VACUUM
VACUUM — garbage-collect and optionally analyze a database
Synopsis
VACUUM [ (option[, ...] ) ] [table_and_columns[, ...] ]VACUUM [ FULL ] [ FREEZE ] [ VERBOSE ] [ ANALYZE ] [table_and_columns[, ...] ]whereoptioncan be one of: FULL FREEZE VERBOSE ANALYZE DISABLE_PAGE_SKIPPINGandtable_and_columnsis:table_name[ (column_name[, ...] ) ]
Description
VACUUM reclaims storage occupied by dead tuples. In normalPostgres Pro operation, tuples that are deleted or obsoleted by an update are not physically removed from their table; they remain present until aVACUUM is done. Therefore it's necessary to doVACUUM periodically, especially on frequently-updated tables.
Without atable_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 aVACUUM and then anANALYZE for each selected table. This is a handy combination form for routine maintenance scripts. SeeANALYZE for more details about its processing.
PlainVACUUM (withoutFULL) 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 anACCESS 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 inPostgreSQL 9.0; the unparenthesized syntax is deprecated.
Parameters
FULLSelects“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.
FREEZESelects aggressive“freezing” of tuples. Specifying
FREEZEis equivalent to performingVACUUMwith thevacuum_freeze_min_age andvacuum_freeze_table_age parameters set to zero. Aggressive freezing is always performed when the table is rewritten, so this option is redundant whenFULLis specified.VERBOSEPrints a detailed vacuum activity report for each table.
ANALYZEUpdates statistics used by the planner to determine the most efficient way to execute a query.
DISABLE_PAGE_SKIPPINGNormally,
VACUUMwill skip pages based on thevisibility 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.table_nameThe 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_nameThe name of a specific column to analyze. Defaults to all columns. If a column list is specified,
ANALYZEmust also be specified.
Outputs
WhenVERBOSE 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-wideVACUUM 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 withGIN indexes,VACUUM (in any form) also completes any pending index insertions, by moving pending index entries to the appropriate places in the mainGIN index structure. SeeSection 62.4.1 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 aVACUUM ANALYZE command for the affected table. This will update the system catalogs with the results of all recent changes, and allow thePostgres Pro query planner to make better choices in planning queries.
TheFULL 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 plainVACUUM 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. SeeSection 18.4.4 for details.
Postgres Pro includes an“autovacuum” facility which can automate routine vacuum maintenance. For more information about automatic and manual vacuuming, seeSection 23.1.
Examples
To clean a single tableonek, analyze it for the optimizer and print a detailed vacuum activity report:
VACUUM (VERBOSE, ANALYZE) onek;
Compatibility
There is noVACUUM statement in the SQL standard.