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This section discusses current restrictions and limitations on MySQL partitioning support.
Prohibited constructs. The following constructs are not permitted in partitioning expressions:
Stored procedures, stored functions, UDFs, or plugins.
Declared variables or user variables.
For a list of SQL functions which are permitted in partitioning expressions, see Section 6.3, “Partitioning Limitations Relating to Functions”.
Arithmetic and logical operators.
Use of the arithmetic operators
* is permitted in
partitioning expressions. However, the result must be an integer
NULL (except in the case of
[LINEAR] KEY partitioning, as discussed
elsewhere in this chapter; see
Chapter 3, Partitioning Types, for more information).
In MySQL 5.5, the
HANDLER statement is not
supported with partitioned tables.
Server SQL mode. Tables employing user-defined partitioning do not preserve the SQL mode in effect at the time that they were created. As discussed in Server SQL Modes, the results of many MySQL functions and operators may change according to the server SQL mode. Therefore, a change in the SQL mode at any time after the creation of partitioned tables may lead to major changes in the behavior of such tables, and could easily lead to corruption or loss of data. For these reasons, it is strongly recommended that you never change the server SQL mode after creating partitioned tables.
Examples. The following examples illustrate some changes in behavior of partitioned tables due to a change in the server SQL mode:
Error handling. Suppose that you create a partitioned table whose partitioning expression is one such as
, as shown here:
mysql> CREATE TABLE tn (c1 INT) -> PARTITION BY LIST(1 DIV c1) ( -> PARTITION p0 VALUES IN (NULL), -> PARTITION p1 VALUES IN (1) -> ); Query OK, 0 rows affected (0.05 sec)
The default behavior for MySQL is to return
NULLfor the result of a division by zero, without producing any errors:
mysql> SELECT @@sql_mode; +------------+ | @@sql_mode | +------------+ | | +------------+ 1 row in set (0.00 sec) mysql> INSERT INTO tn VALUES (NULL), (0), (1); Query OK, 3 rows affected (0.00 sec) Records: 3 Duplicates: 0 Warnings: 0
However, changing the server SQL mode to treat division by zero as an error and to enforce strict error handling causes the same
INSERTstatement to fail, as shown here:
mysql> SET sql_mode='STRICT_ALL_TABLES,ERROR_FOR_DIVISION_BY_ZERO'; Query OK, 0 rows affected (0.00 sec) mysql> INSERT INTO tn VALUES (NULL), (0), (1); ERROR 1365 (22012): Division by 0
Table accessibility. Sometimes a change in the server SQL mode can make partitioned tables unusable. The following
CREATE TABLEstatement can be executed successfully only if the
NO_UNSIGNED_SUBTRACTIONmode is in effect:
mysql> SELECT @@sql_mode; +------------+ | @@sql_mode | +------------+ | | +------------+ 1 row in set (0.00 sec) mysql> CREATE TABLE tu (c1 BIGINT UNSIGNED) -> PARTITION BY RANGE(c1 - 10) ( -> PARTITION p0 VALUES LESS THAN (-5), -> PARTITION p1 VALUES LESS THAN (0), -> PARTITION p2 VALUES LESS THAN (5), -> PARTITION p3 VALUES LESS THAN (10), -> PARTITION p4 VALUES LESS THAN (MAXVALUE) -> ); ERROR 1563 (HY000): Partition constant is out of partition function domain mysql> SET sql_mode='NO_UNSIGNED_SUBTRACTION'; Query OK, 0 rows affected (0.00 sec) mysql> SELECT @@sql_mode; +-------------------------+ | @@sql_mode | +-------------------------+ | NO_UNSIGNED_SUBTRACTION | +-------------------------+ 1 row in set (0.00 sec) mysql> CREATE TABLE tu (c1 BIGINT UNSIGNED) -> PARTITION BY RANGE(c1 - 10) ( -> PARTITION p0 VALUES LESS THAN (-5), -> PARTITION p1 VALUES LESS THAN (0), -> PARTITION p2 VALUES LESS THAN (5), -> PARTITION p3 VALUES LESS THAN (10), -> PARTITION p4 VALUES LESS THAN (MAXVALUE) -> ); Query OK, 0 rows affected (0.05 sec)
If you remove the
NO_UNSIGNED_SUBTRACTIONserver SQL mode after creating
tu, you may no longer be able to access this table:
mysql> SET sql_mode=''; Query OK, 0 rows affected (0.00 sec) mysql> SELECT * FROM tu; ERROR 1563 (HY000): Partition constant is out of partition function domain mysql> INSERT INTO tu VALUES (20); ERROR 1563 (HY000): Partition constant is out of partition function domain
Server SQL modes also impact replication of partitioned tables. Differing SQL modes on master and slave can lead to partitioning expressions being evaluated differently; this can cause the distribution of data among partitions to be different in the master's and slave's copies of a given table, and may even cause inserts into partitioned tables that succeed on the master to fail on the slave. For best results, you should always use the same server SQL mode on the master and on the slave.
