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

On 2020-Aug-17, Ashutosh Sharma wrote:

> > + if (heap_force_opt == HEAP_FORCE_KILL)
> > + ItemIdSetDead(itemid);
> >
> > I think that if the page is an all-visible page, we should clear an
> > all-visible bit on the visibility map corresponding to the page and
> > PD_ALL_VISIBLE on the page header. Otherwise, index only scan would
> > return the wrong results.
>
> I think we should let VACUUM do that. Please note that this module is
> intended to be used only on a damaged relation and should only be
> operated on damaged tuples of such relations. And the execution of any
> of the functions provided by this module on a damaged relation must be
> followed by VACUUM with DISABLE_PAGE_SKIPPING option on that relation.
> This is necessary to bring back a damaged relation to the sane state
> once a surgery is performed on it. I will try to add this note in the
> documentation for this module.

It makes sense to recommend VACUUM after fixing the page, but I agree
with Sawada-san that it would be sensible to reset the VM bit while
doing surgery, since that's the state that the page would be in. We
should certainly *strongly recommend* to do VACUUM DISABLE_PAGE_SKIPPING,
but if users fail to do so, then leaving the VM bit set just means that
we know *for certain* that there will be further corruption as soon as
the XID counter advances sufficiently.

--
Álvaro Herrera https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services