From: Tom Lane <tgl@sss.pgh.pa.us>
To: Greg Stark <gsstark@mit.edu>
Cc: Andrew Sullivan <ajs@crankycanuck.ca>
Cc: pgsql-sql@postgresql.org
Subject: Re: row-attribute in EXPLAIN-output doesn't match count(*)
Date: Thu, 14 Apr 2005 19:26:09 -0400
Message-ID: <27509.1113521169@sss.pgh.pa.us> (raw)
In-Reply-To: <8764ypf8hy.fsf@stark.xeocode.com>
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>
Greg Stark <gsstark@mit.edu> writes:
> This is presumably because of the long-standing issue that Postgres takes the
> snapshot as soon as the BEGIN is issued.
No, we don't set the snapshot until the first DML query is issued. This
is critical for serializable transactions: you have to be able to take
locks before the snapshot is frozen.
There are at least three interesting events involved:
1 BEGIN command issued
2 First lock taken (presumably as a consequence of another command)
3 Transaction snapshot taken (ditto; might be a different command)
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.
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?
regards, tom lane
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To: pgsql-sql@postgresql.org
Cc: tgl@sss.pgh.pa.us, gsstark@mit.edu, ajs@crankycanuck.ca
Subject: Re: row-attribute in EXPLAIN-output doesn't match count(*)
In-Reply-To: <27509.1113521169@sss.pgh.pa.us>
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