Re: crash-safe visibility map, take three

Robert Haas <robertmhaas@gmail.com> writes:
> We're not going to double the cost of VACUUM to get index-only scans.
> And that's exactly what will happen if you do full-page writes of
> every heap page to set a single bit.

It's ridiculous to claim that that "doubles the cost of VACUUM".  In the
worst case, it will add 25% to the cost of setting an all-visible bit on
a page where there is no other work to do.  (You already are writing out
the heap page and the VM page, plus a WAL image of the heap page, so a
WAL image of the VM page adds 25%.  But only if you did not set any
other bits on the same VM page, which is probably not a real common
case.)  Given that VACUUM has a lot of other cleanup besides visibility
bit setting, I'm not convinced that this would even be noticeable.

I think the burden is on people who are proposing complicated mechanisms
to show that there's actually a strong need for them.
        regards, tom lane