MyISAM

MyISAM was the default storage engine for the MySQL relational database management system versions prior to 5.5. It is based on the older ISAM code, but it has many useful extensions.

Filesystem

Each MyISAM table is stored on disk in three files (if it is not partitioned). The files have names that begin with the table name and have an extension to indicate the file type. MySQL uses a .frm file to store the definition of the table, but this file is not a part of the MyISAM engine; instead it is a part of the server. The data file has a .MYD (MYData) extension. The index file has a .MYI (MYIndex) extension.

Features

MyISAM is optimized for environments with heavy read operations, and few writes, or none at all. A typical area in which one could prefer MyISAM is data warehouse, because it involves queries on very big tables, and the update of such tables is done when the database is not in use (usually by night).

The reason MyISAM allows for fast reads is the structure of its indexes: each entry points to a record in the data file, and the pointer is offset from the beginning of the file. This way records can be quickly read, especially when the format is FIXED. Thus, the rows are of constant length. Inserts are easy too, because new rows are appended to the end of the data file. However, delete and update operations are more problematic: deletes must leave an empty space, or the rows' offsets would change; the same goes for updates, as the length of the rows becomes shorter; if the update makes the row longer, the row is fragmented. To defragment rows and claim empty space, the OPTIMIZE TABLE command must be executed. Because of this simple mechanism, usually MyISAM index statistics are quite accurate.