Chapter 6 Restrictions and Limitations on Partitioning

This section discusses the relationship of partitioning keys with primary keys and unique keys. The rule governing this relationship can be expressed as follows: All columns used in the partitioning expression for a partitioned table must be part of every unique key that the table may have.

In other words, every unique key on the table must use every column in the table's partitioning expression. (This also includes the table's primary key, since it is by definition a unique key. This particular case is discussed later in this section.) For example, each of the following table creation statements is invalid:

CREATE TABLE t1 (
    col1 INT NOT NULL,
    col2 DATE NOT NULL,
    col3 INT NOT NULL,
    col4 INT NOT NULL,
    UNIQUE KEY (col1, col2)
)
PARTITION BY HASH(col3)
PARTITIONS 4;
CREATE TABLE t2 (
    col1 INT NOT NULL,
    col2 DATE NOT NULL,
    col3 INT NOT NULL,
    col4 INT NOT NULL,
    UNIQUE KEY (col1),
    UNIQUE KEY (col3)
)
PARTITION BY HASH(col1 + col3)
PARTITIONS 4;

In each case, the proposed table would have at least one unique key that does not include all columns used in the partitioning expression.

Each of the following statements is valid, and represents one way in which the corresponding invalid table creation statement could be made to work:

CREATE TABLE t1 (
    col1 INT NOT NULL,
    col2 DATE NOT NULL,
    col3 INT NOT NULL,
    col4 INT NOT NULL,
    UNIQUE KEY (col1, col2, col3)
)
PARTITION BY HASH(col3)
PARTITIONS 4;
CREATE TABLE t2 (
    col1 INT NOT NULL,
    col2 DATE NOT NULL,
    col3 INT NOT NULL,
    col4 INT NOT NULL,
    UNIQUE KEY (col1, col3)
)
PARTITION BY HASH(col1 + col3)
PARTITIONS 4;

This example shows the error produced in such cases:

mysql> CREATE TABLE t3 (
    ->     col1 INT NOT NULL,
    ->     col2 DATE NOT NULL,
    ->     col3 INT NOT NULL,
    ->     col4 INT NOT NULL,
    ->     UNIQUE KEY (col1, col2),
    ->     UNIQUE KEY (col3)
    -> )
    -> PARTITION BY HASH(col1 + col3)
    -> PARTITIONS 4;
ERROR 1491 (HY000): A PRIMARY KEY must include all columns in the table's partitioning function

The CREATE TABLE statement fails because both col1 and col3 are included in the proposed partitioning key, but neither of these columns is part of both of unique keys on the table. This shows one possible fix for the invalid table definition:

mysql> CREATE TABLE t3 (
    ->     col1 INT NOT NULL,
    ->     col2 DATE NOT NULL,
    ->     col3 INT NOT NULL,
    ->     col4 INT NOT NULL,
    ->     UNIQUE KEY (col1, col2, col3),
    ->     UNIQUE KEY (col3)
    -> )
    -> PARTITION BY HASH(col3)
    -> PARTITIONS 4;
Query OK, 0 rows affected (0.05 sec)

In this case, the proposed partitioning key col3 is part of both unique keys, and the table creation statement succeeds.

The following table cannot be partitioned at all, because there is no way to include in a partitioning key any columns that belong to both unique keys:

CREATE TABLE t4 (
    col1 INT NOT NULL,
    col2 INT NOT NULL,
    col3 INT NOT NULL,
    col4 INT NOT NULL,
    UNIQUE KEY (col1, col3),
    UNIQUE KEY (col2, col4)
);

Since every primary key is by definition a unique key, this restriction also includes the table's primary key, if it has one. For example, the next two statements are invalid:

