On Fri, 2009-07-10 at 08:59 -0400, Bruce Momjian wrote: > > I think it should be possible to use predtest theorem proving to > discard > > the sort/hash step in cases where we can prove the sets are > disjoint. > > Often there are top-level quals that can be compared in the WHERE > > clauses of the sub-queries, so a shallow search could be quite > > profitable in allowing us to rewrite a UNION into a UNION ALL. > > I assume we would still need the distinct removal step; we just avoid > the sort/hash. I mean it seems possible to prove that the distinct removal step is not necessary, by proving that the various sub-queries are already disjoint. It's a common manual optimization, so automating it seems a reasonable future goal. -- Simon Riggs www.2ndQuadrant.com PostgreSQL Training, Services and Support -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
