# MyISAM Overview

The MyISAM storage engine was the default storage engine from MySQL 3.23 until it was replaced as default by [InnoDB](https://mariadb.com/docs/server/server-usage/storage-engines/innodb) in MariaDB and MySQL 5.5. Historically, MyISAM is a replacement for the older ISAM engine, removed in MySQL 4.1.

It's a light, non-transactional engine with great performance, is easy to copy between systems and has a small data footprint.

You're encouraged to rather use the [Aria](https://mariadb.com/docs/server/server-usage/storage-engines/aria) storage engine for new applications, which has even better performance in most cases and the goal of being crash-safe.

A MyISAM table is stored in three files on disk. There's a table definition file with an extension of `.frm`, a data file with the extension `.MYD`, and an index file with the extension `.MYI`.

## MyISAM features

* Does not support [transactions](https://mariadb.com/docs/server/reference/sql-statements/transactions).
* Does not support foreign keys.
* Supports [FULLTEXT indexes](https://mariadb.com/docs/server/ha-and-performance/optimization-and-tuning/optimization-and-indexes/full-text-indexes).
* Supports [GIS](https://mariadb.com/docs/server/reference/sql-structure/geometry) data types.
* Storage limit of 256TB.
* Maximum of 64 indexes per table.
* Maximum of 32 columns per index.
* Maximum index length of 1000 bytes.
* Limit of (232)2 (1.844E+19) rows per table.
* Supports large files up to 63-bits in length where the underlying system supports this.
* All data is stored with the low byte first, so all files will still work if copied to other systems or other machines.
* The data file and the index file can be placed on different devices to improve speed.
* Supports table locking, not row locking.
* Supports a key buffer that is [segmented](https://mariadb.com/docs/server/ha-and-performance/optimization-and-tuning/system-variables/segmented-key-cache) in MariaDB.
* Supports [concurrent inserts](https://mariadb.com/docs/server/reference/sql-statements/data-manipulation/inserting-loading-data/concurrent-inserts).
* Supports fixed length, dynamic and compressed formats - see [MyISAM Storage Formats](https://mariadb.com/docs/server/server-usage/storage-engines/myisam-storage-engine/myisam-storage-formats).
* Numeric index values are stored with the high byte first, which enables more efficient index compression.
* Data values are stored with the low byte first, making it mostly machine and operating system independent. The only exceptions are if a machine doesn't use two's-complement signed integers and the IEEE floating-point format.
* Can be copied between databases or systems with normal system tools, as long as the files are not open on either system. Use [FLUSH\_TABLES](https://mariadb.com/docs/server/reference/sql-statements/administrative-sql-statements/flush-commands/flush) to ensure files are not in use.
* There are a number of tools for working with MyISAM tables. These include:
  * [mariadb-check](https://mariadb.com/docs/server/clients-and-utilities/table-tools/mariadb-check) for checking or repairing
  * [myisamchk](https://mariadb.com/docs/server/clients-and-utilities/myisam-clients-and-utilities/myisamchk) for checking or repairing
  * [myisampack](https://mariadb.com/docs/server/clients-and-utilities/myisam-clients-and-utilities/myisampack) for compressing
* It is possible to build a [MERGE](https://mariadb.com/docs/server/server-usage/storage-engines/merge) table on the top of one or more MyISAM tables.

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