15.1 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 5.7. 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
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;
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 5.1.10, “Server SQL Modes”.
For new tables, MySQL always creates an
.frm file to hold the table and column definitions. The table's index and data may be stored in one or more other files, depending on the storage engine. The server creates the
.frm file above the storage engine level. 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”.