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.
Yep, ideally UNION ALL would be the default behavior, but that standard requires otherwise. Many people don't know that UNION has an extra SORT/UNIQUE step. -- Bruce Momjian <br...@momjian.us> 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@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
