13.1.7.1. ALTER TABLE Partition Operations

Simply using a partition_options clause with ALTER TABLE on a partitioned table repartitions the table according to the partitioning scheme defined by the partition_options. This clause always begins with PARTITION BY, and follows the same syntax and other rules as apply to the partition_options clause for CREATE TABLE (see Section 13.1.17, "CREATE TABLE Syntax", for more detailed information), and can also be used to partition an existing table that is not already partitioned. For example, consider a (nonpartitioned) table defined as shown here:

CREATE TABLE t1 (    id INT,    year_col INT);

This table can be partitioned by HASH, using the id column as the partitioning key, into 8 partitions by means of this statement:

ALTER TABLE t1    PARTITION BY HASH(id)    PARTITIONS 8;

MySQL 5.6.11 and later supports an ALGORITHM option with [SUB]PARTITION BY [LINEAR] KEY. ALGORITHM=1 causes the server to use the same key-hashing functions as MySQL 5.1 when computing the placement of rows in partitions; ALGORITHM=2 means that the server employs the key-hashing functions implemented and used by default for new KEY partitioned tables in MySQL 5.5 and later. (Partitioned tables created with the key-hashing functions employed in MySQL 5.5 and later cannot be used by a MySQL 5.1 server.) Not specifying the option has the same effect as using ALGORITHM=2. This option is intended for use chiefly when upgrading or downgrading [LINEAR] KEY partitioned tables between MySQL 5.1 and later MySQL versions, or for creating tables partitioned by KEY or LINEAR KEY on a MySQL 5.5 or later server which can be used on a MySQL 5.1 server.

To upgrade a KEY partitioned table that was created in MySQL 5.1, first execute SHOW CREATE TABLE and note the the exact columns and number of partitions shown. Now execute an ALTER TABLE statement using exactly the same column list and number of partitions as in the CREATE TABLE statement, while adding ALGORITHM=2 immediately following the PARTITION BY keywords. (You should also include the LINEAR keyword if it was used for the original table definition.) An example from a session in the mysql client is shown here:

mysql> SHOW CREATE TABLE
                    p\G*************************** 1. row ***************************       Table: pCreate Table: CREATE TABLE `p` (  `id` int(11) NOT NULL AUTO_INCREMENT,  `cd` datetime NOT NULL,  PRIMARY KEY (`id`)) ENGINE=InnoDB DEFAULT CHARSET=latin1/*!50100 PARTITION BY LINEAR KEY (id)PARTITIONS 32 */1 row in set (0.00 sec)mysql> ALTER TABLE p PARTITION BY
                    LINEAR KEY ALGORITHM=2 (id) PARTITIONS 32;Query OK, 0 rows affected (5.34 sec)Records: 0  Duplicates: 0  Warnings: 0mysql> SHOW CREATE TABLE p\G*************************** 1. row ***************************       Table: pCreate Table: CREATE TABLE `p` (  `id` int(11) NOT NULL AUTO_INCREMENT,  `cd` datetime NOT NULL,  PRIMARY KEY (`id`)) ENGINE=InnoDB DEFAULT CHARSET=latin1/*!50100 PARTITION BY LINEAR KEY (id)PARTITIONS 32 */1 row in set (0.00 sec)

Downgrading a table created using the default key-hashing used in MySQL 5.5 and later to enable its use by a MySQL 5.1 server is similar, except in this case you should use ALGORITHM=1 to force the table's partitions to be rebuilt using the MySQL 5.1 key-hashing functions. It is recommended that you not do this except when necessary for compatibility with a MySQL 5.1 server, as the improved KEY hashing functions used by default in MySQL 5.5 and later provide fixes for a number of issues found in the older implementation.

The table that results from using an ALTER TABLE ... PARTITION BY statement must follow the same rules as one created using CREATE TABLE ... PARTITION BY. This includes the rules governing the relationship between any unique keys (including any primary key) that the table might have, and the column or columns used in the partitioning expression, as discussed in Section 18.6.1, "Partitioning Keys, Primary Keys, and Unique Keys". The CREATE TABLE ... PARTITION BY rules for specifying the number of partitions also apply to ALTER TABLE ... PARTITION BY.

The partition_definition clause for ALTER TABLE ADD PARTITION supports the same options as the clause of the same name for the CREATE TABLE statement. (See Section 13.1.17, "CREATE TABLE Syntax", for the syntax and description.) Suppose that you have the partitioned table created as shown here:

CREATE TABLE t1 (    id INT,    year_col INT)PARTITION BY RANGE (year_col) (    PARTITION p0 VALUES LESS THAN (1991),    PARTITION p1 VALUES LESS THAN (1995),    PARTITION p2 VALUES LESS THAN (1999));

You can add a new partition p3 to this table for storing values less than 2002 as follows:

ALTER TABLE t1 ADD PARTITION (PARTITION p3 VALUES LESS THAN (2002));

DROP PARTITION can be used to drop one or more RANGE or LIST partitions. This statement cannot be used with HASH or KEY partitions; instead, use COALESCE PARTITION (see below). Any data that was stored in the dropped partitions named in the partition_names list is discarded. For example, given the table t1 defined previously, you can drop the partitions named p0 and p1 as shown here:

ALTER TABLE t1 DROP PARTITION p0, p1;

ADD PARTITION and DROP PARTITION do not currently support IF [NOT] EXISTS.

Renames of partitioned table are supported. You can rename individual partitions indirectly using ALTER TABLE ... REORGANIZE PARTITION; however, this operation makes a copy of the partition's data..

