NAME¶
VACUUM - garbage-collect and optionally analyze a database
SYNOPSIS¶
VACUUM [ ( option [, ...] ) ] [ table_and_columns [, ...] ]
VACUUM [ FULL ] [ FREEZE ] [ VERBOSE ] [ ANALYZE ] [ table_and_columns [, ...] ]
where option can be one of:
FULL [ boolean ]
FREEZE [ boolean ]
VERBOSE [ boolean ]
ANALYZE [ boolean ]
DISABLE_PAGE_SKIPPING [ boolean ]
SKIP_LOCKED [ boolean ]
INDEX_CLEANUP { AUTO | ON | OFF }
PROCESS_MAIN [ boolean ]
PROCESS_TOAST [ boolean ]
TRUNCATE [ boolean ]
PARALLEL integer
SKIP_DATABASE_STATS [ boolean ]
ONLY_DATABASE_STATS [ boolean ]
BUFFER_USAGE_LIMIT size
and table_and_columns is:
table_name [ ( column_name [, ...] ) ]
DESCRIPTION¶
VACUUM reclaims storage occupied by dead tuples. In normal
PostgreSQL operation, tuples that are deleted or obsoleted by an update are
not physically removed from their table; they remain present until a
VACUUM is done. Therefore it's necessary to do VACUUM
periodically, especially on frequently-updated tables.
Without a table_and_columns list, VACUUM processes
every table and materialized view in the current database that the current
user has permission to vacuum. With a list, VACUUM processes only
those table(s).
VACUUM ANALYZE performs a VACUUM and then an
ANALYZE for each selected table. This is a handy combination form for
routine maintenance scripts. See ANALYZE(7) for more details about
its processing.
Plain VACUUM (without FULL) 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 parallel vacuum. To disable this feature, one can use
PARALLEL option and specify parallel workers as zero. VACUUM FULL
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 ACCESS EXCLUSIVE lock on each table
while it is being processed.
When the option list is surrounded by parentheses, the options can
be written in any order. Without parentheses, options must be specified in
exactly the order shown above. The parenthesized syntax was added in
PostgreSQL 9.0; the unparenthesized syntax is deprecated.
PARAMETERS¶
FULL
Selects “full” 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.
FREEZE
Selects aggressive “freezing” of tuples.
Specifying FREEZE is equivalent to performing VACUUM with the
vacuum_freeze_min_age and vacuum_freeze_table_age parameters set to zero.
Aggressive freezing is always performed when the table is rewritten, so this
option is redundant when FULL is specified.
VERBOSE
Prints a detailed vacuum activity report for each
table.
ANALYZE
Updates statistics used by the planner to determine the
most efficient way to execute a query.
DISABLE_PAGE_SKIPPING
Normally, VACUUM will skip pages based on the
visibility map. 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.
SKIP_LOCKED
Specifies that VACUUM 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, VACUUM may still block when
opening the relation's indexes. Additionally, VACUUM ANALYZE may still
block when acquiring sample rows from partitions, table inheritance children,
and some types of foreign tables. Also, while VACUUM ordinarily
processes all partitions of specified partitioned tables, this option will
cause VACUUM to skip all partitions if there is a conflicting lock on
the partitioned table.
INDEX_CLEANUP
Normally,
VACUUM 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
VACUUM
to process indexes when there are more than zero dead tuples. The default is
AUTO, which allows
VACUUM to skip index vacuuming when appropriate. If
INDEX_CLEANUP is set to ON,
VACUUM will conservatively remove all dead
tuples from indexes. This may be useful for backwards compatibility with
earlier releases of PostgreSQL where this was the standard behavior.
INDEX_CLEANUP can also be set to OFF to force VACUUM to
always skip index vacuuming, even when there are many dead tuples in
the table. This may be useful when it is necessary to make VACUUM run
as quickly as possible to avoid imminent transaction ID wraparound (see
Section 25.1.5). However, the wraparound failsafe mechanism
controlled by 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.
This option has no effect for tables that have no index and is
ignored if the FULL option is used. It also has no effect on the transaction
ID wraparound failsafe mechanism. When triggered it will skip index
vacuuming, even when INDEX_CLEANUP is set to ON.
