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17.2.5. Subpartitioning

Subpartitioning — also known as composite partitioning — is the further division of each partition in a partitioned table. For example, consider the following CREATE TABLE statement:

CREATE TABLE ts (id INT, purchased DATE)
    PARTITION BY RANGE(YEAR(purchased))
    SUBPARTITION BY HASH( TO_DAYS(purchased) )
    SUBPARTITIONS 2 (
        PARTITION p0 VALUES LESS THAN (1990),
        PARTITION p1 VALUES LESS THAN (2000),
        PARTITION p2 VALUES LESS THAN MAXVALUE
    );

Table ts has 3 RANGE partitions. Each of these partitions — p0, p1, and p2 — is further divided into 2 subpartitions. In effect, the entire table is divided into 3 * 2 = 6 partitions. However, due to the action of the PARTITION BY RANGE clause, the first 2 of these store only those records with a value less than 1990 in the purchased column.

In MySQL 5.1, it is possible to subpartition tables that are partitioned by RANGE or LIST. Subpartitions may use either HASH or KEY partitioning. This is also known as composite partitioning.

It is also possible to define subpartitions explicitly using SUBPARTITION clauses to specify options for individual subpartitions. For example, a more verbose fashion of creating the same table ts as shown in the previous example would be:

CREATE TABLE ts (id INT, purchased DATE)
    PARTITION BY RANGE( YEAR(purchased) )
    SUBPARTITION BY HASH( TO_DAYS(purchased) ) (
        PARTITION p0 VALUES LESS THAN (1990) (
            SUBPARTITION s0,
            SUBPARTITION s1
        ),
        PARTITION p1 VALUES LESS THAN (2000) (
            SUBPARTITION s2,
            SUBPARTITION s3
        ),
        PARTITION p2 VALUES LESS THAN MAXVALUE (
            SUBPARTITION s4,
            SUBPARTITION s5
        )
    );

Some syntactical items of note:

  • Each partition must have the same number of subpartitions.

  • If you explicitly define any subpartitions using SUBPARTITION on any partition of a partitioned table, you must define them all. In other words, the following statement will fail:

    CREATE TABLE ts (id INT, purchased DATE)
        PARTITION BY RANGE( YEAR(purchased) )
        SUBPARTITION BY HASH( TO_DAYS(purchased) ) (
            PARTITION p0 VALUES LESS THAN (1990) (
                SUBPARTITION s0,
                SUBPARTITION s1
            ),
            PARTITION p1 VALUES LESS THAN (2000),
            PARTITION p2 VALUES LESS THAN MAXVALUE (
                SUBPARTITION s2,
                SUBPARTITION s3
            )
        );
    

    This statement would still fail even if it included a SUBPARTITIONS 2 clause.

  • Each SUBPARTITION clause must include (at a minimum) a name for the subpartition. Otherwise, you may set any desired option for the subpartition or allow it to assume its default setting for that option.

  • Names of subpartitions must be unique within each partition, but do not have to be unique within the table as a whole. For example, the following CREATE TABLE statement is valid:

    CREATE TABLE ts (id INT, purchased DATE)
        PARTITION BY RANGE( YEAR(purchased) )
        SUBPARTITION BY HASH( TO_DAYS(purchased) ) (
            PARTITION p0 VALUES LESS THAN (1990) (
                SUBPARTITION s0,
                SUBPARTITION s1
            ),
            PARTITION p1 VALUES LESS THAN (2000) (
                SUBPARTITION s0,
                SUBPARTITION s1
            ),
            PARTITION p2 VALUES LESS THAN MAXVALUE (
                SUBPARTITION s0,
                SUBPARTITION s1
            )
        );
    

Subpartitions can be used with especially large tables to distribute data and indexes across many disks. Suppose that you have 6 disks mounted as /disk0, /disk1, /disk2, and so on. Now consider the following example:

