Name
VACUUM -- garbage-collect and optionally analyze a databaseSynopsis
VACUUM [ ( { FULL | FREEZE | VERBOSE | ANALYZE | DISABLE_PAGE_SKIPPING } [, ...] ) ] [ table_name [ (column_name [, ...] ) ] ] VACUUM [ FULL ] [ FREEZE ] [ VERBOSE ] [ table_name ] VACUUM [ FULL ] [ FREEZE ] [ VERBOSE ] ANALYZE [ 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.
With no parameter, VACUUM
processes every table in the current database that the current user has permission to vacuum. With a parameter, VACUUM
processes only that table.
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. 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 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 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 the contents of the visibility map are thought to be suspect, which should happen only if there is a hardware or software issue causing database corruption. table_name
-
The name (optionally schema-qualified) of a specific table to vacuum. Defaults to all tables in the current database.
column_name
-
The name of a specific column to analyze. Defaults to all columns. If a column list is specified,
ANALYZE
is implied.
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 63.4.1 for details.
We recommend that active production databases be vacuumed frequently (at least nightly), in order to remove dead rows. After adding or deleting a large number of rows, it might be a good idea to issue a VACUUM ANALYZE
command for the affected table. This will update the system catalogs with the results of all recent changes, and allow the PostgreSQL query planner to make better choices in planning queries.
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.
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. See Section 19.4.4 for details.
PostgreSQL includes an "autovacuum" facility which can automate routine vacuum maintenance. For more information about automatic and manual vacuuming, see Section 24.1.
Examples
The following is an example from running VACUUM
on a table in the regression database:
regression=# VACUUM (VERBOSE, ANALYZE) onek; INFO: vacuuming "public.onek" INFO: index "onek_unique1" now contains 1000 tuples in 14 pages DETAIL: 3000 index tuples were removed. 0 index pages have been deleted, 0 are currently reusable. CPU 0.01s/0.08u sec elapsed 0.18 sec. INFO: index "onek_unique2" now contains 1000 tuples in 16 pages DETAIL: 3000 index tuples were removed. 0 index pages have been deleted, 0 are currently reusable. CPU 0.00s/0.07u sec elapsed 0.23 sec. INFO: index "onek_hundred" now contains 1000 tuples in 13 pages DETAIL: 3000 index tuples were removed. 0 index pages have been deleted, 0 are currently reusable. CPU 0.01s/0.08u sec elapsed 0.17 sec. INFO: index "onek_stringu1" now contains 1000 tuples in 48 pages DETAIL: 3000 index tuples were removed. 0 index pages have been deleted, 0 are currently reusable. CPU 0.01s/0.09u sec elapsed 0.59 sec. INFO: "onek": removed 3000 tuples in 108 pages DETAIL: CPU 0.01s/0.06u sec elapsed 0.07 sec. INFO: "onek": found 3000 removable, 1000 nonremovable tuples in 143 pages DETAIL: 0 dead tuples cannot be removed yet. There were 0 unused item pointers. Skipped 0 pages due to buffer pins. 0 pages are entirely empty. CPU 0.07s/0.39u sec elapsed 1.56 sec. INFO: analyzing "public.onek" INFO: "onek": 36 pages, 1000 rows sampled, 1000 estimated total rows VACUUM
Compatibility
There is no VACUUM
statement in the SQL standard.
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