Mysql：Alternative Storage Engines：可替换存储引擎：mysqld的核心特性

请注意：NO_ENGINE_SUBSTITUTION

Chapter 16 Alternative Storage Engines

Table of Contents

16.1 Setting the Storage Engine

Storage engines are MySQL components that handle the SQL operations for different table types. InnoDB is the default and most general-purpose storage engine, and Oracle recommends using it for tables except for specialized use cases. (The CREATE TABLE statement in MySQL 8.0 creates InnoDB tables by default.)

MySQL Server uses a pluggable storage engine architecture that enables storage engines to be loaded into and unloaded from a running MySQL server.

To determine which storage engines your server supports, use the SHOW ENGINES statement. The value in the Support column indicates whether an engine can be used. A value of YES, NO, or DEFAULT indicates that an engine is available, not available, or available and currently set as the default storage engine.

mysql> SHOW ENGINES\G
*************************** 1. row ***************************
      Engine: PERFORMANCE_SCHEMA
     Support: YES
     Comment: Performance Schema
Transactions: NO
          XA: NO
  Savepoints: NO
*************************** 2. row ***************************
      Engine: InnoDB
     Support: DEFAULT
     Comment: Supports transactions, row-level locking, and foreign keys
Transactions: YES
          XA: YES
  Savepoints: YES
*************************** 3. row ***************************
      Engine: MRG_MYISAM
     Support: YES
     Comment: Collection of identical MyISAM tables
Transactions: NO
          XA: NO
  Savepoints: NO
*************************** 4. row ***************************
      Engine: BLACKHOLE
     Support: YES
     Comment: /dev/null storage engine (anything you write to it disappears)
Transactions: NO
          XA: NO
  Savepoints: NO
*************************** 5. row ***************************
      Engine: MyISAM
     Support: YES
     Comment: MyISAM storage engine
Transactions: NO
          XA: NO
  Savepoints: NO
...

This chapter covers use cases for special-purpose MySQL storage engines. It does not cover the default InnoDB storage engine or the NDB storage engine which are covered in Chapter 15, The InnoDB Storage Engine and Chapter 22, MySQL NDB Cluster 8.0. For advanced users, it also contains a description of the pluggable storage engine architecture (see Section 16.11, “Overview of MySQL Storage Engine Architecture”).

For information about features offered in commercial MySQL Server binaries, see MySQL Editions, on the MySQL website. The storage engines available might depend on which edition of MySQL you are using.

For answers to commonly asked questions about MySQL storage engines, see Section A.2, “MySQL 8.0 FAQ: Storage Engines”.

MySQL 8.0 Supported Storage Engines

• InnoDB: The default storage engine in MySQL 8.0. InnoDB is a transaction-safe (ACID compliant) storage engine for MySQL that has commit, rollback, and crash-recovery capabilities to protect user data. InnoDB row-level locking (without escalation to coarser granularity locks) and Oracle-style consistent nonlocking reads increase multi-user concurrency and performance. InnoDB stores user data in clustered indexes to reduce I/O for common queries based on primary keys. To maintain data integrity, InnoDB also supports FOREIGN KEY referential-integrity constraints. For more information about InnoDB, see Chapter 15, The InnoDB Storage Engine.

• MyISAM: These tables have a small footprint. Table-level locking limits the performance in read/write workloads, so it is often used in read-only or read-mostly workloads in Web and data warehousing configurations.

• Memory: Stores all data in RAM, for fast access in environments that require quick lookups of non-critical data. This engine was formerly known as the HEAP engine. Its use cases are decreasing; InnoDB with its buffer pool memory area provides a general-purpose and durable way to keep most or all data in memory, and NDBCLUSTER provides fast key-value lookups for huge distributed data sets.

• CSV: Its tables are really text files with comma-separated values. CSV tables let you import or dump data in CSV format, to exchange data with scripts and applications that read and write that same format. Because CSV tables are not indexed, you typically keep the data in InnoDB tables during normal operation, and only use CSV tables during the import or export stage.

• Archive: These compact, unindexed tables are intended for storing and retrieving large amounts of seldom-referenced historical, archived, or security audit information.

• Blackhole: The Blackhole storage engine accepts but does not store data, similar to the Unix /dev/null device. Queries always return an empty set. These tables can be used in replication configurations where DML statements are sent to slave servers, but the master server does not keep its own copy of the data.

• NDB (also known as NDBCLUSTER): This clustered database engine is particularly suited for applications that require the highest possible degree of uptime and availability.

• Merge: Enables a MySQL DBA or developer to logically group a series of identical MyISAM tables and reference them as one object. Good for VLDB environments such as data warehousing.

• Federated: Offers the ability to link separate MySQL servers to create one logical database from many physical servers. Very good for distributed or data mart environments.

• Example: This engine serves as an example in the MySQL source code that illustrates how to begin writing new storage engines. It is primarily of interest to developers. The storage engine is a “stub” that does nothing. You can create tables with this engine, but no data can be stored in them or retrieved from them.

You are not restricted to using the same storage engine for an entire server or schema. You can specify the storage engine for any table. For example, an application might use mostly InnoDB tables, with one CSV table for exporting data to a spreadsheet and a few MEMORY tables for temporary workspaces.

