MySQL Reference Manual

ISAM Tables

You can also use the deprecated ISAM table type. This will disappear rather soon (probably in MySQL 4.1) because MyISAM is a better implementation of the same thing. 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.

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. 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.