Add comment to clarify unexpected RECORD type handling in get_name_for_var_field

src/backend/distributed/deparser/ruleutils_18.c

@@ -4880,6 +4880,13 @@ get_name_for_var_field(Var *var, int fieldno,
 		case RTE_VALUES:
 		case RTE_NAMEDTUPLESTORE:
 		case RTE_RESULT:
+
+			/*
+			 * This case should not occur: a column of a table or values list
+			 * shouldn't have type RECORD.  Fall through and fail (most
+			 * likely) at the bottom.
+			 */
+			break;
 		case RTE_SUBQUERY:
 			/* Subselect-in-FROM: examine sub-select's output expr */
 			{