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Space Needed for Keys

My ISAM 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 isfor the worstcase 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 figurę if a string column has a lot

of trailing space or is a VARCHAR column that is not always used to the fuli 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 MylSAM 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 pref ix.