June 2002
Intermediate to advanced
816 pages
20h 46m
English
Content preview from MySQL Reference ManualBecome an O’Reilly member and get unlimited access to this title plus top books and audiobooks from O’Reilly and nearly 200 top publishers, thousands of courses curated by job role, 150+ live events each month,
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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.
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