26.3.2 HASH 和 KEY 分区的管理
Tables which are partitioned by hash or by key are very similar to one another with regard to making changes in a partitioning setup, and both differ in a number of ways from tables which have been partitioned by range or list. For that reason, this section addresses the modification of tables partitioned by hash or by key only. For a discussion of adding and dropping of partitions of tables that are partitioned by range or list, see Section 26.3.1, “Management of RANGE and LIST Partitions”.
You cannot drop partitions from tables that are partitioned by HASH
or KEY
in the same way that you can from tables that are partitioned by RANGE
or LIST
. However, you can merge HASH
or KEY
partitions using ALTER TABLE ... COALESCE PARTITION
. Suppose that a clients
table containing data about clients is divided into 12 partitions, created as shown here:
CREATE TABLE clients (
id INT,
fname VARCHAR(30),
lname VARCHAR(30),
signed DATE
)
PARTITION BY HASH( MONTH(signed) )
PARTITIONS 12;
To reduce the number of partitions from 12 to 8, execute the following ALTER TABLE
statement:
mysql> ALTER TABLE clients COALESCE PARTITION 4;
Query OK, 0 rows affected (0.02 sec)
COALESCE
works equally well with tables that are partitioned by HASH
, KEY
, LINEAR HASH
, or LINEAR KEY
. Here is an example similar to the previous one, differing only in that the table is partitioned by LINEAR KEY
:
mysql> CREATE TABLE clients_lk (
-> id INT,
-> fname VARCHAR(30),
-> lname VARCHAR(30),
-> signed DATE
-> )
-> PARTITION BY LINEAR KEY(signed)
-> PARTITIONS 12;
Query OK, 0 rows affected (0.03 sec)
mysql> ALTER TABLE clients_lk COALESCE PARTITION 4;
Query OK, 0 rows affected (0.06 sec)
Records: 0 Duplicates: 0 Warnings: 0
The number following COALESCE PARTITION
is the number of partitions to merge into the remainder—in other words, it is the number of partitions to remove from the table.
Attempting to remove more partitions than are in the table results in an error like this one:
mysql> ALTER TABLE clients COALESCE PARTITION 18;
ERROR 1478 (HY000): Cannot remove all partitions, use DROP TABLE instead
To increase the number of partitions for the clients
table from 12 to 18, use ALTER TABLE ... ADD PARTITION
as shown here:
ALTER TABLE clients ADD PARTITION PARTITIONS 6;