| PostgreSQL 8.0.1 Documentation | ||||
|---|---|---|---|---|
| Prev | Fast Backward | Fast Forward | Next | |
VACUUM
Synopsis
Description
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 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. 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 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. VACUUM 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.
FREEZE is a special-purpose option that causes tuples to be marked "frozen" as soon as possible, rather than waiting until they are quite old. If this is done when there are no other open transactions in the same database, then it is guaranteed that all tuples in the database are "frozen" and will not be subject to transaction ID wraparound problems, no matter how long the database is left unvacuumed. FREEZE is not recommended for routine use. Its only intended usage is in connection with preparation of user-defined template databases, or other databases that are completely read-only and will not receive routine maintenance VACUUM operations. See Chapter 21 for details.
Parameters
- FULL
Selects "full" vacuum, which may reclaim more space, but takes much longer and exclusively locks the table.
- FREEZE
Selects aggressive "freezing" of tuples.
- 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
The name (optionally schema-qualified) of a specific table to vacuum. Defaults to all tables in the current database.
- column
The name of a specific column to analyze. Defaults to all columns.
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
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 VACUUM 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.
The 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 VACUUM would.
Examples
The following is an example from running VACUUM on a table in the regression database:
