To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Greg Stark <gsstark@mit.edu>,
	Andrew Sullivan <ajs@crankycanuck.ca>, pgsql-sql@postgresql.org
Subject: Re: row-attribute in EXPLAIN-output doesn't match count(*)
References: <200504141838.14705.andreak@officenet.no>
	<425EA496.2030501@archonet.com>
	<200504141921.38476.andreak@officenet.no>
	<20050414173358.GB9027@phlogiston.dyndns.org>
	<8764ypf8hy.fsf@stark.xeocode.com> <27509.1113521169@sss.pgh.pa.us>
In-Reply-To: <27509.1113521169@sss.pgh.pa.us>
From: Greg Stark <gsstark@mit.edu>
Organization: The Emacs Conspiracy; member since 1992
Date: 15 Apr 2005 01:25:08 -0400
Message-ID: <87fyxsehmz.fsf@stark.xeocode.com>
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Tom Lane <tgl@sss.pgh.pa.us> writes:

> We have to start the transaction no later than event #2 since there has
> to be something to hold the lock.  But it'd be easy enough to decouple
> this from BEGIN, and it'd be good enough to solve the "COMMIT;BEGIN"
> problem.

Oh I think I finally figured out what you're saying here. 

So vacuum doesn't really know what tuples are actually visible to the
snapshots actually taken by a transaction? It's making the conservative
estimate that a snapshot could have been taken as early as the start of the
transaction even if no snapshot was taken until later?


> Which of these three times do you think now() ought to correspond to?
> I recall having argued that it should be event #3 since that corresponds
> to the database snapshot you see.  100% backwards compatibility would
> require setting now() at event #1, but will anyone weep if we change that?

I think it would be weird to perform a select and see records with dates after
now(). It would also be weird to perform a select and not see records inserted
before now(). I'm not sure any of the above guarantees those criteria for READ
COMMITTED mode, but I think I'm on the same general path as you.

-- 
greg