Re: Automatic free space map filling

Christopher Browne <cbbrowne(at)acm(dot)org> writes:
> What is unclear to me in the discussion is whether or not this is
> invalidating the item on the TODO list...

No, I don't think any of this is an argument against the
dirty-page-bitmap idea. The amount of foreground effort needed to set a
dirty-page bit is minimal (maybe even zero, if we can make the bgwriter
do it, though I'm pretty suspicious of that idea because I think it
needs to be done immediately when the page is dirtied). I don't see the
dirty-page bitmap as changing the way that VACUUM works in any
fundamental respect --- it will just allow the vacuum process to skip
reading pages that certainly don't need to change.

One point that does need to be considered though is what about
anti-wraparound processing (ie, replacing old XIDs with FrozenXID before
they wrap around)? VACUUM currently is a safe way to handle that,
but if its normal mode of operation stops looking at every tuple then
we're going to have an issue there.

regards, tom lane