From: Tom Lane <tgl@sss.pgh.pa.us>
To: Jon Nelson <jnelson+pgsql@jamponi.net>
Cc: pgsql-performance@postgresql.org
Subject: Re: queries with lots of UNIONed relations
Date: Thu, 13 Jan 2011 15:05:44 -0500
Message-ID: <4879.1294949144@sss.pgh.pa.us> (raw)
In-Reply-To: <AANLkTikM9=JXPwYpAbfQ5KZgenWBCn5_J2=W6MRhq0U0@mail.gmail.com>
References: <AANLkTiktjfaR9_sDU+F3UZhAQn7qrBktXE1EiOX=inzQ@mail.gmail.com>
<19302.1294938802@sss.pgh.pa.us>
<AANLkTikM9=JXPwYpAbfQ5KZgenWBCn5_J2=W6MRhq0U0@mail.gmail.com>
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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To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, jnelson+pgsql@jamponi.net
Subject: Re: queries with lots of UNIONed relations
In-Reply-To: <4879.1294949144@sss.pgh.pa.us>
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