Simon Riggs <simon@xxxxxxxxxxxxxxx> writes: > On Tue, 2009-05-19 at 08:58 -0400, Tom Lane wrote: >> Nonsense. The planner might think some other plan is cheaper, but >> it definitely knows how to do this, and has since at least 8.1. > Please look at Dimitri's plan. If it can remove the pointless sort, why > does it not do so? I haven't followed the whole thread, but the plan in the original post is for a hash join. The planner does not trust a hash join to preserve the order of its left input, because of possible batching. See the discussion a couple of months ago where we considered allowing the planner to disable batching so it *could* assume order preservation, and decided the risk of hashtable bloat was too great. 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
