| From: | Heikki Linnakangas <heikki(at)enterprisedb(dot)com> |
|---|---|
| To: | "Florian G(dot) Pflug" <fgp(at)phlo(dot)org> |
| Cc: | Csaba Nagy <nagy(at)ecircle-ag(dot)com>, Alvaro Herrera <alvherre(at)commandprompt(dot)com>, Gregory Stark <stark(at)enterprisedb(dot)com>, postgres hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: CLUSTER and MVCC |
| Date: | 2007-03-09 15:49:59 |
| Message-ID: | 45F18227.9050503@enterprisedb.com |
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| Thread: | |
| Lists: | pgsql-hackers |
Florian G. Pflug wrote:
> Couldn't HOT in principle deal with this? Let's say you have two
> long-running
> transactions, which see row versions A and D. While those transactions
> are running, the row is constantly updated, leading to row versions B, C
> (before
> the second long-running transaction started), D, E, F, ... Z.
> Now, the versions B,C,E,F,...Z could be removed by HOT or vacuum,
> because they
> are not currently visible, nor will they ever become visible because
> they are
> already deleted.
Yes, you could detect that but you'd need a global view of all snapshots
in the system. I just posted a reply in this thread with more details..
It's not just with HOT, it's the way we determine that a tuple is
vacuumable in general.
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
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