In MySQL 5.6, it is possible to delete rows from selected partitions using the TRUNCATE PARTITION option. This option takes a comma-separated list of one or more partition names. For example, consider the table t1 as defined here:

CREATE TABLE t1 (    id INT,    year_col INT)PARTITION BY RANGE (year_col) (    PARTITION p0 VALUES LESS THAN (1991),    PARTITION p1 VALUES LESS THAN (1995),    PARTITION p2 VALUES LESS THAN (1999),    PARTITION p3 VALUES LESS THAN (2003),    PARTITION p4 VALUES LESS THAN (2007));

To delete all rows from partition p0, you can use the following statement:

ALTER TABLE t1 TRUNCATE PARTITION p0;

The statement just shown has the same effect as the following DELETE statement:

DELETE FROM t1 WHERE year_col < 1991;

When truncating multiple partitions, the partitions do not have to be contiguous: This can greatly simplify delete operations on partitioned tables that would otherwise require very complex WHERE conditions if done with DELETE statements. For example, this statement deletes all rows from partitions p1 and p3:

ALTER TABLE t1 TRUNCATE PARTITION p1, p3;

An equivalent DELETE statement is shown here:

DELETE FROM t1 WHERE     (year_col >= 1991 AND year_col < 1995)    OR    (year_col >= 2003 AND year_col < 2007);

You can also use the ALL keyword in place of the list of partition names; in this case, the statement acts on all partitions in the table.

TRUNCATE PARTITION merely deletes rows; it does not alter the definition of the table itself, or of any of its partitions.

You can verify that the rows were dropped by checking the INFORMATION_SCHEMA.PARTITIONS table, using a query such as this one:

SELECT PARTITION_NAME, TABLE_ROWS     FROM INFORMATION_SCHEMA.PARTITIONS     WHERE TABLE_NAME = 't1';

TRUNCATE PARTITION is supported only for partitioned tables that use the MyISAM, InnoDB, or MEMORY storage engine. It also works on BLACKHOLE tables (but has no effect). It is not supported for ARCHIVE tables.

COALESCE PARTITION can be used with a table that is partitioned by HASH or KEY to reduce the number of partitions by number. Suppose that you have created table t2 using the following definition:

CREATE TABLE t2 (    name VARCHAR (30),    started DATE)PARTITION BY HASH( YEAR(started) )PARTITIONS 6;

You can reduce the number of partitions used by t2 from 6 to 4 using the following statement:

ALTER TABLE t2 COALESCE PARTITION 2;

The data contained in the last number partitions will be merged into the remaining partitions. In this case, partitions 4 and 5 will be merged into the first 4 partitions (the partitions numbered 0, 1, 2, and 3).

To change some but not all the partitions used by a partitioned table, you can use REORGANIZE PARTITION. This statement can be used in several ways:

For more detailed information about and examples of ALTER TABLE ... REORGANIZE PARTITION statements, see Section 18.3.1, "Management of RANGE and LIST Partitions".

It is also possible in MySQL 5.6 to exchange a table partition or subpartition with a table using ALTER TABLE pt EXCHANGE PARTITION p WITH TABLE nt, where pt is the partitioned table and p is the partition or subpartition of pt to be exchanged with unpartitioned table nt, provided that the following statements are true:

Executing ALTER TABLE ... EXCHANGE PARTITION does not invoke any triggers on either the partitioned table or the table to be exchanged.

Any AUTO_INCREMENT columns in the table to be exchanged with a partition are reset.

The IGNORE keyword has no effect when used with ALTER TABLE ... EXCHANGE PARTITION.

For more information about and examples of ALTER TABLE ... EXCHANGE PARTITION, see Section 18.3.3, "Exchanging Partitions and Subpartitions with Tables".

Several additional options provide partition maintenance and repair functionality analogous to that implemented for nonpartitioned tables by statements such as CHECK TABLE and REPAIR TABLE (which are also supported for partitioned tables; see Section 13.7.2, "Table Maintenance Statements" for more information). These include ANALYZE PARTITION, CHECK PARTITION, OPTIMIZE PARTITION, REBUILD PARTITION, and REPAIR PARTITION. Each of these options takes a partition_names clause consisting of one or more names of partitions, separated by commas. The partitions must already exist in the table to be altered. You can also use the ALL keyword in place of partition_names, in which case the statement acts on all partitions in the table. For more information and examples, see Section 18.3.4, "Maintenance of Partitions".

Some MySQL storage engines, such as InnoDB, do not support per-partition optimization. For a partitioned table using such a storage engine, ALTER TABLE ... OPTIMIZE PARTITION rebuilds the entire table. This is a known issue. Beginning with MySQL 5.6.9, running this statement on such a table causes the entire table to rebuilt and analyzed, and an appropriate warning to be issued. (Bug #11751825, Bug #42822)

To work around this problem, use the statements ALTER TABLE ... REBUILD PARTITION and ALTER TABLE ... ANALYZE PARTITION instead.

The ANALYZE PARTITION, CHECK PARTITION, OPTIMIZE PARTITION, and REPAIR PARTITION options are not permitted for tables which are not partitioned.

REMOVE PARTITIONING enables you to remove a table's partitioning without otherwise affecting the table or its data. This option can be combined with other ALTER TABLE options such as those used to add, drop, or rename drop columns or indexes.

Using the ENGINE option with ALTER TABLE changes the storage engine used by the table without affecting the partitioning.