Function get_rte_attribute_is_dropped

src/backend/parser/parse_relation.c:3545–3680

* get_rte_attribute_is_dropped * Check whether attempted attribute ref is to a dropped column */

 *		Check whether attempted attribute ref is to a dropped column
 */
bool
get_rte_attribute_is_dropped(RangeTblEntry *rte, AttrNumber attnum)
{
	bool		result;

	switch (rte->rtekind)
	{
		case RTE_RELATION:
			{
				/*
				 * Plain relation RTE --- get the attribute's catalog entry
				 */
				HeapTuple	tp;
				Form_pg_attribute att_tup;

				tp = SearchSysCache2(ATTNUM,
									 ObjectIdGetDatum(rte->relid),
									 Int16GetDatum(attnum));
				if (!HeapTupleIsValid(tp))	/* shouldn't happen */
					elog(ERROR, "cache lookup failed for attribute %d of relation %u",
						 attnum, rte->relid);
				att_tup = (Form_pg_attribute) GETSTRUCT(tp);
				result = att_tup->attisdropped;
				ReleaseSysCache(tp);
			}
			break;
		case RTE_SUBQUERY:
		case RTE_TABLEFUNC:
		case RTE_VALUES:
		case RTE_CTE:

			/*
			 * Subselect, Table Functions, Values, CTE RTEs never have dropped
			 * columns
			 */
			result = false;
			break;
		case RTE_NAMEDTUPLESTORE:
			{
				/* Check dropped-ness by testing for valid coltype */
				if (attnum <= 0 ||
					attnum > list_length(rte->coltypes))
					elog(ERROR, "invalid varattno %d", attnum);
				result = !OidIsValid((list_nth_oid(rte->coltypes, attnum - 1)));
			}
			break;
		case RTE_JOIN:
			{
				/*
				 * A join RTE would not have dropped columns when constructed,
				 * but one in a stored rule might contain columns that were
				 * dropped from the underlying tables, if said columns are
				 * nowhere explicitly referenced in the rule.  This will be
				 * signaled to us by a null pointer in the joinaliasvars list.
				 */
				Var		   *aliasvar;

				if (attnum <= 0 ||