Function attnumAttName

src/backend/parser/parse_relation.c:3784–3797

* given attribute id, return name of that attribute * * This should only be used if the relation is already * table_open()'ed. Use the cache version get_atttype() * for access to non-opened relations. */

 *	for access to non-opened relations.
 */
const NameData *
attnumAttName(Relation rd, int attid)
{
	if (attid <= 0)
	{
		const FormData_pg_attribute *sysatt;

		sysatt = SystemAttributeDefinition(attid);
		return &sysatt->attname;
	}
	if (attid > rd->rd_att->natts)
		elog(ERROR, "invalid attribute number %d", attid);
	return &TupleDescAttr(rd->rd_att, attid - 1)->attname;
}

/*
 * given attribute id, return type of that attribute