Ubuntu Manpage: VACUUM - garbage-collect and optionally analyze a database

Provided by: postgresql-client-16_16.9-0ubuntu0.24.04.1_amd64

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.