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Re: testing HS/SR - 1 vs 2 performance

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Simon Riggs <simon(at)2ndQuadrant(dot)com>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, Erik Rijkers <er(at)xs4all(dot)nl>, Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: testing HS/SR - 1 vs 2 performance
Date: 2010-04-27 20:18:53
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Lists: pgsql-hackers
Simon Riggs <simon(at)2ndQuadrant(dot)com> writes:
> On Tue, 2010-04-27 at 13:52 -0400, Tom Lane wrote:
>> WTF?  Either the comment is wrong or this should not be an elog
>> condition.

> That section of code has been rewritten many times. I think it is now
> inaccurate and should be removed. I left it there because the
> unfortunate history of the project has been the removal of comments and
> then later rediscovery of the truth, sometimes more than once. I could
> no longer reproduce that error; someone else may know differently.

I haven't tested this, but it appears to me that the failure would occur
in overflow situations.  If we have too many subxacts, we'll generate
XLOG_XACT_ASSIGNMENT, which will cause the subxids to be removed from
KnownAssignedXids[].  Then later when the top-level xact commits or
aborts we'll try to remove them again as a consequence of processing
the top-level's commit/abort record.  No?

			regards, tom lane

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