CREATE TABLE t5 (
    col1 INT NOT NULL,
    col2 DATE NOT NULL,
    col3 INT NOT NULL,
    col4 INT NOT NULL,
    PRIMARY KEY(col1, col2)
)
PARTITION BY HASH(col3)
PARTITIONS 4;
CREATE TABLE t6 (
    col1 INT NOT NULL,
    col2 DATE NOT NULL,
    col3 INT NOT NULL,
    col4 INT NOT NULL,
    PRIMARY KEY(col1, col3),
    UNIQUE KEY(col2)
)
PARTITION BY HASH( YEAR(col2) )
PARTITIONS 4;

In both cases, the primary key does not include all columns referenced in the partitioning expression. However, both of the next two statements are valid:

CREATE TABLE t7 (
    col1 INT NOT NULL,
    col2 DATE NOT NULL,
    col3 INT NOT NULL,
    col4 INT NOT NULL,
    PRIMARY KEY(col1, col2)
)
PARTITION BY HASH(col1 + YEAR(col2))
PARTITIONS 4;
CREATE TABLE t8 (
    col1 INT NOT NULL,
    col2 DATE NOT NULL,
    col3 INT NOT NULL,
    col4 INT NOT NULL,
    PRIMARY KEY(col1, col2, col4),
    UNIQUE KEY(col2, col1)
)
PARTITION BY HASH(col1 + YEAR(col2))
PARTITIONS 4;

If a table has no unique keys—this includes having no primary key—then this restriction does not apply, and you may use any column or columns in the partitioning expression as long as the column type is compatible with the partitioning type.

For the same reason, you cannot later add a unique key to a partitioned table unless the key includes all columns used by the table's partitioning expression. Consider the partitioned table created as shown here:

mysql> CREATE TABLE t_no_pk (c1 INT, c2 INT)
    ->     PARTITION BY RANGE(c1) (
    ->         PARTITION p0 VALUES LESS THAN (10),
    ->         PARTITION p1 VALUES LESS THAN (20),
    ->         PARTITION p2 VALUES LESS THAN (30),
    ->         PARTITION p3 VALUES LESS THAN (40)
    ->     );
Query OK, 0 rows affected (0.12 sec)

It is possible to add a primary key to t_no_pk using either of these ALTER TABLE statements:

#  possible PK
mysql> ALTER TABLE t_no_pk ADD PRIMARY KEY(c1);
Query OK, 0 rows affected (0.13 sec)
Records: 0  Duplicates: 0  Warnings: 0
# drop this PK
mysql> ALTER TABLE t_no_pk DROP PRIMARY KEY;
Query OK, 0 rows affected (0.10 sec)
Records: 0  Duplicates: 0  Warnings: 0
#  use another possible PK
mysql> ALTER TABLE t_no_pk ADD PRIMARY KEY(c1, c2);
Query OK, 0 rows affected (0.12 sec)
Records: 0  Duplicates: 0  Warnings: 0
# drop this PK
mysql> ALTER TABLE t_no_pk DROP PRIMARY KEY;
Query OK, 0 rows affected (0.09 sec)
Records: 0  Duplicates: 0  Warnings: 0

However, the next statement fails, because c1 is part of the partitioning key, but is not part of the proposed primary key:

#  fails with error 1503
mysql> ALTER TABLE t_no_pk ADD PRIMARY KEY(c2);
ERROR 1503 (HY000): A PRIMARY KEY must include all columns in the table's partitioning function

Since t_no_pk has only c1 in its partitioning expression, attempting to adding a unique key on c2 alone fails. However, you can add a unique key that uses both c1 and c2.

These rules also apply to existing nonpartitioned tables that you wish to partition using ALTER TABLE ... PARTITION BY. Consider a table np_pk created as shown here:

mysql> CREATE TABLE np_pk (
    ->     id INT NOT NULL AUTO_INCREMENT,
    ->     name VARCHAR(50),
    ->     added DATE,
    ->     PRIMARY KEY (id)
    -> );
Query OK, 0 rows affected (0.08 sec)

The following ALTER TABLE statement fails with an error, because the added column is not part of any unique key in the table:

mysql> ALTER TABLE np_pk
    ->     PARTITION BY HASH( TO_DAYS(added) )
    ->     PARTITIONS 4;
ERROR 1503 (HY000): A PRIMARY KEY must include all columns in the table's partitioning function