Performance considerations. Some effects of partitioning operations on performance are given in the following list:
File system operations. Partitioning and repartitioning operations (such as
PARTITION BY ...,
REORGANIZE PARTITION, or
REMOVE PARTITIONING) depend on file system operations for their implementation. This means that the speed of these operations is affected by such factors as file system type and characteristics, disk speed, swap space, file handling efficiency of the operating system, and MySQL server options and variables that relate to file handling. In particular, you should make sure that
large_files_supportis enabled and that
open_files_limitis set properly. For partitioned tables using the
MyISAMstorage engine, increasing
myisam_max_sort_file_sizemay improve performance; partitioning and repartitioning operations involving
InnoDBtables may be made more efficient by enabling
See also Maximum number of partitions.
MyISAM and partition file descriptor usage. For a partitioned
MyISAMtable, MySQL uses 2 file descriptors for each partition, for each such table that is open. This means that you need many more file descriptors to perform operations on a partitioned
MyISAMtable than on a table which is identical to it except that the latter table is not partitioned, particularly when performing
twith 100 partitions, such as the table created by this SQL statement:
CREATE TABLE t (c1 VARCHAR(50)) PARTITION BY KEY (c1) PARTITIONS 100 ENGINE=MYISAM;Note
For brevity, we use
KEYpartitioning for the table shown in this example, but file descriptor usage as described here applies to all partitioned
MyISAMtables, regardless of the type of partitioning that is employed. Partitioned tables using other storage engines such as
InnoDBare not affected by this issue.
Now assume that you wish to repartition
tso that it has 101 partitions, using the statement shown here:
ALTER TABLE t PARTITION BY KEY (c1) PARTITIONS 101;
To process this
ALTER TABLEstatement, MySQL uses 402 file descriptors—that is, two for each of the 100 original partitions, plus two for each of the 101 new partitions. This is because all partitions (old and new) must be opened concurrently during the reorganization of the table data. It is recommended that, if you expect to perform such operations, you should make sure that the
open_files_limitsystem variable is not set too low to accommodate them.
Table locks. The process executing a partitioning operation on a table takes a write lock on the table. Reads from such tables are relatively unaffected; pending
UPDATEoperations are performed as soon as the partitioning operation has completed.
Storage engine. Partitioning operations, queries, and update operations generally tend to be faster with
MyISAMtables than with
Indexes; partition pruning. As with nonpartitioned tables, proper use of indexes can speed up queries on partitioned tables significantly. In addition, designing partitioned tables and statements using these tables to take advantage of partition pruning can improve performance dramatically. See Chapter 5, Partition Pruning, for more information.
Performance with LOAD DATA. In MySQL 5.5,
LOAD DATAuses buffering to improve performance. You should be aware that the buffer uses 130 KB memory per partition to achieve this.
Maximum number of partitions.
The maximum possible number of partitions for a given table
(that does not use the
engine) is 1024. This number includes subpartitions.
The maximum possible number of user-defined partitions for a table
NDBCLUSTER storage engine
is determined according to the version of the NDB Cluster software
being used, the number of data nodes, and other factors. See
NDB and user-defined partitioning,
for more information.
If, when creating tables with a large number of partitions (but
less than the maximum), you encounter an error message such as
Got error ... from storage engine: Out of resources
when opening file, you may be able to address the
issue by increasing the value of the
open_files_limit system variable.
However, this is dependent on the operating system, and may not be
possible or advisable on all platforms; see
File Not Found and Similar Errors, for more information.
In some cases, using large numbers (hundreds) of partitions may
also not be advisable due to other concerns, so using more
partitions does not automatically lead to better results.
See also File system operations.
Query cache not supported. The query cache is not supported for partitioned tables. Beginning with MySQL 5.5.23, the query cache is automatically disabled for queries involving partitioned tables, and cannot be enabled for such queries. (Bug #53775)
Per-partition key caches.
In MySQL 5.5, key caches are supported for
MyISAM tables, using
CACHE INDEX and
LOAD INDEX INTO
CACHE statements. Key caches may be defined for one,
several, or all partitions, and indexes for one, several, or all
partitions may be preloaded into key caches.
Foreign keys not supported for partitioned InnoDB tables.
Partitioned tables using the
storage engine do not support foreign keys. More specifically,
this means that the following two statements are true:
No definition of an
InnoDBtable employing user-defined partitioning may contain foreign key references; no
InnoDBtable whose definition contains foreign key references may be partitioned.
InnoDBtable definition may contain a foreign key reference to a user-partitioned table; no
InnoDBtable with user-defined partitioning may contain columns referenced by foreign keys.
The scope of the restrictions just listed includes all tables that
InnoDB storage engine.
statements that would result in tables violating these
restrictions are not allowed.
ALTER TABLE ... ORDER BY.
ALTER TABLE ... ORDER BY
against a partitioned table causes ordering of rows only within
Effects on REPLACE statements by modification of primary keys.
It can be desirable in some cases (see
Section 6.1, “Partitioning Keys, Primary Keys, and Unique Keys”)
to modify a table's primary key. Be aware that, if your
statements and you do this, the results of these statements can
be drastically altered. See REPLACE Statement, for more
information and an example.
