VACUUM
VACUUM — garbage-collect and optionally analyze a database
Synopsis
VACUUM [ (option
[, ...] ) ] [table_and_columns
[, ...] ] whereoption
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
] PARALLELinteger
SKIP_DATABASE_STATS [boolean
] ONLY_DATABASE_STATS [boolean
] BUFFER_USAGE_LIMITsize
andtable_and_columns
is:table_name
[ (column_name
[, ...] ) ]
Description
VACUUM
reclaims storage occupied by dead tuples. In normal Postgres Pro 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 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.
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 performingVACUUM
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 whenFULL
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, whileVACUUM
ordinarily processes all partitions of specified partitioned tables, this option will causeVACUUM
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 forceVACUUM
to process indexes when there are more than zero dead tuples. The default isAUTO
, which allowsVACUUM
to skip index vacuuming when appropriate. IfINDEX_CLEANUP
is set toON
,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 toOFF
to forceVACUUM
to always skip index vacuuming, even when there are many dead tuples in the table. This may be useful when it is necessary to makeVACUUM
run as quickly as possible to avoid imminent transaction ID wraparound (see Section 23.1.5). 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 whenINDEX_CLEANUP
is set toON
.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 correspondingTOAST
table.PROCESS_TOAST
Specifies that
VACUUM
should attempt to process the correspondingTOAST
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 theFULL
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 thevacuum_truncate
option has been set to false for the table to be vacuumed. Setting this option to false may be useful to avoidACCESS EXCLUSIVE
lock on the table that the truncation requires. This option is ignored if theFULL
option is used.PARALLEL
Perform index vacuum and index cleanup phases of
VACUUM
in parallel usinginteger
background workers (for the details of each vacuum phase, please refer to Table 26.46). 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 withPARALLEL
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 ininteger
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 least2
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 theFULL
option.SKIP_DATABASE_STATS
Specifies that
VACUUM
should skip updating the database-wide statistics about oldest unfrozen XIDs. NormallyVACUUM
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 multipleVACUUM
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 manyVACUUM
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 issueVACUUM (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, thetable_and_columns
list must be empty, and no other option may be enabled exceptVERBOSE
.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 aBuffer Access Strategy
. IfANALYZE
is also specified, theBUFFER_USAGE_LIMIT
value is used for both the vacuum and analyze stages. This option can't be used with theFULL
option except ifANALYZE
is also specified. When this option is not specified,VACUUM
uses the value from vacuum_buffer_usage_limit. Higher settings can allowVACUUM
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 is128 kB
and the maximum value is16 GB
.boolean
Specifies whether the selected option should be turned on or off. You can write
TRUE
,ON
, or1
to enable the option, andFALSE
,OFF
, or0
to disable it. Theboolean
value can also be omitted, in which caseTRUE
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), orTB
(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 have the MAINTAIN
privilege on the table. However, database owners are allowed to vacuum all tables in their databases, except shared catalogs. VACUUM
will skip over any tables that the calling user does not have permission to vacuum.
While VACUUM
is running, the search_path is temporarily changed to pg_catalog, pg_temp
.
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 62.4.4.1 for details.
We recommend that all databases be vacuumed regularly in order to remove dead rows. Postgres Pro includes an “autovacuum” facility which can automate routine vacuum maintenance. For more information about automatic and manual vacuuming, see Section 23.1.
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 18.4.4 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 26.4.5 and Section 26.4.2 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.
The following syntax was used before PostgreSQL version 9.0 and is still supported:
VACUUM [ FULL ] [ FREEZE ] [ VERBOSE ] [ ANALYZE ] [ table_and_columns
[, ...] ]
Note that in this syntax, the options must be specified in exactly the order shown.