> What if we drove it off of the PD_ALL_VISIBLE bit on the page itself,
> rather than the visibility map bit? It would be safe to clear the
> visibility map bit without touching the page, but if you clear the
> PD_ALL_VISIBLE bit on the page itself then you set all the hint bits
> and freeze all the tuples. In the case where the visibility map bit
> gets cleared but the page-level bit is still set, a future vacuum can
> notice and reset the visibility map bit. But whenever the visibility
> map bit is set, you know that the page-level bit MUST be set, so you
> needn't vacuum those pages, even for anti-wraparound: you know they'll
> be frozen when and if they ever get written again.
How does that get us out of reading and writing old pages, though? If
we're going to set a bit on them, we might as well freeze them.
-- -- Josh Berkus PostgreSQL Experts Inc.
http://www.pgexperts.com