14 Storage Engines and Table Types
* myisam-storage-engine The `MyISAM' Storage Engine
* innodb The `InnoDB' Storage Engine
* merge-storage-engine The `MERGE' Storage Engine
* memory-storage-engine The `MEMORY' (`HEAP') Storage Engine
* bdb-storage-engine The `BDB' (`BerkeleyDB') Storage Engine
* example-storage-engine The `EXAMPLE' Storage Engine
* federated-storage-engine The `FEDERATED' Storage Engine
* archive-storage-engine The `ARCHIVE' Storage Engine
* csv-storage-engine The `CSV' Storage Engine
* blackhole-storage-engine The `BLACKHOLE' Storage Engine
MySQL supports several storage engines that act as handlers for
different table types. MySQL storage engines include both those that
handle transaction-safe tables and those that handle
* `MyISAM' manages non-transactional tables. It provides high-speed
storage and retrieval, as well as fulltext searching capabilities.
`MyISAM' is supported in all MySQL configurations, and is the
default storage engine unless you have configured MySQL to use a
different one by default.
* The `MEMORY' storage engine provides in-memory tables. The `MERGE'
storage engine allows a collection of identical `MyISAM' tables to
be handled as a single table. Like `MyISAM', the `MEMORY' and
`MERGE' storage engines handle non-transactional tables, and both
are also included in MySQL by default.
* The `MEMORY' storage engine formerly was known as the
* The `InnoDB' and `BDB' storage engines provide transaction-safe
tables. `BDB' is included in MySQL-Max binary distributions on
those operating systems that support it. `InnoDB' is also included
by default in all MySQL 5.0 binary distributions. In source
distributions, you can enable or disable either engine by
configuring MySQL as you like.
* The `EXAMPLE' storage engine is a `stub' engine that does nothing.
You can create tables with this engine, but no data can be stored
in them or retrieved from them. The purpose of this engine is to
serve as an example in the MySQL source code that illustrates how
to begin writing new storage engines. As such, it is primarily of
interest to developers.
* `NDB Cluster' is the storage engine used by MySQL Cluster to
implement tables that are partitioned over many computers. It is
available in MySQL-Max 5.0 binary distributions. This storage
engine is currently supported on Linux, Solaris, and Mac OS X
only. We intend to add support for this engine on other platforms,
including Windows, in future MySQL releases.
* The `ARCHIVE' storage engine is used for storing large amounts of
data without indexes with a very small footprint.
* The `CSV' storage engine stores data in text files using
comma-separated values format.
* The `BLACKHOLE' storage engine accepts but does not store data and
retrievals always return an empty set.
* The `FEDERATED' storage engine was added in MySQL 5.0.3. This
engine stores data in a remote database. Currently, it works with
MySQL only, using the MySQL C Client API. In future releases, we
intend to enable it to connect to other data sources using other
drivers or client connection methods.
This chapter describes each of the MySQL storage engines except for
`NDB Cluster', which is covered in ndbcluster.
When you create a new table, you can specify which storage engine to
use by adding an `ENGINE' or `TYPE' table option to the `CREATE TABLE'
CREATE TABLE t (i INT) ENGINE = INNODB;
CREATE TABLE t (i INT) TYPE = MEMORY;
The older term `TYPE' is supported as a synonym for `ENGINE' for
backward compatibility, but `ENGINE' is the preferred term and `TYPE'
If you omit the `ENGINE' or `TYPE' option, the default storage engine
is used. Normally, this is `MyISAM', but you can change it by using the
-default-storage-engine or -default-table-type server startup option,
or by setting the `storage_engine' or `table_type' system variable.
When MySQL is installed on Windows using the MySQL Configuration
Wizard, the `InnoDB' storage engine can be selected as the default
instead of `MyISAM'. See mysql-config-wizard-database-usage.
To convert a table from one type to another, use an `ALTER TABLE'
statement that indicates the new type:
ALTER TABLE t ENGINE = MYISAM;
ALTER TABLE t TYPE = BDB;
See create-table, and alter-table.
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, usually `MyISAM'. This behavior is convenient
when you want to copy tables between MySQL servers that support
different storage engines. (For example, in a replication setup,
perhaps your master server supports transactional storage engines for
increased safety, but the slave servers use only non-transactional
storage engines for greater speed.)
This automatic substitution of the default storage engine for
unavailable engines can be confusing for new MySQL users. A warning is
generated whenever a storage engine is automatically changed.
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.
A database may contain tables of different types. That is, tables need
not all be created with the same storage engine.
Transaction-safe tables (TSTs) have several advantages over
non-transaction-safe tables (NTSTs):
* They are safer. Even if MySQL crashes or you get hardware
problems, you can get your data back, either by automatic recovery
or from a backup plus the transaction log.
* You can combine many statements and accept them all at the same
time with the `COMMIT' statement (if autocommit is disabled).
* You can execute `ROLLBACK' to ignore your changes (if autocommit
* If an update fails, all of your changes are reverted. (With
non-transaction-safe tables, all changes that have taken place are
* Transaction-safe storage engines can provide better concurrency
for tables that get many updates concurrently with reads.
You can combine transaction-safe and non-transaction-safe tables in the
same statements to get the best of both worlds. However, although MySQL
supports several transaction-safe storage engines, for best results,
you should not mix different storage engines within a transaction with
autocommit disabled. For example, if you do this, changes to
non-transaction-safe tables still are committed immediately and cannot
be rolled back. For information about this and other problems that can
occur in transactions that use mixed storage engines, see
Non-transaction-safe tables have several advantages of their own, all
of which occur because there is no transaction overhead:
* Much faster
* Lower disk space requirements
* Less memory required to perform updates
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