PROCESS_MAIN
Specifies that VACUUM 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 TOAST table.
PROCESS_TOAST
Specifies that VACUUM should attempt to process
the corresponding TOAST 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 FULL option is used.
TRUNCATE
Specifies that VACUUM 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 vacuum_truncate option has been
set to false for the table to be vacuumed. Setting this option to false may be
useful to avoid ACCESS EXCLUSIVE lock on the table that the truncation
requires. This option is ignored if the FULL option is used.
PARALLEL
Perform index vacuum and index cleanup phases of
VACUUM in parallel using integer background workers (for the
details of each vacuum phase, please refer to Table 28.45). 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 PARALLEL option if any which is further limited by
max_parallel_maintenance_workers. An index can participate in parallel vacuum
if and only if the size of the index is more than
min_parallel_index_scan_size. Please note that it is not guaranteed that the
number of parallel workers specified in integer 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 2 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 FULL option.
SKIP_DATABASE_STATS
Specifies that VACUUM should skip updating the
database-wide statistics about oldest unfrozen XIDs. Normally VACUUM
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 VACUUM 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
VACUUM 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 VACUUM
(ONLY_DATABASE_STATS) afterwards.
ONLY_DATABASE_STATS
Specifies that VACUUM should do nothing except
update the database-wide statistics about oldest unfrozen XIDs. When this
option is specified, the table_and_columns list must be empty, and no
other option may be enabled except VERBOSE.
BUFFER_USAGE_LIMIT
Specifies the Buffer Access Strategy ring buffer size for
VACUUM. This size is used to calculate the number of shared buffers
which will be reused as part of this strategy. 0 disables use of a Buffer
Access Strategy. If ANALYZE is also specified, the
BUFFER_USAGE_LIMIT value is used for both the vacuum and analyze
stages. This option can't be used with the FULL option except if
ANALYZE is also specified. When this option is not specified,
VACUUM uses the value from vacuum_buffer_usage_limit. Higher settings
can allow VACUUM 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 128 kB and the maximum value is 16 GB.
boolean
Specifies whether the selected option should be turned on
or off. You can write TRUE, ON, or 1 to enable the option, and FALSE, OFF, or
0 to disable it. The boolean value can also be omitted, in which case
TRUE is assumed.
integer
Specifies a non-negative integer value passed to the
selected option.
size
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: B (bytes), kB (kilobytes), MB (megabytes),
GB (gigabytes), or TB (terabytes).
table_name
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.
column_name
The name of a specific column to analyze. Defaults to all
columns. If a column list is specified, ANALYZE must also be specified.
OUTPUTS¶
When VERBOSE is specified, VACUUM emits progress messages
to indicate which table is currently being processed. Various statistics
about the tables are printed as well.
NOTES¶
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 VACUUM can only be performed
by a superuser.) VACUUM will skip over any tables that the calling
user does not have permission to vacuum.
VACUUM cannot be executed inside a transaction block.
For tables with GIN indexes, VACUUM (in any form) also
completes any pending index insertions, by moving pending index entries to
the appropriate places in the main GIN index structure. See
Section 70.4.1 for details.
We recommend that all databases be vacuumed regularly in order to
remove dead rows. PostgreSQL includes an “autovacuum” facility
which can automate routine vacuum maintenance. For more information about
automatic and manual vacuuming, see Section 25.1.
The FULL 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. VACUUM
FULL will usually shrink the table more than a plain VACUUM
would.
The PARALLEL option is used only for vacuum purposes. If
this option is specified with the ANALYZE option, it does not affect
ANALYZE.
VACUUM 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 Section 20.4.4 for details.
Each backend running VACUUM without the FULL option will
report its progress in the pg_stat_progress_vacuum view. Backends running
VACUUM FULL will instead report their progress in the
pg_stat_progress_cluster view. See Section 28.4.5 and
Section 28.4.2 for details.
EXAMPLES¶
To clean a single table onek, analyze it for the optimizer and
print a detailed vacuum activity report:
VACUUM (VERBOSE, ANALYZE) onek;
COMPATIBILITY¶
There is no VACUUM statement in the SQL standard.
SEE ALSO¶
vacuumdb(1), Section 20.4.4, Section 25.1.6,
Section 28.4.5, Section 28.4.2
