(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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(mysql.info.gz) MyISAM table formats