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<!--
doc/src/sgml/ref/vacuum.sgml
PostgreSQL documentation
-->
<refentry id="sql-vacuum">
<indexterm zone="sql-vacuum">
<primary>VACUUM</primary>
</indexterm>
<refmeta>
<refentrytitle>VACUUM</refentrytitle>
<manvolnum>7</manvolnum>
<refmiscinfo>SQL - Language Statements</refmiscinfo>
</refmeta>
<refnamediv>
<refname>VACUUM</refname>
<refpurpose>garbage-collect and optionally analyze a database</refpurpose>
</refnamediv>
<refsynopsisdiv>
<synopsis>
VACUUM [ ( <replaceable class="parameter">option</replaceable> [, ...] ) ] [ <replaceable class="parameter">table_and_columns</replaceable> [, ...] ]
<phrase>where <replaceable class="parameter">option</replaceable> can be one of:</phrase>
FULL [ <replaceable class="parameter">boolean</replaceable> ]
FREEZE [ <replaceable class="parameter">boolean</replaceable> ]
VERBOSE [ <replaceable class="parameter">boolean</replaceable> ]
ANALYZE [ <replaceable class="parameter">boolean</replaceable> ]
DISABLE_PAGE_SKIPPING [ <replaceable class="parameter">boolean</replaceable> ]
SKIP_LOCKED [ <replaceable class="parameter">boolean</replaceable> ]
INDEX_CLEANUP { AUTO | ON | OFF }
PROCESS_MAIN [ <replaceable class="parameter">boolean</replaceable> ]
PROCESS_TOAST [ <replaceable class="parameter">boolean</replaceable> ]
TRUNCATE [ <replaceable class="parameter">boolean</replaceable> ]
PARALLEL <replaceable class="parameter">integer</replaceable>
SKIP_DATABASE_STATS [ <replaceable class="parameter">boolean</replaceable> ]
ONLY_DATABASE_STATS [ <replaceable class="parameter">boolean</replaceable> ]
BUFFER_USAGE_LIMIT <replaceable class="parameter">size</replaceable>
<phrase>and <replaceable class="parameter">table_and_columns</replaceable> is:</phrase>
<replaceable class="parameter">table_name</replaceable> [ ( <replaceable class="parameter">column_name</replaceable> [, ...] ) ]
</synopsis>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para>
<command>VACUUM</command> reclaims storage occupied by dead tuples.
In normal <productname>PostgreSQL</productname> operation, tuples that
are deleted or obsoleted by an update are not physically removed from
their table; they remain present until a <command>VACUUM</command> is
done. Therefore it's necessary to do <command>VACUUM</command>
periodically, especially on frequently-updated tables.
</para>
<para>
Without a <replaceable class="parameter">table_and_columns</replaceable>
list, <command>VACUUM</command> processes every table and materialized view
in the current database that the current user has permission to vacuum.
With a list, <command>VACUUM</command> processes only those table(s).
</para>
<para>
<command>VACUUM ANALYZE</command> performs a <command>VACUUM</command>
and then an <command>ANALYZE</command> for each selected table. This
is a handy combination form for routine maintenance scripts. See
<xref linkend="sql-analyze"/>
for more details about its processing.
</para>
<para>
Plain <command>VACUUM</command> (without <literal>FULL</literal>) simply reclaims
space and makes it
available for re-use. This form of the command can operate in parallel
with normal reading and writing of the table, as an exclusive lock
is not obtained. However, extra space is not returned to the operating
system (in most cases); it's just kept available for re-use within the
same table. It also allows us to leverage multiple CPUs in order to process
indexes. This feature is known as <firstterm>parallel vacuum</firstterm>.
To disable this feature, one can use <literal>PARALLEL</literal> option and
specify parallel workers as zero. <command>VACUUM FULL</command> rewrites
the entire contents of the table into a new disk file with no extra space,
allowing unused space to be returned to the operating system. This form is
much slower and requires an <literal>ACCESS EXCLUSIVE</literal> lock on
each table while it is being processed.
</para>
</refsect1>
<refsect1>
<title>Parameters</title>
<variablelist>
<varlistentry>
<term><literal>FULL</literal></term>
<listitem>
<para>
Selects <quote>full</quote> vacuum, which can reclaim more
space, but takes much longer and exclusively locks the table.
This method also requires extra disk space, since it writes a
new copy of the table and doesn't release the old copy until
the operation is complete. Usually this should only be used when a
significant amount of space needs to be reclaimed from within the table.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>FREEZE</literal></term>
<listitem>
<para>
Selects aggressive <quote>freezing</quote> of tuples.
