Re: Status of 8.3 patches

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Heikki Linnakangas <heikki(at)enterprisedb(dot)com>
Cc: Bruce Momjian <bruce(at)momjian(dot)us>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>, Neil Conway <neilc(at)samurai(dot)com>, ITAGAKI Takahiro <itagaki(dot)takahiro(at)oss(dot)ntt(dot)co(dot)jp>
Subject: Re: Status of 8.3 patches
Date: 2007-08-21 04:35:37
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Heikki Linnakangas <heikki(at)enterprisedb(dot)com> writes:
> Bruce Momjian wrote:
>> o Error correction for n_dead_tuples
>> This shows as waiting on another patch. Again, I am thinking to
>> keep it for 8.4.

> It was waiting on the "vacuum oldestxmin" patch, which didn't make it to
> 8.3. I don't care for the patch myself, but it was submitted well before
> feature freeze and deserves a review. It looks good to me at first glance.

This patch was originally submitted before we realized that pg_stats
failed to distinguish the effects of committed vs rolled-back
transactions (which was fixed about three months ago); and we also
recently fixed several other bugs such as losing stats data for shared
catalogs. So there's a significant probability that the errors it was
trying to compensate for are already fixed.

Also, I'm still quite unhappy that the patch converts the tracking of
n_dead_tuples into a dead-reckoning system in which incremental changes
are continually applied without any feedback that'd prevent the value
from diverging arbitrarily far from reality. Murphy's law says that
the value *will* diverge, if you don't have any negative feedback
in the loop to force it to track reality.

There may be something to be done here, but there's not any evidence
at hand that CVS HEAD actually suffers from a problem in tracking
n_dead_tuples, and even if it does I do not think that this particular
patch is a good fix.

regards, tom lane

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