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. -- 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