Specifying <literal>FREEZE</literal> is equivalent to performing
<command>VACUUM</command> with the
<xref linkend="guc-vacuum-freeze-min-age"/> and
<xref linkend="guc-vacuum-freeze-table-age"/> parameters
set to zero. Aggressive freezing is always performed when the
table is rewritten, so this option is redundant when <literal>FULL</literal>
is specified.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>VERBOSE</literal></term>
<listitem>
<para>
Prints a detailed vacuum activity report for each table.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>ANALYZE</literal></term>
<listitem>
<para>
Updates statistics used by the planner to determine the most
efficient way to execute a query.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>DISABLE_PAGE_SKIPPING</literal></term>
<listitem>
<para>
Normally, <command>VACUUM</command> will skip pages based on the <link
linkend="vacuum-for-visibility-map">visibility map</link>. Pages where
all tuples are known to be frozen can always be skipped, and those
where all tuples are known to be visible to all transactions may be
skipped except when performing an aggressive vacuum. Furthermore,
except when performing an aggressive vacuum, some pages may be skipped
in order to avoid waiting for other sessions to finish using them.
This option disables all page-skipping behavior, and is intended to
be used only when the contents of the visibility map are
suspect, which should happen only if there is a hardware or software
issue causing database corruption.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>SKIP_LOCKED</literal></term>
<listitem>
<para>
Specifies that <command>VACUUM</command> should not wait for any
conflicting locks to be released when beginning work on a relation:
if a relation cannot be locked immediately without waiting, the relation
is skipped. Note that even with this option,
<command>VACUUM</command> may still block when opening the relation's
indexes. Additionally, <command>VACUUM ANALYZE</command> may still
block when acquiring sample rows from partitions, table inheritance
children, and some types of foreign tables. Also, while
<command>VACUUM</command> ordinarily processes all partitions of
specified partitioned tables, this option will cause
<command>VACUUM</command> to skip all partitions if there is a
conflicting lock on the partitioned table.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>INDEX_CLEANUP</literal></term>
<listitem>
<para>
Normally, <command>VACUUM</command> will skip index vacuuming
when there are very few dead tuples in the table. The cost of
processing all of the table's indexes is expected to greatly
exceed the benefit of removing dead index tuples when this
happens. This option can be used to force
<command>VACUUM</command> to process indexes when there are more
than zero dead tuples. The default is <literal>AUTO</literal>,
which allows <command>VACUUM</command> to skip index vacuuming
when appropriate. If <literal>INDEX_CLEANUP</literal> is set to
<literal>ON</literal>, <command>VACUUM</command> will
conservatively remove all dead tuples from indexes. This may be
useful for backwards compatibility with earlier releases of
<productname>PostgreSQL</productname> where this was the
standard behavior.
</para>
<para>
<literal>INDEX_CLEANUP</literal> can also be set to
<literal>OFF</literal> to force <command>VACUUM</command> to
<emphasis>always</emphasis> skip index vacuuming, even when
there are many dead tuples in the table. This may be useful
when it is necessary to make <command>VACUUM</command> run as
quickly as possible to avoid imminent transaction ID wraparound
(see <xref linkend="vacuum-for-wraparound"/>). However, the
wraparound failsafe mechanism controlled by <xref
linkend="guc-vacuum-failsafe-age"/> will generally trigger
automatically to avoid transaction ID wraparound failure, and
should be preferred. If index cleanup is not performed
regularly, performance may suffer, because as the table is
modified indexes will accumulate dead tuples and the table
itself will accumulate dead line pointers that cannot be removed
until index cleanup is completed.
</para>
<para>
This option has no effect for tables that have no index and is
ignored if the <literal>FULL</literal> option is used. It also
has no effect on the transaction ID wraparound failsafe
mechanism. When triggered it will skip index vacuuming, even
when <literal>INDEX_CLEANUP</literal> is set to
<literal>ON</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>PROCESS_MAIN</literal></term>
<listitem>
<para>
Specifies that <command>VACUUM</command> should attempt to process the
main relation. This is usually the desired behavior and is the default.
Setting this option to false may be useful when it is only necessary to
vacuum a relation's corresponding <literal>TOAST</literal> table.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>PROCESS_TOAST</literal></term>
<listitem>
<para>
Specifies that <command>VACUUM</command> should attempt to process the
corresponding <literal>TOAST</literal> table for each relation, if one
exists. This is usually the desired behavior and is the default.