Choosing a Storage Engine

The various storage engines provided with MySQL are designed with different use cases in mind. The following table provides an overview of some storage engines provided with MySQL, with clarifying notes following the table.

Table 16.1 Storage Engines Feature Summary

Feature	MyISAM	Memory	InnoDB	Archive	NDB
B-tree indexes	Yes	Yes	Yes	No	No
Backup/point-in-time recovery (note 1)	Yes	Yes	Yes	Yes	Yes
Cluster database support	No	No	No	No	Yes
Clustered indexes	No	No	Yes	No	No
Compressed data	Yes (note 2)	No	Yes	Yes	No
Data caches	No	N/A	Yes	No	Yes
Encrypted data	Yes (note 3)	Yes (note 3)	Yes (note 4)	Yes (note 3)	Yes (note 3)
Foreign key support	No	No	Yes	No	Yes (note 5)
Full-text search indexes	Yes	No	Yes (note 6)	No	No
Geospatial data type support	Yes	No	Yes	Yes	Yes
Geospatial indexing support	Yes	No	Yes (note 7)	No	No
Hash indexes	No	Yes	No (note 8)	No	Yes
Index caches	Yes	N/A	Yes	No	Yes
Locking granularity	Table	Table	Row	Row	Row
MVCC	No	No	Yes	No	No
Replication support (note 1)	Yes	Limited (note 9)	Yes	Yes	Yes
Storage limits	256TB	RAM	64TB	None	384EB
T-tree indexes	No	No	No	No	Yes
Transactions	No	No	Yes	No	Yes
Update statistics for data dictionary	Yes	Yes	Yes	Yes	Yes

 

Notes:

1. Implemented in the server, rather than in the storage engine.

2. Compressed MyISAM tables are supported only when using the compressed row format. Tables using the compressed row format with MyISAM are read only.

3. Implemented in the server via encryption functions.

4. Implemented in the server via encryption functions; In MySQL 5.7 and later, data-at-rest tablespace encryption is supported.

5. Support for foreign keys is available in MySQL Cluster NDB 7.3 and later.

6. InnoDB support for FULLTEXT indexes is available in MySQL 5.6 and later.

7. InnoDB support for geospatial indexing is available in MySQL 5.7 and later.

8. InnoDB utilizes hash indexes internally for its Adaptive Hash Index feature.

9. See the discussion later in this section.

16.1 Setting the Storage Engine

When you create a new table, you can specify which storage engine to use by adding an ENGINE table option to the CREATE TABLE statement:

-- ENGINE=INNODB not needed unless you have set a different
-- default storage engine.
CREATE TABLE t1 (i INT) ENGINE = INNODB;
-- Simple table definitions can be switched from one to another.
CREATE TABLE t2 (i INT) ENGINE = CSV;
CREATE TABLE t3 (i INT) ENGINE = MEMORY;

When you omit the ENGINE option, the default storage engine is used. The default engine is InnoDB in MySQL 8.0. You can specify the default engine by using the --default-storage-engine server startup option, or by setting the default-storage-engine option in the my.cnf configuration file.

You can set the default storage engine for the current session by setting the default_storage_engine variable:

SET default_storage_engine=NDBCLUSTER;

The storage engine for TEMPORARY tables created with CREATE TEMPORARY TABLE can be set separately from the engine for permanent tables by setting the default_tmp_storage_engine, either at startup or at runtime.

To convert a table from one storage engine to another, use an ALTER TABLE statement that indicates the new engine:

ALTER TABLE t ENGINE = InnoDB;

See Section 13.1.20, “CREATE TABLE Statement”, and Section 13.1.9, “ALTER TABLE Statement”.

If you try to use a storage engine that is not compiled in or that is compiled in but deactivated, MySQL instead creates a table using the default storage engine. For example, in a replication setup, perhaps your master server uses InnoDB tables for maximum safety, but the slave servers use other storage engines for speed at the expense of durability or concurrency.

By default, a warning is generated whenever CREATE TABLE or ALTER TABLE cannot use the default storage engine. To prevent confusing, unintended behavior if the desired engine is unavailable, enable the NO_ENGINE_SUBSTITUTION SQL mode. If the desired engine is unavailable, this setting produces an error instead of a warning, and the table is not created or altered. See Section 5.1.11, “Server SQL Modes”.

MySQL may store a table's index and data in one or more other files, depending on the storage engine. Table and column definitions are stored in the MySQL data dictionary. Individual storage engines create any additional files required for the tables that they manage. If a table name contains special characters, the names for the table files contain encoded versions of those characters as described in Section 9.2.4, “Mapping of Identifiers to File Names”.

 

• NO_ENGINE_SUBSTITUTION

  Control automatic substitution of the default storage engine when a statement such as CREATE TABLE or ALTER TABLE specifies a storage engine that is disabled or not compiled in.

  By default, NO_ENGINE_SUBSTITUTION is enabled.

  Because storage engines can be pluggable at runtime, unavailable engines are treated the same way:

  With NO_ENGINE_SUBSTITUTION disabled, for CREATE TABLE the default engine is used and a warning occurs if the desired engine is unavailable. For ALTER TABLE, a warning occurs and the table is not altered.

  With NO_ENGINE_SUBSTITUTION enabled, an error occurs and the table is not created or altered if the desired engine is unavailable.