Re: Add 64-bit XIDs into PostgreSQL 15

On Mon, Nov 28, 2022 at 4:09 PM Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
> Granted, the specifics of the current XidStopLimit mechanism are
> unlikely to directly carry over to 64-bit XIDs. XidStopLimit is
> structured in a way that doesn't actually consider freeze debt in
> units like unfrozen pages. Like Chris, I just don't see why the patch
> obviates the need for something like XidStopLimit, since the patch
> doesn't remove freezing. An improved XidStopLimit mechanism might even
> end up kicking in *before* the oldest relfrozenxid reached 2 billion
> XIDs, depending on the specifics.

What is the purpose of using 64-bit XIDs, if not to avoid having to
stop the world when we run short of XIDs?

I'd say that if this patch, or any patch with broadly similar goals,
fails to remove xidStopLimit, it might as well not exist.

xidStopLimit is not a general defense against falling too far behind
on freezing, and in general, there is no reason to think that we need
such a defense. xidStopLimit is a defense against reusing the same XID
and thus causing irreversible database corruption. When that
possibility no longer exists, it has outlived its usefulness and we
should be absolutely delighted to bid it adieu.

It seems like you and Chris are proposing the moral equivalent of
paying off your home loan but still sending a check to the mortgage
company every month just to be sure they don't get mad.

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