(mysql.info.gz) Key space
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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 will also be prefix compressed. Space compression makes the
index file smaller than the worst-case figure if the 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
`PACK_KEYS=1' when you create the table. This helps when you have many
integer keys that have an identical prefix when the numbers are stored
high-byte first.
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