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Re: BUG #4945: Parallel update(s) gone wild

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: "Dan Boeriu" <dan(dot)boeriu(at)roost(dot)com>
Cc: "Robert Haas" <robertmhaas(at)gmail(dot)com>, "Craig Ringer" <craig(at)postnewspapers(dot)com(dot)au>, "PostgreSQL bugs" <pgsql-bugs(at)postgresql(dot)org>
Subject: Re: BUG #4945: Parallel update(s) gone wild
Date: 2009-07-30 21:34:48
Message-ID: 4174.1248989688@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-bugs
"Dan Boeriu" <dan(dot)boeriu(at)roost(dot)com> writes:
> Attached is the reproducible test case - I was able to reproduce the problem on 32 and 64 bit 8.3.6 and 8.4.0 RedHat 5.3 kernel 2.6.18-128.1.16.el5 #1 SMP

I looked at this a bit.  It's the same issue discussed at
http://archives.postgresql.org/pgsql-bugs/2008-09/msg00045.php
namely, that the second update finds itself trying to update a large
number of tuples that were already updated since its snapshot was taken.
That means it has to re-verify that the updated versions of those tuples
meet its WHERE qualification.  That's done by a function EvalPlanQual
that's pretty darn inefficient for complex queries like this one.
It's essentially redoing the join (and recomputing the whole sub-SELECT)
for each row that needs to be updated.

Someday I'd like us to redesign that mechanism, but don't hold
your breath ...

			regards, tom lane

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Next:From: Dan BoeriuDate: 2009-07-30 22:11:39
Subject: Re: BUG #4945: Parallel update(s) gone wild
Previous:From: Pavel StehuleDate: 2009-07-30 18:45:30
Subject: Re: fix: plpgsql: return query and dropped columns problem

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