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

Provided by: postgresql-client-17_17.2-1_amd64

NAME

       VACUUM - garbage-collect and optionally analyze a database

SYNOPSIS

       VACUUM [ ( option [, ...] ) ] [ 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.

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 24.1.5). 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 27.46). 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 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 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 64.4.4.1 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 24.1.

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 19.4.4 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 27.4.5 and Section 27.4.2 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.