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

Provided by: postgres-xc-client_1.1-2ubuntu2_amd64

NAME

       VACUUM - garbage-collect and optionally analyze a database

SYNOPSIS

       VACUUM [ ( { FULL | FREEZE | VERBOSE | ANALYZE } [, ...] ) ] [ table_name [ (column_name [, ...] ) ] ]
       VACUUM [ FULL ] [ FREEZE ] [ VERBOSE ] [ table_name ]
       VACUUM [ FULL ] [ FREEZE ] [ VERBOSE ] ANALYZE [ table_name [ (column_name [, ...] ) ] ]

DESCRIPTION

Note

The following description applies both to Postgres-XC and PostgreSQL if not described explicitly.

VACUUM reclaims storage occupied by dead tuples. In normal Postgres-XC 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.

With no parameter, VACUUM processes every table in the current database that the current user has
permission to vacuum. With a parameter, VACUUM processes only that table.

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. 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 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.

Note

The following description applies only to Postgres-XC

In Postgres-XC, VACUUM will be performed on all the Datanodes as well.

PARAMETERS

           Note

           The following description applies both to Postgres-XC and PostgreSQL if not described explicitly.

       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 parameter set to zero.

       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.

       table_name
           The name (optionally schema-qualified) of a specific table to vacuum. Defaults to all tables in the
           current database.

       column_name
           The name of a specific column to analyze. Defaults to all columns. If a column list is specified,
           ANALYZE is implied.

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 57.3.1, “GIN
Fast Update Technique”, in the documentation 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 Postgres-XC 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.

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. See Section
17.4.4, “Cost-based Vacuum Delay”, in the documentation for details.

Postgres-XC includes an “autovacuum” facility which can automate routine vacuum maintenance. For more
information about automatic and manual vacuuming, see Section 22.1, “Routine Vacuuming”, in the
documentation.

EXAMPLES

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

COMPATIBILITY

       There is no VACUUM statement in the SQL standard.