Re: Pruning never visible changes

On Thu, 22 Sept 2022 at 15:16, Alvaro Herrera <alvherre@alvh.no-ip.org> wrote:
>
> On 2022-Sep-22, Simon Riggs wrote:
>
> > On Mon, 19 Sept 2022 at 00:16, Greg Stark <stark@mit.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.

Oh, completely agreed.

I will submit a modified patch that adds a test case and just a
comment to explain why we can't remove such rows.

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