Re: 64-bit XIDs in deleted nbtree pages

On Mon, Feb 22, 2021 at 4:21 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> The 0001 patch looks good to me. In the documentation, I think we need
> to update the following paragraph in the description of
> vacuum_cleanup_index_scale_factor:

Good point. I think that the structure should make the page deletion
triggering condition have only secondary importance -- it is only
described at all to be complete and exhaustive. The
vacuum_cleanup_index_scale_factor-related threshold is all that users
will really care about in this area.

The reasons for this are: it's pretty rare to have many page
deletions, but never again delete/non-hot update even one single
tuple. But when that happens, it's *much* rarer still to *also* have
inserts, that might actually benefit from recycling the deleted page.
So it's very narrow.

I think that I'll add a "Note" box that talks about the page deletion
stuff, right at the end. It's actually kind of an awkward thing to
describe, and yet I think we still need to describe it.

I also think that the existing documentation should clearly point out
that the vacuum_cleanup_index_scale_factor only gets considered when
there are no updates or deletes since the last VACUUM -- that seems
like an existing problem worth fixing now. It's way too unclear that
this setting only really concerns append-only tables.

--
Peter Geoghegan