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.