CREATE TABLE ts (id INT, purchased DATE)
    PARTITION BY RANGE( YEAR(purchased) )
    SUBPARTITION BY HASH( TO_DAYS(purchased) ) (
        PARTITION p0 VALUES LESS THAN (1990) (
            SUBPARTITION s0 
                DATA DIRECTORY = '/disk0/data' 
                INDEX DIRECTORY = '/disk0/idx',
            SUBPARTITION s1 
                DATA DIRECTORY = '/disk1/data' 
                INDEX DIRECTORY = '/disk1/idx'
        ),
        PARTITION p1 VALUES LESS THAN (2000) (
            SUBPARTITION s0 
                DATA DIRECTORY = '/disk2/data' 
                INDEX DIRECTORY = '/disk2/idx',
            SUBPARTITION s1 
                DATA DIRECTORY = '/disk3/data' 
                INDEX DIRECTORY = '/disk3/idx'
        ),
        PARTITION p2 VALUES LESS THAN MAXVALUE (
            SUBPARTITION s0 
                DATA DIRECTORY = '/disk4/data' 
                INDEX DIRECTORY = '/disk4/idx',
            SUBPARTITION s1 
                DATA DIRECTORY = '/disk5/data' 
                INDEX DIRECTORY = '/disk5/idx'
        )
    );

In this case, a separate disk is used for the data and for the indexes of each RANGE. Many other variations are possible; another example might be:

CREATE TABLE ts (id INT, purchased DATE)
    PARTITION BY RANGE(YEAR(purchased))
    SUBPARTITION BY HASH( TO_DAYS(purchased) ) (
        PARTITION p0 VALUES LESS THAN (1990) (
            SUBPARTITION s0a 
                DATA DIRECTORY = '/disk0' 
                INDEX DIRECTORY = '/disk1',
            SUBPARTITION s0b 
                DATA DIRECTORY = '/disk2' 
                INDEX DIRECTORY = '/disk3'
        ),
        PARTITION p1 VALUES LESS THAN (2000) (
            SUBPARTITION s1a 
                DATA DIRECTORY = '/disk4/data' 
                INDEX DIRECTORY = '/disk4/idx',
            SUBPARTITION s1b 
                DATA DIRECTORY = '/disk5/data' 
                INDEX DIRECTORY = '/disk5/idx'
        ),
        PARTITION p2 VALUES LESS THAN MAXVALUE (
            SUBPARTITION s2a,
            SUBPARTITION s2b
        )
    );

Here, the storage is as follows:

  • Rows with purchased dates from before 1990 take up a vast amount of space, so are split up 4 ways, with a separate disk dedicated to the data and to the indexes for each of the two subpartitions (s0a and s0b) making up partition p0. In other words:

    • The data for subpartition s0a is stored on /disk0.

    • The indexes for subpartition s0a are stored on /disk1.

    • The data for subpartition s0b is stored on /disk2.

    • The indexes for subpartition s0b are stored on /disk3.

  • Rows containing dates ranging from 1990 to 1999 (partition p1) do not require as much room as those from before 1990. These are split between 2 disks (/disk4 and /disk5) rather than 4 disks as with the legacy records stored in p0:

    • Data and indexes belonging to p1's first subpartition (s1a) are stored on /disk4 — the data in /disk4/data, and the indexes in /disk4/idx.

    • Data and indexes belonging to p1's second subpartition (s1b) are stored on /disk5 — the data in /disk5/data, and the indexes in /disk5/idx.

  • Rows reflecting dates from the year 2000 to the present (partition p2) do not take up as much space as required by either of the two previous ranges. Currently, it is sufficient to store all of these in the default location.

    In future, when the number of purchases for the decade beginning with the year 2000 grows to a point where the default location no longer provides sufficient space, the corresponding rows can be moved using an ALTER TABLE ... REORGANIZE PARTITION statement. See Section 17.3, “Partition Management”, for an explanation of how this can be done.


 
 
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