Partitioned tables do not support
indexes or searches. This includes partitioned tables employing
MyISAM storage engine.
Columns with spatial data types such as
GEOMETRY cannot be used in partitioned
It is not possible to partition the log tables; an
TABLE ... PARTITION BY ... statement on such a table
fails with an error.
Data type of partitioning key.
A partitioning key must be either an integer column or an
expression that resolves to an integer. Expressions employing
ENUM columns cannot be used. The
column or expression value may also be
(See Section 3.7, “How MySQL Partitioning Handles NULL”.)
There are two exceptions to this restriction:
When partitioning by
[LINEAR] KEY, it is possible to use columns of any valid MySQL data type other than
BLOBas partitioning keys, because MySQL's internal key-hashing functions produce the correct data type from these types. For example, the following two
CREATE TABLEstatements are valid:
CREATE TABLE tkc (c1 CHAR) PARTITION BY KEY(c1) PARTITIONS 4; CREATE TABLE tke ( c1 ENUM('red', 'orange', 'yellow', 'green', 'blue', 'indigo', 'violet') ) PARTITION BY LINEAR KEY(c1) PARTITIONS 6;
CREATE TABLE rc (c1 INT, c2 DATE) PARTITION BY RANGE COLUMNS(c2) ( PARTITION p0 VALUES LESS THAN('1990-01-01'), PARTITION p1 VALUES LESS THAN('1995-01-01'), PARTITION p2 VALUES LESS THAN('2000-01-01'), PARTITION p3 VALUES LESS THAN('2005-01-01'), PARTITION p4 VALUES LESS THAN(MAXVALUE) ); CREATE TABLE lc (c1 INT, c2 CHAR(1)) PARTITION BY LIST COLUMNS(c2) ( PARTITION p0 VALUES IN('a', 'd', 'g', 'j', 'm', 'p', 's', 'v', 'y'), PARTITION p1 VALUES IN('b', 'e', 'h', 'k', 'n', 'q', 't', 'w', 'z'), PARTITION p2 VALUES IN('c', 'f', 'i', 'l', 'o', 'r', 'u', 'x', NULL) );
SUBPARTITION BY KEY requires that the
subpartitioning column or columns be specified explicitly, unlike
the case with
PARTITION BY KEY, where it can be
omitted (in which case the table's primary key column is used
by default). Consider the table created by this statement:
CREATE TABLE ts ( id INT NOT NULL AUTO_INCREMENT PRIMARY KEY, name VARCHAR(30) );
You can create a table having the same columns, partitioned by
KEY, using a statement such as this one:
CREATE TABLE ts ( id INT NOT NULL AUTO_INCREMENT PRIMARY KEY, name VARCHAR(30) ) PARTITION BY KEY() PARTITIONS 4;
The previous statement is treated as though it had been written like this, with the table's primary key column used as the partitioning column:
CREATE TABLE ts ( id INT NOT NULL AUTO_INCREMENT PRIMARY KEY, name VARCHAR(30) ) PARTITION BY KEY(id) PARTITIONS 4;
However, the following statement that attempts to create a subpartitioned table using the default column as the subpartitioning column fails, and the column must be specified for the statement to succeed, as shown here:
mysql> CREATE TABLE ts ( -> id INT NOT NULL AUTO_INCREMENT PRIMARY KEY, -> name VARCHAR(30) -> ) -> PARTITION BY RANGE(id) -> SUBPARTITION BY KEY() -> SUBPARTITIONS 4 -> ( -> PARTITION p0 VALUES LESS THAN (100), -> PARTITION p1 VALUES LESS THAN (MAXVALUE) -> ); ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ') mysql> CREATE TABLE ts ( -> id INT NOT NULL AUTO_INCREMENT PRIMARY KEY, -> name VARCHAR(30) -> ) -> PARTITION BY RANGE(id) -> SUBPARTITION BY KEY(id) -> SUBPARTITIONS 4 -> ( -> PARTITION p0 VALUES LESS THAN (100), -> PARTITION p1 VALUES LESS THAN (MAXVALUE) -> ); Query OK, 0 rows affected (0.07 sec)
This is a known issue (see Bug #51470).
DELAYED option not supported.
INSERT DELAYED to insert
rows into a partitioned table is not supported. Attempting to do
so fails with an error.
DATA DIRECTORY and INDEX DIRECTORY options.
DATA DIRECTORY and
DIRECTORY are subject to the following restrictions
when used with partitioned tables:
INDEX DIRECTORYoptions are ignored (see Bug #32091).
On Windows, the
INDEX DIRECTORYoptions are not supported for individual partitions or subpartitions (Bug #30459).
In addition, you can use
ALTER TABLE ... REBUILD
PARTITION to rebuild one or more partitions of a
ALTER TABLE ... REORGANIZE
PARTITION also causes partitions to be rebuilt. See
ALTER TABLE Statement, for more information about these
mysqlcheck, myisamchk, and myisampack are not supported with partitioned tables.