Setting this option to false may be useful when it is only necessary to
vacuum the main relation. This option is required when the
<literal>FULL</literal> option is used.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>TRUNCATE</literal></term>
<listitem>
<para>
Specifies that <command>VACUUM</command> should attempt to
truncate off any empty pages at the end of the table and allow
the disk space for the truncated pages to be returned to
the operating system. This is normally the desired behavior
and is the default unless the <literal>vacuum_truncate</literal>
option has been set to false for the table to be vacuumed.
Setting this option to false may be useful to avoid
<literal>ACCESS EXCLUSIVE</literal> lock on the table that
the truncation requires. This option is ignored if the
<literal>FULL</literal> option is used.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>PARALLEL</literal></term>
<listitem>
<para>
Perform index vacuum and index cleanup phases of <command>VACUUM</command>
in parallel using <replaceable class="parameter">integer</replaceable>
background workers (for the details of each vacuum phase, please
refer to <xref linkend="vacuum-phases"/>). The number of workers used
to perform the operation is equal to the number of indexes on the
relation that support parallel vacuum which is limited by the number of
workers specified with <literal>PARALLEL</literal> option if any which is
further limited by <xref linkend="guc-max-parallel-maintenance-workers"/>.
An index can participate in parallel vacuum if and only if the size of the
index is more than <xref linkend="guc-min-parallel-index-scan-size"/>.
Please note that it is not guaranteed that the number of parallel workers
specified in <replaceable class="parameter">integer</replaceable> will be
used during execution. It is possible for a vacuum to run with fewer
workers than specified, or even with no workers at all. Only one worker
can be used per index. So parallel workers are launched only when there
are at least <literal>2</literal> indexes in the table. Workers for
vacuum are launched before the start of each phase and exit at the end of
the phase. These behaviors might change in a future release. This
option can't be used with the <literal>FULL</literal> option.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>SKIP_DATABASE_STATS</literal></term>
<listitem>
<para>
Specifies that <command>VACUUM</command> should skip updating the
database-wide statistics about oldest unfrozen XIDs. Normally
<command>VACUUM</command> will update these statistics once at the
end of the command. However, this can take awhile in a database
with a very large number of tables, and it will accomplish nothing
unless the table that had contained the oldest unfrozen XID was
among those vacuumed. Moreover, if multiple <command>VACUUM</command>
commands are issued in parallel, only one of them can update the
database-wide statistics at a time. Therefore, if an application
intends to issue a series of many <command>VACUUM</command>
commands, it can be helpful to set this option in all but the last
such command; or set it in all the commands and separately
issue <literal>VACUUM (ONLY_DATABASE_STATS)</literal> afterwards.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>ONLY_DATABASE_STATS</literal></term>
<listitem>
<para>
Specifies that <command>VACUUM</command> should do nothing except
update the database-wide statistics about oldest unfrozen XIDs.
When this option is specified,
the <replaceable class="parameter">table_and_columns</replaceable>
list must be empty, and no other option may be enabled
except <literal>VERBOSE</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>BUFFER_USAGE_LIMIT</literal></term>
<listitem>
<para>
Specifies the
<glossterm linkend="glossary-buffer-access-strategy">Buffer Access Strategy</glossterm>
ring buffer size for <command>VACUUM</command>. This size is used to
calculate the number of shared buffers which will be reused as part of
this strategy. <literal>0</literal> disables use of a
<literal>Buffer Access Strategy</literal>. If <option>ANALYZE</option>
is also specified, the <option>BUFFER_USAGE_LIMIT</option> value is used
for both the vacuum and analyze stages. This option can't be used with
the <option>FULL</option> option except if <option>ANALYZE</option> is
also specified. When this option is not specified,
<command>VACUUM</command> uses the value from
<xref linkend="guc-vacuum-buffer-usage-limit"/>. Higher settings can
allow <command>VACUUM</command> to run more quickly, but having too
large a setting may cause too many other useful pages to be evicted from
shared buffers. The minimum value is <literal>128 kB</literal> and the
maximum value is <literal>16 GB</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">integer</replaceable></term>
<listitem>
<para>
Specifies a non-negative integer value passed to the selected option.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="parameter">size</replaceable></term>
<listitem>
<para>
Specifies an amount of memory in kilobytes. Sizes may also be specified
as a string containing the numerical size followed by any one of the
following memory units: <literal>B</literal> (bytes),
<literal>kB</literal> (kilobytes), <literal>MB</literal> (megabytes),
<literal>GB</literal> (gigabytes), or <literal>TB</literal> (terabytes).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="parameter">table_name</replaceable></term>
<listitem>
<para>
The name (optionally schema-qualified) of a specific table or
materialized view to vacuum. If the specified table is a partitioned
table, all of its leaf partitions are vacuumed.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="parameter">column_name</replaceable></term>
<listitem>
<para>
The name of a specific column to analyze. Defaults to all columns.
