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23.1 Overview of Partitioning in MySQL

Inhoudsopgave

This section provides a conceptual overview of partitioning in MySQL 8.0.

For information on partitioning restrictions and feature limitations, see Section 23.6, “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 file system, hardware or even both. In MySQL, the InnoDB storage engine has long supported the notion of a tablespace (see Section 15.6.3, “Tablespaces”), 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 8.12.2, “Using Symbolic Links”, for an explanation of how this is done).

Partitioning takes this notion a step further, by enabling you to distribute portions of individual tables across a file system 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 a column value, a function acting on one or more column values, or a set of one or more column values, depending on the type of partitioning that is used.

In the case of RANGE, LIST, and [LINEAR] HASH partitioning, the value of the partitioning column 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 nonconstant and nonrandom. It may not contain any queries, but may use an SQL expression that is valid in MySQL, as long as that expression returns either NULL or an integer intval such that 

-MAXVALUE <= intval <= MAXVALUE

(MAXVALUE is used to represent the least upper bound for the type of integer in question. -MAXVALUE represents the greatest lower bound.)

For [LINEAR] KEY, RANGE COLUMNS, and LIST COLUMNS partitioning, the partitioning expression consists of a list of one or more columns.

For [LINEAR] KEY partitioning, the partitioning function is supplied by MySQL.

For more information about permitted partitioning column types and partitioning functions, see Section 23.2, “Partitioning Types”, as well as Section 13.1.20, “CREATE TABLE Syntax”, which provides partitioning syntax descriptions and additional examples. For information about restrictions on partitioning functions, see Section 23.6.3, “Partitioning Limitations Relating to Functions”.

This is known as horizontal partitioning—that is, different rows of a table may be assigned to different physical partitions. MySQL 8.0 does not support vertical partitioning, in which different columns of a table are assigned to different physical partitions. There are no plans at this time to introduce vertical partitioning into MySQL.

For creating partitioned tables, you must use a storage engine that supports them. In MySQL 8.0, all partitions of the same partitioned table must use the same storage engine. 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.

In MySQL 8.0, the only storage engines that support partitioning are InnoDB and NDB. Partitioning cannot be used with storage engines that do not support it; these include the MyISAM, MERGE, CSV, and FEDERATED storage engines.

Partitioning by KEY or LINEAR KEY is possible with NDB, but other types of user-defined partitioning are not supported for tables using this storage engine. In addition, an NDB table that employs user-defined partitioning must have an explicit primary key, and any columns referenced in the table's partitioning expression must be part of the primary key. However, if no columns are listed in the PARTITION BY KEY or PARTITION BY LINEAR KEY clause of the CREATE TABLE or ALTER TABLE statement used to create or modify a user-partitioned NDB table, then the table is not required to have an explicit primary key. For more information, see Section 22.1.7.1, “Noncompliance with SQL Syntax in NDB Cluster”.

When creating a partitioned table, the default storage engine is used just as when creating any other table; to override this behavior, it is necessary only to use the [STORAGE] ENGINE option just as you would for a table that is not partitioned. The target storage engine must provide native partitioning support, or the statement fails. 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 (regardless of the value of default_storage_engine):

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

Each PARTITION clause can include a [STORAGE] ENGINE option, but in MySQL 8.0 this has no effect.

Unless otherwise specified, the remaining examples in this discussion assume that default_storage_engine is InnoDB.

Important

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.

Only the DATA DIRECTORY option is supported for individual partitions and subpartitions of InnoDB tables.

All columns used in the table's partitioning expression must be part of every unique key that the table may have, including any primary key. This means that a table such as this one, created by the following SQL statement, cannot be partitioned:

  1. CREATE TABLE tnp (
  2.     id INT NOT NULL AUTO_INCREMENT,
  3.     ref BIGINT NOT NULL,
  4.     name VARCHAR(255),
  5.     PRIMARY KEY pk (id),
  6.     UNIQUE KEY uk (name)
  7. );

Because the keys pk and uk have no columns in common, there are no columns available for use in a partitioning expression. Possible workarounds in this situation include adding the name column to the table's primary key, adding the id column to uk, or simply removing the unique key altogether. See Section 23.6.1, “Partitioning Keys, Primary Keys, and Unique Keys”, for more information.

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 23.3, “Partition Management”, for more information on these options.

The MAX_ROWS option can also be useful for creating NDB Cluster tables with extra partitions, thus allowing for greater storage of hash indexes. See the documentation for the DataMemory data node configuration parameter, as well as Section 22.1.2, “NDB Cluster Nodes, Node Groups, Replicas, and Partitions”, for more information.

Some advantages of partitioning are listed here:

  • Partitioning makes it possible to store more data in one table than can be held on a single disk or file system partition.

  • Data that loses its usefulness can often be easily removed from a partitioned table by dropping the partition (or partitions) containing only that data. Conversely, the process of adding new data can in some cases be greatly facilitated by adding one or more new partitions for storing specifically 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, which automatically excludes 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 often used when the partitioning scheme was first set up. This ability to exclude non-matching partitions (and thus any rows they contain) is often referred to as partition pruning. For more information, see Section 23.4, “Partition Pruning”.

    In addition, MySQL supports explicit partition selection for queries. For example, SELECT * FROM t PARTITION (p0,p1) WHERE c < 5 selects only those rows in partitions p0 and p1 that match the WHERE condition. In this case, MySQL does not check any other partitions of table t; this can greatly speed up queries when you already know which partition or partitions you wish to examine. Partition selection is also supported for the data modification statements DELETE, INSERT, REPLACE, UPDATE, and LOAD DATA, LOAD XML. See the descriptions of these statements for more information and examples.


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