| 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 |
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| 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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