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Re: [Fwd: Re: Outer joins and Seq scans]

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Sami Dalouche <skoobi(at)free(dot)fr>
Cc: pgsql-performance <pgsql-performance(at)postgresql(dot)org>
Subject: Re: [Fwd: Re: Outer joins and Seq scans]
Date: 2007-10-31 22:05:16
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Lists: pgsql-performance
Sami Dalouche <skoobi(at)free(dot)fr> writes:
> -- For some reason, my message doesn't seem to go through the mailing
> list, so I am trying without any attachment

Please don't do that, at least not that way.  These explain outputs have
been line-wrapped to the point of utter unreadability.

The main problem looks to me that you're trying to do a 25-way join.
You'll want to increase join_collapse_limit and maybe fool with the
geqo parameters.  I fear you won't get a plan in a sane amount of time
if you try to do the full query as a single exhaustive search.  You
can either raise join_collapse_limit all the way and trust geqo to
find a decent plan repeatably (not a real safe assumption unfortunately)
or raise both join_collapse_limit and geqo_threshold to some
intermediate level and hope that a slightly wider partial plan search
will find the plan you need.

It's also possible that you're just stuck and the outer join is
inherently harder to execute.  I didn't study the query closely enough
to see if it's joining to any left join right-hand-sides, or anything
else that would forbid picking a nice join order.

			regards, tom lane

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