Re: Overhead of union versus union all

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.

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Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

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