> If you have separate indexes on ssn and id_num, MySQL may be able to > efficiently use an index merge optimization . See > http://dev.mysql.com/doc/refman/5.0/en/index-merge-optimization.html. > This is only in 5.0+ - on older versions of MySQL you may find a union > more efficient.
And in newer versions, too. The optimizer frequently underestimates the cost of the merge operation and the required random I/O for row lookups. So, yes it can "use" an index merge, but... efficiency is another question. I've seen table scans outperform a two-way index merge by orders of magnitude. -- MySQL General Mailing List For list archives: http://lists.mysql.com/mysql To unsubscribe: http://lists.mysql.com/mysql?unsub=arch...@jab.org
