MySQL :: MySQL 8.0 Reference Manual :: 26.2.5 KEY Partitioning (original) (raw)

KEY takes only a list of zero or more column names. Any columns used as the partitioning key must comprise part or all of the table's primary key, if the table has one. Where no column name is specified as the partitioning key, the table's primary key is used, if there is one. For example, the followingCREATE TABLE statement is valid in MySQL 8.0:

CREATE TABLE k1 (
    id INT NOT NULL PRIMARY KEY,
    name VARCHAR(20)
)
PARTITION BY KEY()
PARTITIONS 2;

If there is no primary key but there is a unique key, then the unique key is used for the partitioning key:

CREATE TABLE k1 (
    id INT NOT NULL,
    name VARCHAR(20),
    UNIQUE KEY (id)
)
PARTITION BY KEY()
PARTITIONS 2;

However, if the unique key column were not defined asNOT NULL, then the previous statement would fail.

In both of these cases, the partitioning key is theid column, even though it is not shown in the output of SHOW CREATE TABLE or in thePARTITION_EXPRESSION column of the Information Schema PARTITIONS table.

Unlike the case with other partitioning types, columns used for partitioning by KEY are not restricted to integer or NULL values. For example, the following CREATE TABLE statement is valid:

CREATE TABLE tm1 (
    s1 CHAR(32) PRIMARY KEY
)
PARTITION BY KEY(s1)
PARTITIONS 10;

The preceding statement would not be valid, were a different partitioning type to be specified. (In this case, simply using PARTITION BY KEY() would also be valid and have the same effect as PARTITION BY KEY(s1), sinces1 is the table's primary key.)

For additional information about this issue, seeSection 26.6, “Restrictions and Limitations on Partitioning”.

Columns with index prefixes are not supported in partitioning keys. This means thatCHAR,VARCHAR,BINARY, andVARBINARY columns can be used in a partitioning key, as long as they do not employ prefixes; because a prefix must be specified forBLOB andTEXT columns in index definitions, it is not possible to use columns of these two types in partitioning keys. Prior to MySQL 8.0.21, columns using prefixes were permitted when creating, altering, or upgrading a partitioned table, even though they were not included in the table's partitioning key; in MySQL 8.0.21 and later, this permissive behavior is deprecated, and the server displays appropriate warnings or errors when one or more such columns are used. SeeColumn index prefixes not supported for key partitioning, for more information and examples.

Note

Tables using the NDB storage engine are implicitly partitioned byKEY, using the table's primary key as the partitioning key (as with other MySQL storage engines). In the event that the NDB Cluster table has no explicit primary key, the “hidden” primary key generated by the NDB storage engine for each NDB Cluster table is used as the partitioning key.

If you define an explicit partitioning scheme for anNDB table, the table must have an explicit primary key, and any columns used in the partitioning expression must be part of this key. However, if the table uses an “empty” partitioning expression—that is, PARTITION BY KEY() with no column references—then no explicit primary key is required.

You can observe this partitioning using thendb_desc utility (with the-p option).

Important

For a key-partitioned table, you cannot execute anALTER TABLE DROP PRIMARY KEY, as doing so generates the error ERROR 1466 (HY000): Field in list of fields for partition function not found in table. This is not an issue for NDB Cluster tables which are partitioned by KEY; in such cases, the table is reorganized using the“hidden” primary key as the table's new partitioning key. See Chapter 25, MySQL NDB Cluster 8.0.