If a column list is specified, <literal>ANALYZE</literal> must also be
specified.
</para>
</listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1>
<title>Outputs</title>
<para>
When <literal>VERBOSE</literal> is specified, <command>VACUUM</command> 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>
To vacuum a table, one must ordinarily be the table's owner or a
superuser. However, database owners are allowed to
vacuum all tables in their databases, except shared catalogs.
(The restriction for shared catalogs means that a true database-wide
<command>VACUUM</command> can only be performed by a superuser.)
<command>VACUUM</command> will skip over any tables that the calling user
does not have permission to vacuum.
</para>
<para>
While <command>VACUUM</command> is running, the <xref
linkend="guc-search-path"/> is temporarily changed to <literal>pg_catalog,
pg_temp</literal>.
</para>
<para>
<command>VACUUM</command> cannot be executed inside a transaction block.
</para>
<para>
For tables with <acronym>GIN</acronym> indexes, <command>VACUUM</command> (in
any form) also completes any pending index insertions, by moving pending
index entries to the appropriate places in the main <acronym>GIN</acronym> index
structure. See <xref linkend="gin-fast-update"/> for details.
</para>
<para>
We recommend that all databases be vacuumed regularly in
order to remove dead rows. <productname>PostgreSQL</productname> includes
an <quote>autovacuum</quote> facility which can automate routine vacuum
maintenance. For more information about automatic and manual vacuuming,
see <xref linkend="routine-vacuuming"/>.
</para>
<para>
The <option>FULL</option> option is not recommended for routine use,
but might be useful in special cases. An example is when you have deleted
or updated most of the rows in a table and would like the table to
physically shrink to occupy less disk space and allow faster table
scans. <command>VACUUM FULL</command> will usually shrink the table
more than a plain <command>VACUUM</command> would.
</para>
<para>
The <option>PARALLEL</option> option is used only for vacuum purposes.
If this option is specified with the <option>ANALYZE</option> option,
it does not affect <option>ANALYZE</option>.
</para>
<para>
<command>VACUUM</command> causes a substantial increase in I/O traffic,
which might cause poor performance for other active sessions. Therefore,
it is sometimes advisable to use the cost-based vacuum delay feature. For
parallel vacuum, each worker sleeps in proportion to the work done by that
worker. See <xref linkend="runtime-config-resource-vacuum-cost"/> for
details.
</para>
<para>
Each backend running <command>VACUUM</command> without the
<literal>FULL</literal> option will report its progress in the
<structname>pg_stat_progress_vacuum</structname> view. Backends running
<command>VACUUM FULL</command> will instead report their progress in the
<structname>pg_stat_progress_cluster</structname> view. See
<xref linkend="vacuum-progress-reporting"/> and
<xref linkend="cluster-progress-reporting"/> for details.
</para>
</refsect1>
<refsect1>
<title>Examples</title>
<para>
To clean a single table <literal>onek</literal>, analyze it for
the optimizer and print a detailed vacuum activity report:
<programlisting>
VACUUM (VERBOSE, ANALYZE) onek;
</programlisting></para>
</refsect1>
<refsect1>
<title>Compatibility</title>
<para>
There is no <command>VACUUM</command> statement in the SQL standard.
</para>
<para>
The following syntax was used before <productname>PostgreSQL</productname>
version 9.0 and is still supported:
<synopsis>
VACUUM [ FULL ] [ FREEZE ] [ VERBOSE ] [ ANALYZE ] [ <replaceable class="parameter">table_and_columns</replaceable> [, ...] ]
</synopsis>
Note that in this syntax, the options must be specified in exactly the order
shown.
</para>
</refsect1>
<refsect1>
<title>See Also</title>
<simplelist type="inline">
<member><xref linkend="app-vacuumdb"/></member>
<member><xref linkend="runtime-config-resource-vacuum-cost"/></member>
<member><xref linkend="autovacuum"/></member>
<member><xref linkend="vacuum-progress-reporting"/></member>
<member><xref linkend="cluster-progress-reporting"/></member>
</simplelist>
</refsect1>
</refentry>
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