Function list_difference

src/backend/nodes/list.c:1156–1176

* Return a list that contains all the cells in list1 that are not in * list2. The returned list is freshly allocated via palloc(), but the * cells themselves point to the same objects as the cells of the * input lists. * * This variant works on lists of pointers, and determines list * membership via equal() */

 * membership via equal()
 */
List *
list_difference(const List *list1, const List *list2)
{
	const ListCell *cell;
	List	   *result = NIL;

	Assert(IsPointerList(list1));
	Assert(IsPointerList(list2));

	if (list2 == NIL)
		return list_copy(list1);

	foreach(cell, list1)
	{
		if (!list_member(list2, lfirst(cell)))
			result = lappend(result, lfirst(cell));
	}

	check_list_invariants(result);
	return result;
}

/*
 * This variant of list_difference() determines list membership via