Re: Pruning never visible changes

On Mon, 19 Sept 2022 at 00:16, Greg Stark <stark(at)mit(dot)edu> wrote:
>
> On Fri, 16 Sept 2022 at 10:27, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> >
> > Simon Riggs <simon(dot)riggs(at)enterprisedb(dot)com> writes:
> > > A user asked me whether we prune never visible changes, such as
> > > BEGIN;
> > > INSERT...
> > > UPDATE.. (same row)
> > > COMMIT;
> >
> > Didn't we just have this discussion in another thread?
>
> Well..... not "just" :)
>
> commit 44e4bbf75d56e643b6afefd5cdcffccb68cce414
> Author: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
> Date: Fri Apr 29 16:29:42 2011 -0400
>
> Remove special case for xmin == xmax in HeapTupleSatisfiesVacuum().
>
> VACUUM was willing to remove a committed-dead tuple immediately if it was
> deleted by the same transaction that inserted it. The idea is that such a
> tuple could never have been visible to any other transaction, so we don't
> need to keep it around to satisfy MVCC snapshots. However, there was
> already an exception for tuples that are part of an update chain, and this
> exception created a problem: we might remove TOAST tuples (which are never
> part of an update chain) while their parent tuple stayed around (if it was
> part of an update chain). This didn't pose a problem for most things,
> since the parent tuple is indeed dead: no snapshot will ever consider it
> visible. But MVCC-safe CLUSTER had a problem, since it will try to copy
> RECENTLY_DEAD tuples to the new table. It then has to copy their TOAST
> data too, and would fail if VACUUM had already removed the toast tuples.
>
> Easiest fix is to get rid of the special case for xmin == xmax. This may
> delay reclaiming dead space for a little bit in some cases, but it's by far
> the most reliable way to fix the issue.
>
> Per bug #5998 from Mark Reid. Back-patch to 8.3, which is the oldest
> version with MVCC-safe CLUSTER.

Good research Greg, thank you. Only took 10 years for me to notice it
was gone ;-)

--
Simon Riggs http://www.EnterpriseDB.com/