Simon Riggs wrote: > > 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. I am confused what sub-queries produce _distinct_ output. I know there are some that produce _ordered_ output. -- Bruce Momjian <bruce@xxxxxxxxxx> http://momjian.us EnterpriseDB http://enterprisedb.com + If your life is a hard drive, Christ can be your backup. + -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
