VACUUM — garbage-collect and optionally analyze a database
VACUUM [ (option[, ...] ) ] [table_and_columns[, ...] ] VACUUM [ FULL ] [ FREEZE ] [ VERBOSE ] [ ANALYZE ] [table_and_columns[, ...] ] whereoptioncan be one of: FULL [boolean] FREEZE [boolean] VERBOSE [boolean] ANALYZE [boolean] DISABLE_PAGE_SKIPPING [boolean] SKIP_LOCKED [boolean] INDEX_CLEANUP [boolean] TRUNCATE [boolean] PARALLELintegerandtable_and_columnsis:table_name[ (column_name[, ...] ) ]
VACUUM reclaims storage occupied by dead tuples.
In normal LightDB 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.
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 LightDB 9.0; the unparenthesized syntax is deprecated.
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.
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.
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_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
Specifies that VACUUM should attempt to remove
index entries pointing to dead tuples. This is normally the desired
behavior and is the default unless the
vacuum_index_cleanup option has been set to false
for the table to be vacuumed. Setting this option to false may be
useful when it is necessary to make vacuum run as quickly as possible,
for example to avoid imminent transaction ID wraparound
(see Section 22.1.5). However, 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 do not have an index and is ignored if 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 25.37). 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.
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.
integerSpecifies a non-negative integer value passed to the selected option.
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_name
The name of a specific column to analyze. Defaults to all columns.
If a column list is specified, ANALYZE must also be
specified.
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.
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 64.4.1 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
LightDB 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.
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 17.4.4 for
details.
LightDB includes an “autovacuum” facility which can automate routine vacuum maintenance. For more information about automatic and manual vacuuming, see Section 22.1.
To clean a single table onek, analyze it for
the optimizer and print a detailed vacuum activity report:
VACUUM (VERBOSE, ANALYZE) onek;
There is no VACUUM statement in the SQL standard.