the contents
of tables in the database, and stores the results in the <a class="link" href="catalog-pg-statistic.html" title="52.50. pg_statistic"><code class="structname">pg_statistic</code></a>
system catalog. Subsequently, the query planner uses these
statistics to help determine the most efficient execution plans for
queries.
</p><p>
Without a <em class="replaceable"><code>table_and_columns</code></em>
list, <code class="command">ANALYZE</code> processes every table and materialized view
in the current database that the current user has permission to analyze.
With a list, <code class="command">ANALYZE</code> processes only those table(s).
It is further possible to give a list of column names for a table,
in which case only the statistics for those columns are collected.
</p><p>
When the option list is surrounded by parentheses, the options can be
written in any order. The parenthesized syntax was added in
<span class="productname">PostgreSQL</span> 11; the unparenthesized syntax
is deprecated.
</p></div><div class="refsect1" id="id-1.9.3.46.6"><h2>Parameters</h2><div class="variablelist"><dl class="variablelist"><dt><span class="term"><code class="literal">VERBOSE</code></span></dt><dd><p>
Enables display of progress messages.
</p></dd><dt><span class="term"><em class="replaceable"><code>table_name</code></em></span></dt><dd><p>
The name (possibly schema-qualified) of a specific table to
analyze. If omitted, all regular tables, partitioned tables, and
materialized views in the current database are analyzed (but not
foreign tables). If the specified table is a partitioned table, both the
inheritance statistics of the partitioned table as a whole and
statistics of the individual partitions are updated.
</p></dd><dt><span class="term"><em class="replaceable"><code>column_name</code></em></span></dt><dd><p>
The name of a specific column to analyze. Defaults to all columns.
</p></dd></dl></div></div><div class="refsect1" id="id-1.9.3.46.7"><h2>Outputs</h2><p>
When <code class="literal">VERBOSE</code> is specified, <code class="command">ANALYZE</code> emits
progress messages to indicate which table is currently being
processed. Various statistics about the tables are printed as well.
</p></div><div class="refsect1" id="id-1.9.3.46.8"><h2>Notes</h2><p>
Foreign tables are analyzed only when explicitly selected. Not all
foreign data wrappers support <code class="command">ANALYZE</code>. If the table's
wrapper does not support <code class="command">ANALYZE</code>, the command prints a
warning and does nothing.
</p><p>
In the default <span class="productname">PostgreSQL</span> configuration,
the autovacuum daemon (see <a class="xref" href="routine-vacuuming.html#AUTOVACUUM" title="24.1.6. The Autovacuum Daemon">Section 24.1.6</a>)
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 <code class="command">ANALYZE</code> 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 for read-mostly databases is to run <a class="xref" href="sql-vacuum.html" title="VACUUM"><span class="refentrytitle">VACUUM</span></a>
and <code class="command">ANALYZE</code> once a day during a low-usage time of day.
(This will not be sufficient if there is heavy update activity.)
</p><p>
<code class="command">ANALYZE</code>
requires only a read lock on the target table, so it can run in
parallel with other activity on the table.
</p><p>
The statistics collected by <code class="command">ANALYZE</code> 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
<code class="command">ANALYZE</code> 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 <a class="xref" href="maintenance.html" title="Chapter 24. Routine Database Maintenance Tasks">Chapter 24</a>.
</p><p>
For large tables, <code class="command">ANALYZE</code> 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 <code class="command">ANALYZE</code> 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
<a class="xref" href="sql-explain.html" title="EXPLAIN"><span class="refentrytitle">EXPLAIN</span></a>.
In rare situations, this non-determinism will cause the planner's
choices of query plans to change after <code class="command">ANALYZE</code> is run.
To avoid this, raise the amount of statistics collected by
<code class="command">ANALYZE</code>, as described below.
</p><p>
The extent of analysis can be controlled by adjusting the
<a class="xref" href="runtime-config-query.html#GUC-DEFAULT-STATISTICS-TARGET">default_statistics_target</a> configuration variable, or
on a column-by-column basis by setting the per-column statistics
target with <code class="command">ALTER TABLE ... ALTER COLUMN ... SET
STATISTICS</code> (see <a class="xref" href="sql-altertable.html" title="ALTER TABLE"><span class="refentrytitle">ALTER TABLE</span></a>).
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
<code class="command">ANALYZE</code> and the amount of space occupied in
<code class="literal">pg_statistic</code>. 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 <code class="literal">WHERE</code>, <code class="literal">GROUP BY</code>,
or <code class="literal">ORDER BY</code> clauses of queries, since the planner will
have no use for statistics on such columns.
</p><p>
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 <code class="command">ANALYZE</code>.
</p><p>
One of the values estimated by <code class="command">ANALYZE</code> 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
<code class="command">ALTER TABLE ... ALTER COLUMN ... SET (n_distinct = ...)</code>
(see <a class="xref" href="sql-altertable.html" title="ALTER TABLE"><span class="refentrytitle">ALTER TABLE</span></a>).
</p><p>
If the table being analyzed has one or more children,
<code class="command">ANALYZE</code> will gather statistics twice: once on the
rows of the parent table only, and a second time on the rows of the
parent table with all of its children. This second set of statistics
is needed when planning queries that traverse the entire inheritance
tree. The autovacuum daemon, however, will only consider inserts or
updates on the parent table itself when deciding whether to trigger an
automatic analyze for that table. If that table is rarely inserted into
or updated, the inheritance statistics will not be up to date unless you
run <code class="command">ANALYZE</code> manually.
</p><p>
If any of the child tables are foreign tables whose foreign data wrappers
do not support <code class="command">ANALYZE</code>, those child tables are ignored while
gathering inheritance statistics.
</p><p>
If the table being analyzed is completely empty, <code class="command">ANALYZE</code>
will not record new statistics for that table. Any existing statistics
will be retained.
</p></div><div class="refsect1" id="id-1.9.3.46.9"><h2>Compatibility</h2><p>
There is no <code class="command">ANALYZE</code> statement in the SQL standard.
</p></div><div class="refsect1" id="id-1.9.3.46.10"><h2>See Also</h2><span class="simplelist"><a class="xref" href="sql-vacuum.html" title="VACUUM"><span class="refentrytitle">VACUUM</span></a>, <a class="xref" href="app-vacuumdb.html" title="vacuumdb"><span class="refentrytitle"><span class="application">vacuumdb</span></span></a>, <a class="xref" href="runtime-config-resource.html#RUNTIME-CONFIG-RESOURCE-VACUUM-COST" title="19.4.4. Cost-based Vacuum Delay">Section 19.4.4</a>, <a class="xref" href="routine-vacuuming.html#AUTOVACUUM" title="24.1.6. The Autovacuum Daemon">Section 24.1.6</a></span></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="sql-alterview.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="sql-commands.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="sql-begin.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">ALTER VIEW </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> BEGIN</td></tr></table></div></body></html>
