VACUUM

Description

VACUUM recycles storage space occupied by tables or B-Tree indexes. In normal database operation, rows that have been deleted are not physically removed from their table; they remain present until a VACUUM is done. Therefore, it is necessary to do VACUUM periodically, especially on frequently-updated tables.

Precautions

• If no table is specified, VACUUM processes the tables on which the user has the corresponding permission in the current database. With a parameter, VACUUM processes only that table.
• To perform VACUUM operation on a table, you must be a table owner or a user granted the VACUUM permission on the table. When the separation of duties is disabled, system administrators have this permission by default. 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 executed by the system administrator). VACUUM skips over any tables that the calling user does not have the permission to vacuum.
• VACUUM cannot be executed inside a transaction block.
• It is recommended 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 run VACUUM ANALYZE for the affected table. This will update the system catalogs with the results of all recent changes, and allow the query planner to make better choices in planning queries.
• FULL is recommended only in special scenarios. For example, you wish to physically narrow the table to decrease the occupied disk space after deleting most rows of a table. VACUUM FULL usually shrinks a table more than VACUUM does. The FULL option does not clear indexes. You are advised to periodically use the REINDEX statement. If the physical space usage does not decrease after you run the statement, check whether there are other active transactions (that have started before you delete data transactions and not ended before you run VACUUM FULL). If there are such transactions, run this statement again when the transactions quit.
• VACUUM FULL returns the free space in the table to the tablespace by rebuilding the table. During the rebuilding, extra storage space equivalent to the valid data size in the table is required. For a non-segment-page table, after VACUUM FULL is executed, the physical files occupied by the original table are deleted, and the physical file space occupied by the original table is returned to the OS. For a segment-page table, after VACUUM FULL is executed, the physical space occupied by the original table is returned to the segment-page data file instead of the OS.
• 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.
• When VERBOSE is specified, VACUUM prints progress messages to indicate which table is currently being processed. Various statistics about the tables are printed as well.
• When the option list is surrounded by parentheses, the options can be written in any order. If there are no brackets, the options must be given in the order displayed in the syntax.
• VACUUM and VACUUM FULL clear deleted tuples after the delay specified by vacuum_defer_cleanup_age.
• VACUUM ANALYZE executes a VACUUM operation and then an ANALYZE operation for each selected table. This is a handy combination form for routine maintenance scripts.
• Plain VACUUM (without FULL) simply recycles space and makes it available for reuse. This form of statement can operate in parallel with normal reading and writing of the table, as an exclusive lock is not obtained. VACUUM FULL executes wider processing, including moving rows across blocks to compress tables so they occupy the minimum number of disk blocks. This form is much slower and requires an exclusive lock on each table while it is being processed.
• A deadlock may occur when multiple VACUUM FULL statements are executed simultaneously.
• If the xc_maintenance_mode parameter is not enabled, the VACUUM FULL operation will skip all system catalogs.
• If you run VACUUM FULL immediately after running DELETE, the space will not be recycled. After running DELETE, execute 1000 non-SELECT transactions, or wait for 1s and then execute one transaction. Then, run VACUUM FULL to the space.
• During VACUUM FULL, an exclusive lock is added to the table. Therefore, you are advised not to run VACUUM FULL during peak hours because it may cause long waiting time or service interruption.
• For Ustore, the behavior of manual VACUUM is the same as that in Astore. Locks are obtained to clear heap tables and indexes. In Ustore, AUTOVACUUM only clears GPIs of partitioned tables, updates FSMs of heap tables, and recycles index pages.
• When VACUUM FULL is executed, partitions are traversed for clearance and GPIs are rebuilt after partition clearance. Therefore, if there are a large number of partitions, you are advised to delete GPIs first and rebuild indexes after VACUUM FULL is executed. In this way, the execution time of VACUUM FULL is reduced.

Syntax

• Recycle space and update statistics information, without requirements for keyword orders.

  VACUUM [ ( { FULL | FREEZE | VERBOSE | {ANALYZE | ANALYSE }} [,...] ) ]
      [ table_name [ (column_name [, ...] ) ] [ PARTITION ( partition_name ) | SUBPARTITION ( subpartition_name ) ] ];

• Recycle space, without updating statistics information.

  VACUUM [ FULL [COMPACT] ] [ FREEZE ] [ VERBOSE ] [ table_name 
  [ PARTITION ( partition_name )  | SUBPARTITION ( subpartition_name ) ] ];

• Recycle space and update statistics information, and require keywords in order.

