On Thu, Jul 9, 2009 at 7:58 PM, Bruce Momjian<bruce@xxxxxxxxxx> wrote: > Scott Bailey wrote: >> Alvaro Herrera wrote: >> > Tim Keitt wrote: >> >> I am combining query results that I know are disjoint. I'm wondering >> >> how much overhead there is in calling union versus union all. (Just >> >> curious really; I can't see a reason not to use union all.) >> > >> > UNION needs to uniquify the output, for which it plasters an additional >> > sort step, whereas UNION ALL does not need to uniquify its output and >> > thus it can avoid the sort step. Using UNION ALL is recommended >> > wherever possible. >> > >> >> I think I read somewhere that as of 8.4 it no longer required the sort >> step, due to the improvements in hashing. Here it is >> >> http://wiki.postgresql.org/wiki/WhatsNew84#Performance > > Oh, yea, hashing is used in some cases rather than sort. I assume sort > is still used if the hash exceeds workmem size. The important point being that it's still more expensive than a plain union all thought, right? -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
