You have not given complete steps to reproduce the problem where instead of the index scan, a sequential scan would be picked. I have tried to reproduce by extending your steps but didn't see the problem. Let me know if I am missing something.
I think the steps you shared are what I had in mind.
I have debugged the above example and it uses an index scan during apply without your latest change which is what I expected. AFAICS, the use of IdxIsRelationIdentityOrPK() is to decide whether we will do tuples_equal() or not during the index scan and I see it gives the correct results with the example you provided.
Right, I got confused. IdxIsRelationIdentityOrPK is only called within
RelationFindReplTupleByIndex(). And, yes, it only impacts tuples_equal.
But, still, it feels safer to keep as the current patch if we don't change the
name of the function.
I really don't have any strong opinions for either way, only a slight preference
to keep as v35 for future callers not to get confused as we do here.
