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
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