On Tue, Jan 13, 2009 at 7:07 PM, Baron Schwartz <ba...@xaprb.com> wrote: >> 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. > These appeared to be high selectivity indexes, but perhaps I assumed too much. :)
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