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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Jon Nelson <jnelson+pgsql@jamponi.net>
Cc: pgsql-performance@postgresql.org
Subject: Re: queries with lots of UNIONed relations
Date: Thu, 13 Jan 2011 17:26:22 -0500
Message-ID: <11563.1294957582@sss.pgh.pa.us> (raw)
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Robert Haas <robertmhaas@gmail.com> writes:
> On Thu, Jan 13, 2011 at 3:12 PM, Jon Nelson <jnelson+pgsql@jamponi.net> wrote:
>> I still think that having UNION do de-duplication of each contributory
>> relation is a beneficial thing to consider -- especially if postgresql
>> thinks the uniqueness is not very high.

> This might be worth a TODO.

I don't believe there is any case where hashing each individual relation
is a win compared to hashing them all together.  If the optimizer were
smart enough to be considering the situation as a whole, it would always
do the latter.

			regards, tom lane



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