Excerpts from Robert Haas's message of mié jun 02 14:16:33 -0400 2010:
> We could, but I think we'd be better off just freezing at the time we
> mark the page PD_ALL_VISIBLE and then using the visibility map for
> both purposes. Keeping around the old xmin values after every tuple
> on the page is visible to every running transaction is useful only for
> forensics, and building a whole new freeze map just to retain that
> information longer (and eventually force a massive anti-wraparound
> vacuum) seems like overkill.
Reducing the xid wraparound horizon "a bit" is reasonable, but moving it
all the way forward to OldestXmin is a bit much, methinks.
Besides, there's another argument for not freezing tuples immediately:
they may be updated shortly thereafter, causing extra churn for no gain.
I'd prefer a setting that would tell the system to freeze all tuples
that fall within a safety range whenever any tuple in the page is frozen
-- weren't you working on a patch to do this? (was it Jeff Davis?)
(BTW maybe instead of separate visibility and freeze maps we could have
two bits in the visibility map?)
--
Álvaro Herrera <alvherre@commandprompt.com>
The PostgreSQL Company - Command Prompt, Inc.
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