However, this statement using the id column for the partitioning column is valid, as shown here:

mysql> ALTER TABLE np_pk
    ->     PARTITION BY HASH(id)
    ->     PARTITIONS 4;
Query OK, 0 rows affected (0.11 sec)
Records: 0  Duplicates: 0  Warnings: 0

In the case of np_pk, the only column that may be used as part of a partitioning expression is id; if you wish to partition this table using any other column or columns in the partitioning expression, you must first modify the table, either by adding the desired column or columns to the primary key, or by dropping the primary key altogether.

The following limitations apply to the use of storage engines with user-defined partitioning of tables.

MERGE storage engine.  User-defined partitioning and the MERGE storage engine are not compatible. Tables using the MERGE storage engine cannot be partitioned. Partitioned tables cannot be merged.

FEDERATED storage engine.  Partitioning of FEDERATED tables is not supported; it is not possible to create partitioned FEDERATED tables.

CSV storage engine.  Partitioned tables using the CSV storage engine are not supported; it is not possible to create partitioned CSV tables.

InnoDB storage engine.  InnoDB foreign keys and MySQL partitioning are not compatible. Partitioned InnoDB tables cannot have foreign key references, nor can they have columns referenced by foreign keys. InnoDB tables which have or which are referenced by foreign keys cannot be partitioned.

In addition, ALTER TABLE ... OPTIMIZE PARTITION does not work correctly with partitioned tables that use the InnoDB storage engine. Use ALTER TABLE ... REBUILD PARTITION and ALTER TABLE ... ANALYZE PARTITION, instead, for such tables. For more information, see ALTER TABLE Partition Operations.

User-defined partitioning and the NDB storage engine (MySQL Cluster).  Partitioning by KEY (including LINEAR KEY) is the only type of partitioning supported for the NDB storage engine. It is not possible under normal circumstances in MySQL Cluster NDB 7.4 to create a MySQL Cluster table using any partitioning type other than [LINEAR] KEY, and attempting to do so fails with an error.

Exception (not for production): It is possible to override this restriction by setting the new system variable on MySQL Cluster SQL nodes to ON. If you choose to do this, you should be aware that tables using partitioning types other than [LINEAR] KEY are not supported in production. In such cases, you can create and use tables with partitioning types other than KEY or LINEAR KEY, but you do this entirely at your own risk.

The maximum number of partitions that can be defined for an NDB table depends on the number of data nodes and node groups in the cluster, the version of the MySQL Cluster software in use, and other factors. See NDB and user-defined partitioning, for more information.

The maximum amount of fixed-size data that can be stored per partition in an NDB table is 16 GB.

CREATE TABLE and ALTER TABLE statements that would cause a user-partitioned NDB table not to meet either or both of the following two requirements are not permitted, and fail with an error:

Exception.  If a user-partitioned NDB table is created using an empty column-list (that is, using PARTITION BY KEY() or PARTITION BY LINEAR KEY()), then no explicit primary key is required.

Upgrading partitioned tables.  When performing an upgrade, tables which are partitioned by KEY and which use any storage engine other than NDB must be dumped and reloaded.

Same storage engine for all partitions.  All partitions of a partitioned table must use the same storage engine and it must be the same storage engine used by the table as a whole. In addition, if one does not specify an engine on the table level, then one must do either of the following when creating or altering a partitioned table:

This section discusses limitations in MySQL Partitioning relating specifically to functions used in partitioning expressions.

Only the MySQL functions shown in the following table are allowed in partitioning expressions.

In MySQL 5.6, range optimization can be used for the TO_DAYS(), TO_SECONDS(), and YEAR() functions. In addition, beginning with MySQL 5.6.3, UNIX_TIMESTAMP() is treated as monotonic in partitioning expressions. See Chapter 5, Partition Pruning, for more information.

