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Re: advancing snapshot's xmin

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Gregory Stark <stark(at)enterprisedb(dot)com>
Cc: "Heikki Linnakangas" <heikki(at)enterprisedb(dot)com>, "Neil Conway" <neilc(at)samurai(dot)com>, "Alvaro Herrera" <alvherre(at)commandprompt(dot)com>, "Pg Hackers" <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: advancing snapshot's xmin
Date: 2008-03-26 15:58:52
Message-ID: 22651.1206547132@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Gregory Stark <stark(at)enterprisedb(dot)com> writes:
> Uhm, yeah, I somehow didn't write was I was thinking. I didn't mean to say we
> would be taking a new snapshot for each INSERT but that we would be resetting
> xmin for each INSERT. Whereas currently we only set xmin once when we set the
> serializable snapshot.

Right, but setting xmin within GetSnapshotData is essentially free.
What I'm envisioning is that we lose the notion of "this is a
serializable snapshot" that that function currently has, and just
give it the rule "if MyProc->xmin is currently zero, then set it".
Then the only additional mechanism needed is for the snapshot
manager to detect when all snapshots are gone and zero out
MyProc->xmin --- that would happen sometime during command shutdown,
and per current discussion it shouldn't need a lock.

			regards, tom lane

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