Under wal_level=logical, when saving old tuples, always save OID.

There's no real point in not doing this.  It doesn't cost anything
in performance or space.  So let's go wild.

Andres Freund, with substantial editing as to style by me.

src/backend/access/heap/heapam.c

@@ -6638,7 +6638,6 @@ ExtractReplicaIdentity(Relation relation, HeapTuple tp, bool key_changed, bool *
 	TupleDesc	idx_desc;
 	char		replident = relation->rd_rel->relreplident;
 	HeapTuple	key_tuple = NULL;
-	bool		copy_oid = false;
 	bool		nulls[MaxHeapAttributeNumber];
 	Datum		values[MaxHeapAttributeNumber];
 	int			natt;
@@ -6697,20 +6696,30 @@ ExtractReplicaIdentity(Relation relation, HeapTuple tp, bool key_changed, bool *
 	{
 		int attno = idx_rel->rd_index->indkey.values[natt];
 
-		if (attno == ObjectIdAttributeNumber)
-			copy_oid = true;
-		else if (attno < 0)
+		if (attno < 0)
+		{
+			/*
+			 * The OID column can appear in an index definition, but that's
+			 * OK, becuse we always copy the OID if present (see below).
+			 * Other system columns may not.
+			 */
+			if (attno == ObjectIdAttributeNumber)
+				continue;
 			elog(ERROR, "system column in index");
-		else
-			nulls[attno - 1] = false;
+		}
+		nulls[attno - 1] = false;
 	}
 
 	key_tuple = heap_form_tuple(desc, values, nulls);
 	*copy = true;
 	RelationClose(idx_rel);
 
-	/* XXX: we could also do this unconditionally, the space is used anyway */
-	if (copy_oid)
+	/*
+	 * Always copy oids if the table has them, even if not included in the
+	 * index. The space in the logged tuple is used anyway, so there's little
+	 * point in not including the information.
+	 */
+	if (relation->rd_rel->relhasoids)
 		HeapTupleSetOid(key_tuple, HeapTupleGetOid(tp));
 
 	/*