Function cmp_orderbyvals

src/backend/executor/nodeIndexscan.c:408–438

* Compare ORDER BY expression values. */

 * Compare ORDER BY expression values.
 */
static int
cmp_orderbyvals(const Datum *adist, const bool *anulls,
				const Datum *bdist, const bool *bnulls,
				IndexScanState *node)
{
	int			i;
	int			result;

	for (i = 0; i < node->iss_NumOrderByKeys; i++)
	{
		SortSupport ssup = &node->iss_SortSupport[i];

		/*
		 * Handle nulls.  We only need to support NULLS LAST ordering, because
		 * match_pathkeys_to_index() doesn't consider indexorderby
		 * implementation otherwise.
		 */
		if (anulls[i] && !bnulls[i])
			return 1;
		else if (!anulls[i] && bnulls[i])
			return -1;
		else if (anulls[i] && bnulls[i])
			return 0;

		result = ssup->comparator(adist[i], bdist[i], ssup);
		if (result != 0)
			return result;
	}

	return 0;
}

/*
 * Pairing heap provides getting topmost (greatest) element while KNN provides