Alvaro Herrera wrote:
> 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?)
Yeah, the two-bits idea was suggested during the conversation core had
about the issue.
-- Bruce Momjian <bruce@momjian.us> http://momjian.us EnterpriseDB
http://enterprisedb.com
+ None of us is going to be here forever. +