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