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To: Robert Haas <robertmhaas@gmail.com>
cc: Jon Nelson <jnelson+pgsql@jamponi.net>, pgsql-performance@postgresql.org
Subject: Re: queries with lots of UNIONed relations 
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Comments: In-reply-to Robert Haas <robertmhaas@gmail.com>
	message dated "Thu, 13 Jan 2011 17:42:41 -0500"
Date: Thu, 13 Jan 2011 18:07:25 -0500
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From: Tom Lane <tgl@sss.pgh.pa.us>
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Robert Haas <robertmhaas@gmail.com> writes:
> Yeah, I'm all wet, because you'd still have to re-de-duplicate at the
> end.  But then why did the OP get a speedup?  *scratches head*

He was reporting that 2 levels of hashing was faster than sort+uniq
(with the sorts swapping to disk, no doubt).  One level of hashing
should be faster yet, but maybe not by enough to be obvious as long
as you don't start to swap.

			regards, tom lane