Re: recovering from "found xmin ... from before relfrozenxid ..."

On Tue, Aug 25, 2020 at 11:51 PM Robert Haas <robertmhaas@gmail.com> wrote:
>
> On Tue, Aug 25, 2020 at 8:17 AM Masahiko Sawada
> <masahiko.sawada@2ndquadrant.com> wrote:
> > + <note>
> > + <para>
> > +  While performing surgery on a damaged relation, we must not be doing anything
> > +  else on that relation in parallel. This is to ensure that when we are
> > +  operating on a damaged tuple there is no other transaction trying to modify
> > +  that tuple.
> > + </para>
> > + </note>
> >
> > If we prefer to avoid concurrent operations on the target relation why
> > don't we use AccessExclusiveLock?
>
> I disagree with the content of the note. It's up to the user whether
> to perform any concurrent operations on the target relation, and in
> many cases it would be fine to do so. Users who can afford to take the
> table off-line to repair the problem don't really need this tool in
> the first place.
>

The only reason I added this note was to ensure that we do not revive
the tuple that is deleted but not yet vacuumed. There is one
corner-case scenario as reported by you in - [1] where you have
explained a scenario under which vacuum can report "found xmin ...
from before relfrozenxid ..." sort of error for the  deleted tuples.
And as per the explanation provided there, it can happen when there
are multiple transactions operating on the same tuple. However, I
think we can take care of this scenario by doing some code changes in
heap_force_freeze to identify the deleted tuples and maybe skip such
tuples. So, yeah, I will do the code changes for handling this and
remove the note added in the documentation. Thank you Robert and
Masahiko-san for pointing this out.

[1] - https://www.postgresql.org/message-id/CA%2BTgmobfJ8CkabKJZ-1FGfvbSz%2Bb8bBX807Y6hHEtVfzVe%2Bg6A%40mail.gmail.com

--
With Regards,
Ashutosh Sharma
EnterpriseDB:http://www.enterprisedb.com