Re: crash-safe visibility map, take three

On 01.12.2010 18:25, Robert Haas wrote:
> I think we can improve this a bit further by also introducing a
> HEAP_XMIN_FROZEN bit that we set in lieu of overwriting XMIN with
> FrozenXID. This allows us to freeze tuples aggressively - if we want
> - without losing any forensic information. We can then modify the
> above algorithm slightly, so that when we observe that a page is all
> visible, we not only set PD_ALL_VISIBLE on the page but also
> HEAP_XMIN_FROZEN on each tuple.

Hmm, actually, if we're willing to believe PD_ALL_VISIBLE in the page
header over the xmin/xmax on the tuples, we could simply not bother
doing anti-wraparound vacuums for pages that have the flag set. I'm not
sure what changes that would require outside heapam.c, as we'd have to
be careful to not trust the xmin/xmax if the flag was set.

The first update on the page that clears the flag would need to freeze
all the tuples in that scheme.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com