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Re: Searching for the cause of a bad plan

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Simon Riggs <simon(at)2ndquadrant(dot)com>
Cc: Csaba Nagy <nagy(at)ecircle-ag(dot)com>, postgres performance list <pgsql-performance(at)postgresql(dot)org>
Subject: Re: Searching for the cause of a bad plan
Date: 2007-09-21 17:53:40
Message-ID: 13644.1190397220@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-performance
Simon Riggs <simon(at)2ndquadrant(dot)com> writes:
> On Fri, 2007-09-21 at 12:08 -0400, Tom Lane wrote:
>> The reason you get a bad plan is that this rowcount estimate is so far
>> off:

> That's true, but its not relevant,

Yes it is --- the reason it wants to use a hashjoin instead of a
nestloop is exactly that it thinks the loop would iterate too many
times.  (Ten is already too many in this case --- if it had estimated
five rows out of the join, it'd have gone with the nestloop, since
the cost estimate difference at the top level is less than 2x.)

			regards, tom lane

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