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Re: queries with lots of UNIONed relations - Mailing list pgsql-performance

From Tom Lane
Subject Re: queries with lots of UNIONed relations
Date
Msg-id 4879.1294949144@sss.pgh.pa.us
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In response to Re: queries with lots of UNIONed relations  (Jon Nelson <jnelson+pgsql@jamponi.net>)
Responses Re: queries with lots of UNIONed relations  (Jon Nelson <jnelson+pgsql@jamponi.net>)
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Jon Nelson <jnelson+pgsql@jamponi.net> writes:
> Your comment regarding "each individual de-duplication looked like it
> would fit in work_mem" doesn't really make sense, exactly. Maybe I'm
> misunderstanding you.

Yeah.  What I was suggesting was to NOT add the DISTINCT's, but instead
raise work_mem high enough so you get just one HashAggregation step at
the top level.  (Although I think this only works in 8.4 and up.)
That should be faster than two levels of de-duplication.

            regards, tom lane

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