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Re: Redundant sub query triggers slow nested loop left join

From: "henk de wit" <henk53602(at)hotmail(dot)com>
To: tgl(at)sss(dot)pgh(dot)pa(dot)us
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: Redundant sub query triggers slow nested loop left join
Date: 2007-04-22 16:17:40
Message-ID: BAY106-F7E7209A4ED3C87F1CAFC1F5540@phx.gbl (view raw or flat)
Thread:
Lists: pgsql-performance
>Since you have two redundant tests, the selectivity is being
>double-counted, leading to a too-small rows estimate and a not very
>appropriate choice of join plan.

I see, thanks for the explanation. I did notice though that in the second 
case, with 1 redundant test removed, the estimate is still low:

"Hash Left Join (cost=1449.99..2392.68 rows=2 width=714) (actual 
time=24.257..25.292 rows=553 loops=1)"

In that case the prediction is 2 rows, which is only 1 row more than in the 
previous case. Yet the plan is much better and performance improved 
dramatically. Is there a reason/explanation for that?

>FWIW, CVS HEAD does get rid of the duplicate conditions for the common
>case of mergejoinable equality operators --- but it's not explicitly
>looking for duplicate conditions, rather this is falling out of a new
>method for making transitive equality deductions.

This sounds very interesting Tom. Is there some documentation somewhere 
where I can read about this new method?

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Next:From: Tom LaneDate: 2007-04-22 17:53:26
Subject: Re: Redundant sub query triggers slow nested loop left join
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Subject: Re: Large objetcs performance

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