On Tue, Nov 27, 2007 at 11:51:40PM +0100, cluster wrote: > A thing that strikes me is the following. As you can see I have the > constraint: q.status = 1. Only a small subset of the data set has this > status. I have an index on q.status but for some reason this is not used. > Instead the constraint are ensured with a "Filter: (q.status = 1)" in an > index scan for the primary key in the "q" table. If the small subset having > q.status = 1 could be isolated quickly using an index, I would expect the > query to perform better. I just don't know why the planner doesn't use the > index on q.status. An index scan (as opposed to a bitmap index scan) can only use one index at a time, so it will choose the most selective one. Here it quite correctly recognizes that there will only be one matching record for the given question_id, so it uses the primary key instead. You could make an index on (question_id,status) (or a partial index on question id, with status=1 as the filter), but I'm not sure how much it would help you unless the questions table is extremely big. It doesn't appear to be; in fact, it appears to be all in RAM, so that's not your bottleneck. /* Steinar */ -- Homepage: http://www.sesse.net/ ---------------------------(end of broadcast)--------------------------- TIP 5: don't forget to increase your free space map settings
