Re: HOT chain validation in verify_heapam()

I disapprove of ignoring the HEAP_COMBOCID flag. Emitting a message
claiming that the CID has a certain value when that's actually a combo
CID is misleading, so at least a different message wording is needed
in such cases. But it's also not clear to me that the newer update has
to have a higher combo CID, because combo CIDs can be reused. If you
have multiple cursors open in the same transaction, the updates can be
interleaved, and it seems to me that it might be possible for an older
CID to have created a certain combo CID after a newer CID, and then
both cursors could update the same page in succession and end up with
combo CIDs out of numerical order. Unless somebody can make a
convincing argument that this isn't possible, I think we should just
skip this check for cases where either tuple has a combo CID.

+            if (TransactionIdEquals(pred_xmin, curr_xmin) &&
+                (TransactionIdEquals(curr_xmin, curr_xmax) ||
+                 !TransactionIdIsValid(curr_xmax)) && pred_cmin >= curr_cmin)

I don't understand the reason for the middle part of this condition --
TransactionIdEquals(curr_xmin, curr_xmax) ||
!TransactionIdIsValid(curr_xmax). I suppose the comment is meant to
explain this, but I still don't get it. If a tuple with XMIN 12345
CMIN 2 is updated to produce a tuple with XMIN 12345 CMIN 1, that's
corruption, regardless of what the XMAX of the second tuple may happen
to be.