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17.1. Overview of Partitioning in MySQL

This section provides a conceptual overview of partitioning in MySQL 5.1.

For information on partitioning restrictions and feature limitations, see Section 17.4, “Restrictions and Limitations on Partitioning”.

The SQL standard does not provide much in the way of guidance regarding the physical aspects of data storage. The SQL language itself is intended to work independently of any data structures or media underlying the schemas, tables, rows, or columns with which it works. Nonetheless, most advanced database management systems have evolved some means of determining the physical location to be used for storing specific pieces of data in terms of the filesystem, hardware or even both. In MySQL, the InnoDB storage engine has long supported the notion of a tablespace, and the MySQL Server, even prior to the introduction of partitioning, could be configured to employ different physical directories for storing different databases (see Section 7.6.1, “Using Symbolic Links”, for an explanation of how this is done).

Partitioning takes this notion a step further, by allowing you to distribute portions of individual tables across a filesystem according to rules which you can set largely as needed. In effect, different portions of a table are stored as separate tables in different locations. The user-selected rule by which the division of data is accomplished is known as a partitioning function, which in MySQL can be the modulus, simple matching against a set of ranges or value lists, an internal hashing function, or a linear hashing function. The function is selected according to the partitioning type specified by the user, and takes as its parameter the value of a user-supplied expression. This expression can be either an integer column value, or a function acting on one or more column values and returning an integer. The value of this expression is passed to the partitioning function, which returns an integer value representing the number of the partition in which that particular record should be stored. This function must be non-constant and non-random. It may not contain any queries, but may use virtually any SQL expression that is valid in MySQL, so long as that expression returns a positive integer less than MAXVALUE (the greatest possible positive integer). Examples of partitioning functions can be found in the discussions of partitioning types later in this chapter (see Section 17.2, “Partition Types”), as well as in the partitioning syntax descriptions given in Section 13.1.5, “CREATE TABLE Syntax”.

This is known as horizontal partitioning — that is, different rows of a table may be assigned to different physical partitions. MySQL 5.1 does not support vertical partitioning, in which different columns of a table are assigned to different physical partitions. There are not at this time any plans to introduce vertical partitioning into MySQL 5.1.

Partitioning support is included in the -max releases of MySQL 5.1 (that is, the 5.1 -max binaries will be built with --with-partition). If the MySQL binary is built with partitioning support, nothing further needs to be done in order to enable it (for example, no special entries are required in your my.cnf file). You can determine whether your MySQL server supports partitioning by means of a SHOW VARIABLES command such as this one:

mysql> SHOW VARIABLES LIKE '%partition%';

+-------------------+-------+
| Variable_name     | Value |
+-------------------+-------+
| have_partitioning | YES   |
+-------------------+-------+
1 row in set (0.00 sec)

If you do not see the have_partitioning variable with the value YES listed as shown above in the output of an appropriate SHOW VARIABLES, then your version of MySQL does not support partitioning.

Prior to MySQL 5.1.6, this variable was named have_partition_engine. (Bug #16718)

For creating partitioned tables, you can use any storage engine that is supported by your MySQL server; the MySQL partitioning engine runs in a separate layer and can interact with any of these. In MySQL 5.1, all partitions of the same partitioned table must use the same storage engine; for example, you cannot use MyISAM for one partition and InnoDB for another. However, there is nothing preventing you from using different storage engines for different partitioned tables on the same MySQL server or even in the same database.

Note: Prior to MySQL 5.1.6, it was not feasible to create a partitioned table using the BLACKHOLE storage engine. (Bug #14524).

To employ a particular storage engine for a partitioned table, it is necessary only to use the [STORAGE] ENGINE option just as you would for a non-partitioned table. However, you should keep in mind that [STORAGE] ENGINE (and other table options) need to be listed before any partitioning options are used in a CREATE TABLE statement. This example shows how to create a table that is partitioned by hash into 6 partitions and which uses the InnoDB storage engine:

CREATE TABLE ti (id INT, amount DECIMAL(7,2), tr_date DATE)
    ENGINE=INNODB
    PARTITION BY HASH( MONTH(tr_date) )
    PARTITIONS 6;

(Note that each PARTITION clause can include a [STORAGE] ENGINE option, but in MySQL 5.1 this has no effect.)

It is possible to create partitioned temporary tables; however, the lifetime of such tables is only as long as the current MySQL session. This is the same as for non-partitioned temporary tables.

Note: Partitioning applies to all data and indexes of a table; you cannot partition only the data and not the indexes, or vice versa, nor can you partition only a portion of the table.

Data and indexes for each partition can be assigned to a specific directory using the DATA DIRECTORY and INDEX DIRECTORY options for the PARTITION clause of the CREATE TABLE statement used to create the partitioned table. In addition, MAX_ROWS and MIN_ROWS can be used to determine the maximum and minimum numbers of rows, respectively, that can be stored in each partition. See Section 17.3, “Partition Management”, for more information on these options. Note: This particular feature is currently nonfunctional due to Bug #13250; we should have this fixed by the time the first 5.1 binaries are made available.

Some of the advantages of partitioning include:

  • Being able to store more data in one table than can be held on a single disk or filesystem partition.

  • Data that loses its usefulness can often be easily be removed from the table by dropping the partition containing only that data. Conversely, the process of adding new data can in some cases be greatly facilitated by adding a new partition specifically for that data.

  • Some queries can be greatly optimized in virtue of the fact that data satisfying a given WHERE clause can be stored only on one or more partitions, thereby excluding any remaining partitions from the search. Because partitions can be altered after a partitioned table has been created, you can reorganize your data to enhance frequent queries that may not have been so when the partitioning scheme was first set up. This capability, sometimes referred to as partition pruning, was implemented in MySQL 5.1.6.

Other benefits usually associated with partitioning include those in the following list. These features are not currently implemented in MySQL Partitioning, but are high on our list of priorities; we hope to include them in the 5.1 production release.

  • Queries involving aggregate functions such as SUM() and COUNT() can easily be parallelized. A simple example of such a query might be SELECT salesperson_id, COUNT(orders) as order_total FROM sales GROUP BY salesperson_id;. By “parallelized,” we mean that the query can be run simultaneously on each partition, and the final result obtained merely by summing the results obtained for all partitions.

  • Achieving greater query throughput in virtue of spreading data seeks over multiple disks.

Be sure to check this page and chapter frequently for updates as Partitioning development for MySQL 5.1 continues.


 
 
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