Currently the new verify_heapam() follows ctid chains when XMAX_INVALID is set
and expects to find an item it can dereference - but I don't think that's
something we can rely on: Afaics HOT pruning can break chains, but doesn't
reset xmax.
We have below code which I think takes care of xmin and xmax matching and if they match then only we add them to the predecessor array.
/*
* If the next line pointer is a redirect, or if it's a tuple
* but the XMAX of this tuple doesn't match the XMIN of the next
* tuple, then the two aren't part of the same update chain and
* there is nothing more to do.
*/
if (ItemIdIsRedirected(next_lp))
continue;
curr_htup = (HeapTupleHeader) PageGetItem(
ctx.page, curr_lp);
curr_xmax = HeapTupleHeaderGetUpdateXid(curr_htup);
next_htup = (HeapTupleHeader) PageGetItem(
ctx.page, next_lp);
next_xmin = HeapTupleHeaderGetXmin(next_htup);
if (!TransactionIdIsValid(curr_xmax) ||
!TransactionIdEquals(curr_xmax, next_xmin))
continue;