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

Parameters

• FULL

  Selects "FULL" vacuum, which can recycle more space, but takes much longer and exclusively locks the table.

  

  Using FULL will cause statistics missing. To collect statistics, add the keyword ANALYZE to VACUUM FULL.

• FREEZE

  Is equivalent to running VACUUM with the vacuum_freeze_min_age parameter set to 0.

• VERBOSE

  Prints a detailed VACUUM activity report for each table.

• ANALYZE | ANALYSE

  Updates statistics used by the planner to determine the most efficient way to execute a query.

  

  VACUUM is also triggered when autovacuum is set to analyze for a Ustore partitioned table.

• table_name

  Specifies the name (optionally schema-qualified) of a specific table to vacuum.

  Value range: name of a specific table to vacuum Defaults are all tables in the current database.

• column_name

  Specifies the name of the column to be analyzed. This parameter must be used together with ANALYZE.

  Value range: name of the column to be analyzed Defaults are all columns.

  

  The mechanism of the VACUUM ANALYZE statement is to execute VACUUM and ANALYZE in sequence. Therefore, if column_name is incorrect, VACUUM may be successfully executed but ANALYZE may fail to be executed. For a partitioned table, ANALYZE may fail to be executed after VACUUM is successfully executed on a partition.

• PARTITION

  COMPACT and PARTITION cannot be used at the same time.

• partition_name

  Specifies the level-1 partition name of the table to be cleared. If it is left empty, all level-1 partitions are cleared.

• subpartition_name

  Specifies the level-2 partition name of the table to be cleared. If it is left empty, all level-2 partitions are cleared.

Examples

• VACUUM

  1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

  -- Create the table tbl_test and insert data into the tables. gaussdb=# CREATE TABLE tbl_test(c1 int); gaussdb=# INSERT INTO tbl_test VALUES (1); -- View the data and the CTID of the data. gaussdb=# SELECT ctid,* FROM tbl_test; ctid | c1 -------+---- (0,1) | 1 (1 row) -- Delete the data record. gaussdb=# DELETE FROM tbl_test; -- Insert a data record again. It is found that a new ctid is used. gaussdb=# INSERT INTO tbl_test VALUES (2); gaussdb=# SELECT ctid,* FROM tbl_test; ctid | c1 -------+---- (0,2) | 2 (1 row) -- After the VACUUM statement is executed, the old space is reused when data is inserted. gaussdb=# VACUUM ANALYZE tbl_test; gaussdb=# INSERT INTO tbl_test VALUES (3); gaussdb=# SELECT ctid,* FROM tbl_test; ctid | c1 -------+---- (0,1) | 3 (0,2) | 2 (2 rows) -- Delete the table. gaussdb=# DROP TABLE tbl_test;

• VACUUM FULL

  -- Create a table.
  gaussdb=# CREATE TABLE tbl_test2(c1 int);
  
  -- Insert 100,000 data records and check the table size.
  gaussdb=# INSERT INTO tbl_test2 VALUES (generate_series(1,100000));
  gaussdb=# SELECT 'tbl_test2' AS tablename, pg_size_pretty(pg_relation_size('tbl_test2')) AS size;
   tablename |  size   
  -----------+---------
   tbl_test2 | 3048 kB
  (1 row)
  
  -- Delete data and check the table size.
  gaussdb=# DELETE FROM tbl_test2;
  gaussdb=# SELECT 'tbl_test2' AS tablename, pg_size_pretty(pg_relation_size('tbl_test2')) AS size;
   tablename |  size   
  -----------+---------
   tbl_test2 | 3048 kB
  (1 row)
  
  -- Use VACUUM FULL to reclaim space and check the table size.
  gaussdb=# VACUUM FULL ANALYZE tbl_test2;
  gaussdb=# SELECT 'tbl_test2' AS tablename, pg_size_pretty(pg_relation_size('tbl_test2')) AS size;
   tablename |  size   
  -----------+---------
   tbl_test2 | 0 bytes
  (1 row)
  
  
  -- Delete.
  gaussdb=# DROP TABLE tbl_test2;

Suggestions

• VACUUM
  • VACUUM cannot be executed inside a transaction block.
  • It is recommended that active production databases be vacuumed frequently (at least nightly), in order to remove dead rows. It is strongly recommended that you run VACUUM ANALYZE after adding or deleting a large number of records.
  • FULL is recommended only in special scenarios. For example, you wish to physically narrow the table to decrease the occupied disk space after deleting most rows of a table.