"Tomas Vondra" <tv@xxxxxxxx> writes: > I've run into a strange plan difference on 9.1.9 - the first query does > "DISTINCT" by doing a GROUP BY on the columns (both INT). ... > Now, this takes ~45 seconds to execute, but after rewriting the query to > use the regular DISTINCT it suddenly switches to HashAggregate with ~1/3 > the cost (although it produces the same output, AFAIK), and it executes in > ~15 seconds. [ scratches head... ] I guess you're running into some corner case where choose_hashed_grouping and choose_hashed_distinct make different choices. It's going to be tough to debug without a test case though. I couldn't reproduce the behavior in a few tries here. > BTW I can't test this on 9.2 or 9.3 easily, as this is our production > environment and I can't just export the data. I've tried to simulate this > but so far no luck. I suppose they won't yet you step through those two functions with a debugger either ... regards, tom lane -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
