On 8/13/13, Robert James <srobertjames@xxxxxxxxx> wrote: > I noticed that when I have an index on (a,b) of table t, and I do an > SELECT * FROM t ORDER BY a ASC, it doesn't use the index. When I > create a new index of only a, it does use the index. Why is that? > > And, more importantly, when I do a query involving a merge join of > table t, which requires sorting table t, the planner does the sort > manually using quicksort, not using the index. The time that step > takes is identical to the ORDER BY without using the index. What do I > need to do to have Postgres use the index for the merge join? > > (Postgres 8.3) Interestingly enough, in the JOIN query, if I replace "t" with: (SELECT f1, f2 FROM t ORDER BY f1 ASC) AS t_ Postgres does use the index, getting the query done in half the time! -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
