Re: HOT chain validation in verify_heapam()

> 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".