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

On Tue, Aug 18, 2020 at 9:44 PM Alvaro Herrera <alvherre(at)2ndquadrant(dot)com> wrote:
>
> 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.

Sure, I will try to do that change but I would still recommend to
always run VACUUM with DISABLE_PAGE_SKIPPING option on the relation
that underwent surgery.

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.
>

Yeah, I've already added a note for this in the documentation:

Note: "After a surgery is performed on a damaged relation using this
module, we must run VACUUM with DISABLE_PAGE_SKIPPING option on that
relation to bring it back into a sane state."

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