Re: HOT chain validation in verify_heapam()

Hi,

On 2022-11-17 21:33:17 +0530, Himanshu Upadhyaya wrote:
> On Tue, Nov 15, 2022 at 3:32 AM Andres Freund <andres@anarazel.de> wrote:
> > > Furthermore, it is
> > > possible that successor[x] = successor[x'] since the page might be
> > corrupted
> > > and we haven't checked otherwise.
> > >
> > > predecessor[y] = x means that successor[x] = y but in addition we've
> > > checked that y is sane, and that x.xmax=y.xmin. If there are multiple
> > > tuples for which these conditions hold, we've issued complaints about
> > > all but one and entered the last into the predecessor array.
> >
> > As shown by the isolationtester test I just posted, this doesn't quite work
> > right now. Probably fixable.
> >
> > I don't think we can follow non-HOT ctid chains if they're older than the
> > xmin
> > horizon, including all cases of xmin being frozen. There's just nothing
> > guaranteeing that the tuples are actually "related".
> >
> I understand the problem with frozen tuples but don't understand the
> concern with non-HOT chains,
> could you please help with some explanation around it?

I think there might be cases where following non-HOT ctid-chains across tuples
within a page will trigger spurious errors, if the tuple versions are older
than the xmin horizon. But it's a bit hard to say without seeing the code with
a bunch of the other bugs fixed.

Greetings,

Andres Freund