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 <bruce@momjian.us> http://momjian.us
EnterpriseDB http://enterprisedb.com
+ If your life is a hard drive, Christ can be your backup. +
