MyISAM tables use B-tree indexes. You can
roughly calculate the size for the index file as
(key_length+4)/0.67, summed over all keys.
This is for the worst case when all keys are inserted in sorted
order and the table doesn't have any compressed keys.
String indexes are space compressed. If the first index part is
a string, it is also prefix compressed. Space compression makes
the index file smaller than the worst-case figure if a string
column has a lot of trailing space or is a
VARCHAR column that is not always
used to the full length. Prefix compression is used on keys that
start with a string. Prefix compression helps if there are many
strings with an identical prefix.
MyISAM tables, you can also prefix
compress numbers by specifying the
PACK_KEYS=1 table option when you create the
table. Numbers are stored with the high byte first, so this
helps when you have many integer keys that have an identical