Difference between creating index on a prefix of a string column or the whole column if the actual content length is very limited
<p>Assume there is a column <code>A</code> of type <code>varchar(255)</code> and the actual length of content of this column is about 20 characters for all rows.  Is there any difference between creating index on the whole <code>A</code> column or a 20-character prefix of the <code>A</code> column, regarding mysql's performance (time and space)?  (The storage engine being InnoDB.)</p>