Subpartitioning—also known ascomposite partitioning—is the further division of each partition in a partitioned table. Consider the followingCREATE TABLE
statement:
创建表ts (INT id,购买日期)分区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 );
Tablets
has 3RANGE
partitions. Each of these partitions—p0
,p1
, andp2
—is further divided into 2 subpartitions. In effect, the entire table is divided into3 * 2 = 6
partitions. However, due to the action of thePARTITION BY RANGE
clause, the first 2 of these store only those records with a value less than 1990 in thepurchased
column.
It is possible to subpartition tables that are partitioned byRANGE
orLIST
. Subpartitions may use eitherHASH
orKEY
partitioning. This is also known ascomposite partitioning.
SUBPARTITION BY HASH
andSUBPARTITION BY KEY
generally follow the same syntax rules asPARTITION BY HASH
andPARTITION BY KEY
, respectively. An exception to this is thatSUBPARTITION BY KEY
(unlikePARTITION BY KEY
) does not currently support a default column, so the column used for this purpose must be specified, even if the table has an explicit primary key. This is a known issue which we are working to address; seeIssues with subpartitions, for more information and an example.
It is also possible to define subpartitions explicitly usingSUBPARTITION
clauses to specify options for individual subpartitions. For example, a more verbose fashion of creating the same tablets
as shown in the previous example would be:
创建表ts (INT id,购买日期)分区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 are listed here:
每一个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 fails:创建表ts (INT id,购买日期)分区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 used
SUBPARTITIONS 2
.每一个
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.Subpartition names must be unique across the entire table. For example, the following
CREATE TABLE
statement is valid:创建表ts (INT id,购买日期)分区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 ) );