|PostgreSQL Reference Manual - Volume 1 - SQL Language Reference|
by The PostgreSQL Global Development Group
Paperback (6"x9"), 716 pages
RRP £32.00 ($49.95)
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VACUUM -- garbage-collect and optionally analyze a database
VACUUM [ FULL ] [ FREEZE ] [ VERBOSE ] [ table ] VACUUM [ FULL ] [ FREEZE ] [ VERBOSE ] ANALYZE [ table [ (column [, ...] ) ] ]
VACUUM reclaims storage occupied by deleted 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
done. Therefore it's necessary to do
periodically, especially on frequently-updated tables.
With no parameter,
VACUUM processes every table in the
current database. With a parameter,
only that table.
VACUUM ANALYZE performs a
and then an
ANALYZE for each selected table. This
is a handy combination form for routine maintenance scripts. See
for more details about its processing.
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.
FULL does more extensive processing, including moving of tuples
across blocks to try to compact the table to the minimum number of disk
blocks. This form is much slower and requires an exclusive lock on each
table while it is being processed.
- Selects “full” vacuum, which may reclaim more space, but takes much longer and exclusively locks the table.
Selects aggressive “freezing” of tuples.
FREEZEis equivalent to performing
vacuum_freeze_min_ageparameter set to zero. The
FREEZEoption is deprecated and will be removed in a future release; set the parameter instead.
- Prints a detailed vacuum activity report for each table.
- Updates statistics used by the planner to determine the most efficient way to execute a query.
- The name (optionally schema-qualified) of a specific table to vacuum. Defaults to all tables in the current database.
- The name of a specific column to analyze. Defaults to all columns.
VERBOSE is specified,
progress messages to indicate which table is currently being
processed. Various statistics about the tables are printed as well.
VACUUM cannot be executed inside a transaction block.
We recommend that active production databases be
vacuumed frequently (at least nightly), in order to
remove expired rows. After adding or deleting a large number
of rows, it may be a good idea to issue a
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.
FULL option is not recommended for routine use,
but may be useful in special cases. An example is when you have deleted
most of the rows in a table and would like the table to physically shrink
to occupy less disk space.
VACUUM FULL will usually
shrink the table more than a plain
FULL option does not shrink indexes; a periodic
REINDEX is still recommended. In fact, it is often faster
to drop all indexes,
VACUUM FULL, and recreate the indexes.
VACUUM causes a substantial increase in I/O traffic,
which can cause poor performance for other active sessions. Therefore,
it is sometimes advisable to use the cost-based vacuum delay feature.
See Volume 3: Cost-Based Vacuum Delay for details.
PostgreSQL includes an “autovacuum” facility which can automate routine vacuum maintenance. For more information about automatic and manual vacuuming, see Volume 3: Routine Vacuuming.
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. 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
There is no
VACUUM statement in the SQL standard.
|ISBN 0954612027||PostgreSQL Reference Manual - Volume 1 - SQL Language Reference||See the print edition|