7.3 ISAM Tables

The deprecated ISAM table type will disappear in MySQL version 5.0. In MySQL 4.1 it's included in the source but not compiled anymore. MyISAM is a better implementation of this table handler and you should convert all ISAM tables to MySAM tables as soon as possible.

ISAM uses a B-tree index. The index is stored in a file with the `.ISM' extension, and the data is stored in a file with the `.ISD' extension. You can check/repair ISAM tables with the isamchk utility. See section 4.4.6.7 Using myisamchk for Crash Recovery.

ISAM has the following features/properties:

• Compressed and fixed-length keys
• Fixed and dynamic record length
• 16 keys with 16 key parts/key
• Max key length 256 (default)
• Data is stored in machine format; this is fast, but is machine/OS dependent.

Most of the things true for MyISAM tables are also true for ISAM tables. See section 7.1 MyISAM Tables. The major differences compared to MyISAM tables are:

• ISAM tables are not binary portable across OS/Platforms.
• Can't handle tables > 4G.
• Only support prefix compression on strings.
• Smaller key limits.
• Dynamic tables get more fragmented.
• Tables are compressed with pack_isam rather than with myisampack.

If you want to convert an ISAM table to a MyISAM table so that you can use utilities such as mysqlcheck, use an ALTER TABLE statement:

mysql> ALTER TABLE tbl_name TYPE = MYISAM;

The embedded MySQL versions doesn't support ISAM tables.

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