Space Needed for Keys-opracowanie

Space Needed for Keys
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.
In 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 prefix.
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