VACUUM

Garbage-collects and optionally analyzes a database.

Synopsis

VACUUM [({ FULL | FREEZE | VERBOSE | ANALYZE } [, ...])] [<table> [(<column> [, ...] )]]

VACUUM [FULL] [FREEZE] [VERBOSE] [<table>]

VACUUM [FULL] [FREEZE] [VERBOSE] ANALYZE
              [<table> [(<column> [, ...] )]]

Description

VACUUM reclaims storage occupied by deleted tuples. In normal LightDB-A Database operation, tuples that are deleted or obsoleted by an update are not physically removed from their table; they remain present on disk until a VACUUM is done. Therefore it is 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.

VACUUM (without FULL) marks deleted and obsoleted data in tables and indexes for future reuse and reclaims space for re-use only if the space is at the end of the table and an exclusive table lock can be easily obtained. Unused space at the start or middle of a table remains as is. With heap tables, 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.

With append-optimized tables, VACUUM compacts a table by first vacuuming the indexes, then compacting each segment file in turn, and finally vacuuming auxiliary relations and updating statistics. On each segment, visible rows are copied from the current segment file to a new segment file, and then the current segment file is scheduled to be dropped and the new segment file is made available. Plain VACUUM of an append-optimized table allows scans, inserts, deletes, and updates of the table while a segment file is compacted. However, an Access Exclusive lock is taken briefly to drop the current segment file and activate the new segment file.

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 Access Exclusive lock on each table while it is being processed. The Access Exclusive lock guarantees that the holder is the only transaction accessing the table in any way.

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-A Database 6.0; the unparenthesized syntax is deprecated.

Important For information on the use of VACUUM, VACUUM FULL, and VACUUM ANALYZE, see Notes.

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.

Parameters

FULL : Selects a full vacuum, which may 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 : Specifying FREEZE is equivalent to performing VACUUM with the vacuum_freeze_min_age server configuration parameter set to zero. See Server Configuration Parameters for information about vacuum_freeze_min_age.

VERBOSE : Prints a detailed vacuum activity report for each table.

ANALYZE : Updates statistics used by the planner to determine the most efficient way to run 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. If a column list is specified, ANALYZE is implied.

Notes

VACUUM cannot be run inside a transaction block.

Vacuum active databases frequently (at least nightly), in order to remove expired rows. After adding or deleting a large number of rows, running the VACUUM ANALYZE command for the affected table might be useful. This updates the system catalogs with the results of all recent changes, and allows the LightDB-A Database query optimizer to make better choices in planning queries.

Important PostgreSQL has a separate optional server process called the autovacuum daemon, whose purpose is to automate the execution of VACUUM and ANALYZE commands. LightDB-A Database enables the autovacuum daemon to perform VACUUM operations only on the LightDB-A Database template database template0. Autovacuum is enabled for template0 because connections are not allowed to template0. The autovacuum daemon performs VACUUM operations on template0 to manage transaction IDs (XIDs) and help avoid transaction ID wraparound issues in template0.

Manual VACUUM operations must be performed in user-defined databases to manage transaction IDs (XIDs) in those databases.

VACUUM causes a substantial increase in I/O traffic, which can cause poor performance for other active sessions. Therefore, it is advisable to vacuum the database at low usage times.

VACUUM commands skip external and foreign tables.

VACUUM FULL reclaims all expired row space, however it requires an exclusive lock on each table being processed, is a very expensive operation, and might take a long time to complete on large, distributed LightDB-A Database tables. Perform VACUUM FULL operations during database maintenance periods.

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.

As an alternative to VACUUM FULL, you can re-create the table with a CREATE TABLE AS statement and drop the old table.

For append-optimized tables, VACUUM requires enough available disk space to accommodate the new segment file during the VACUUM process. If the ratio of hidden rows to total rows in a segment file is less than a threshold value (10, by default), the segment file is not compacted. The threshold value can be configured with the gp_appendonly_compaction_threshold server configuration parameter. VACUUM FULL ignores the threshold and rewrites the segment file regardless of the ratio. VACUUM can be deactivated for append-optimized tables using the gp_appendonly_compaction server configuration parameter. See Server Configuration Parameters for information about the server configuration parameters.

If a concurrent serializable transaction is detected when an append-optimized table is being vacuumed, the current and subsequent segment files are not compacted. If a segment file has been compacted but a concurrent serializable transaction is detected in the transaction that drops the original segment file, the drop is skipped. This could leave one or two segment files in an “awaiting drop” state after the vacuum has completed.

For more information about concurrency control in LightDB-A Database, see “Routine System Maintenance Tasks” in LightDB-A Database Administrator Guide.

Examples

To clean a single table onek, analyze it for the optimizer and print a detailed vacuum activity report:

VACUUM (VERBOSE, ANALYZE) onek;

Vacuum all tables in the current database:

VACUUM;

Vacuum a specific table only:

VACUUM (VERBOSE, ANALYZE) mytable;

Vacuum all tables in the current database and collect statistics for the query optimizer:

VACUUM ANALYZE;

Compatibility

There is no VACUUM statement in the SQL standard.