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14.1.2 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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