Re: Overhauling "Routine Vacuuming" docs, particularly its handling of freezing

On Mon, May 1, 2023 at 12:01 PM Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
> > If the XID space were non-circular, it could
> > never try to reuse the XID values that have previously been used, and
> > this entire class of problems would go away. Because it is circular,
> > it's possible for the XID counter to arrive back at a place that it's
> > been before i.e. it can wrap around.
>
> But integer wrap around isn't really aligned with anything important.
> xidStopLimit will kick in when we're only halfway towards literal
> integer wrap around. Users have practical concerns about avoiding
> xidStopLimit -- what a world without xidStopLimit looks like just
> doesn't matter. Just having some vague awareness of truncated XIDs
> being insufficient at some point is all you really need, even if
> you're an advanced user.

I disagree. If you start the cluster in single-user mode, you can
actually wrap it around, unless something has changed that I don't
know about.

I'm not trying to debate the details of the patch, which I have not
read. I am saying that, while wraparound is perhaps not a perfect term
for what's happening, it is not, in my opinion, a bad term either. I
don't think it's accurate to imagine that this is a 64-bit counter
where we only store 32 bits on disk. We're trying to retcon that into
being true, but we'd have to work significantly harder to actually
make it true.

--
Robert Haas
EDB: http://www.enterprisedb.com