Re: Pruning never visible changes

On 2022-Sep-22, Simon Riggs wrote:

> On Mon, 19 Sept 2022 at 00:16, Greg Stark <stark(at)mit(dot)edu> wrote:

> > 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.

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

But this begs the question: is the proposed change safe, given that
ancient consideration? I don't think TOAST issues have been mentioned
in this thread so far, so I wonder if there is a test case that verifies
that this problem doesn't occur for some reason.

--
Álvaro Herrera PostgreSQL Developer — https://www.EnterpriseDB.com/