On Thu, Jun 17, 2010 at 09:07:56AM -0500, Black, Michael (IS) scratched on the wall: > I sense the beginning of a "How to use indexes" page. > > Things you need know: > Only one index is used per query (or insert??).
It isn't that bad. In general, it is "one index PER TABLE, per query." But it isn't even that bad. Sub-queries can often use their own set of indexes, or any other situation when it is cheaper to extract a bunch of small data sets and combine then, rather than perform an scan, can use multiple indexes associated to the same table. The easiest case to understand is a chain of OR'ed WHERE conditions... if we assume each OR will produce a moderate to small sub-set, it can be cheaper to use an index to generate each sub-set and then OR the sub-sets, rather than do a full table scan and apply the OR chain to each row. Yeah, it gets complex real fast.... -j -- Jay A. Kreibich < J A Y @ K R E I B I.C H > "Intelligence is like underwear: it is important that you have it, but showing it to the wrong people has the tendency to make them feel uncomfortable." -- Angela Johnson _______________________________________________ sqlite-users mailing list sqlite-users@sqlite.org http://sqlite.org:8080/cgi-bin/mailman/listinfo/sqlite-users
