The MyISAM storage engine was the default storage engine from MySQL 3.23 until it was replaced by 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 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
- Does not support transactions.
- Does not support foreign keys.
- Supports FULLTEXT indexes.
- Supports GIS 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 in MariaDB.
- Supports concurrent inserts.
- Supports fixed length, dynamic and compressed formats - see 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 to ensure files are not in use.
- There are a number of tools for working with MyISAM tables. These include:
- It is possible to build a MERGE table on the top of one or more MyISAM tables.