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Re: hashjoin chosen over 1000x faster plan

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Simon Riggs <simon(at)2ndquadrant(dot)com>
Cc: Kevin Grittner <Kevin(dot)Grittner(at)wicourts(dot)gov>, pgsql-performance(at)postgresql(dot)org
Subject: Re: hashjoin chosen over 1000x faster plan
Date: 2007-10-10 18:07:30
Message-ID: 20980.1192039650@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-performance
Simon Riggs <simon(at)2ndquadrant(dot)com> writes:
> Basically the planner doesn't ever optimise for the possibility of the
> never-executed case because even a single row returned would destroy
> that assumption. 

It's worse than that: the outer subplan *does* return some rows.
I suppose that all of them had NULLs in the join keys, which means
that (since 8.1 or so) nodeMergejoin discards them as unmatchable.
Had even one been non-NULL the expensive subplan would have been run.

This seems like too much of a corner case to justify adding a lot of
machinery for.

			regards, tom lane

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