CEILING() and FLOOR().  Each of these functions returns an integer only if it is passed an argument of an exact numeric type, such as one of the INT types or DECIMAL. This means, for example, that the following CREATE TABLE statement fails with an error, as shown here:

mysql> CREATE TABLE t (c FLOAT) PARTITION BY LIST( FLOOR(c) )(
    ->     PARTITION p0 VALUES IN (1,3,5),
    ->     PARTITION p1 VALUES IN (2,4,6)
    -> );
ERROR 1490 (HY000): The PARTITION function returns the wrong type

EXTRACT() function with WEEK specifier.  The value returned by the EXTRACT() function, when used as EXTRACT(WEEK FROM col), depends on the value of the default_week_format system variable. For this reason, beginning with MySQL 5.6.2, EXTRACT() is no longer permitted as a partitioning function when it specifies the unit as WEEK. (Bug #54483)

See Mathematical Functions, for more information about the return types of these functions, as well as Numeric Types.

In MySQL 5.6.5 and earlier, for storage engines such as MyISAM that actually execute table-level locks when executing DML or DDL statements, such a statement affecting a partitioned table imposed a lock on the table as a whole; that is, all partitions were locked until the statement was finished. MySQL 5.6.6 implements partition lock pruning, which eliminates unneeded locks in many cases. In MySQL 5.6.6 and later, most statements reading from or updating a partitioned MyISAM table cause only the effected partitions to be locked. For example, prior to MySQL 5.6.6, a SELECT from a partitioned MyISAM table caused a lock on the entire table; in MySQL 5.6.6 and later, only those partitions actually containing rows that satisfy the SELECT statement's WHERE condition are locked. This has the effect of increasing the speed and efficiency of concurrent operations on partitioned MyISAM tables. This improvement becomes particularly noticeable when working with MyISAM tables that have many (32 or more) partitions.

This change in behavior does not have any impact on statements effecting partitioned tables using storage engines such as InnoDB, that employ row-level locking and do not actually perform (or need to perform) the locks prior to partition pruning.

The next few paragraphs discuss the effects of partition lock pruning for various MySQL statements on tables using storage engines that employ table-level locks.

Effects on DML statements

SELECT statements (including those containing unions or joins) now lock only those partitions that actually need to be read. This also applies to SELECT ... PARTITION.

An UPDATE prunes locks only for tables on which no partitioning columns are updated.

REPLACE and INSERT now lock only those partitions having rows to be inserted or replaced. However, if an AUTO_INCREMENT value is generated for any partitioning column then all partitions are locked.

INSERT ... ON DUPLICATE KEY UPDATE is pruned as long as no partitioning column is updated.

INSERT ... SELECT now locks only those partitions in the source table that need to be read, although all partitions in the target table are locked.

Locks imposed by LOAD DATA statements on partitioned tables cannot be pruned.

The presence of BEFORE INSERT or BEFORE UPDATE triggers using any partitioning column of a partitioned table means that locks on INSERT and UPDATE statements updating this table cannot be pruned, since the trigger can alter its values: A BEFORE INSERT trigger on any of the table's partitioning columns means that locks set by INSERT or REPLACE cannot be pruned, since the BEFORE INSERT trigger may change a row's partitioning columns before the row is inserted, forcing the row into a different partition than it would be otherwise. A BEFORE UPDATE trigger on a partitioning column means that locks imposed by UPDATE or INSERT ... ON DUPLICATE KEY UPDATE cannot be pruned.

Affected DDL statements

CREATE VIEW no longer causes any locks.

ALTER TABLE ... EXCHANGE PARTITION now prunes locks; only the exchanged table and the exchanged partition are locked.

ALTER TABLE ... TRUNCATE PARTITION now prunes locks; only the partitions to be emptied are locked.

ALTER TABLE statements still take metadata locks on the table level.

Other statements

LOCK TABLES cannot prune partition locks.

CALL stored_procedure(expr) supports lock pruning, but evaluating expr does not.

DO and SET statements do not support partitioning lock pruning.