Function PrepareSortSupportFromOrderingOp

src/backend/utils/sort/sortsupport.c:134–151

* Fill in SortSupport given an ordering operator (btree "<" or ">" operator). * * Caller must previously have zeroed the SortSupportData structure and then * filled in ssup_cxt, ssup_collation, and ssup_nulls_first. This will fill * in ssup_reverse as well as the comparator function pointer. */

 * in ssup_reverse as well as the comparator function pointer.
 */
void
PrepareSortSupportFromOrderingOp(Oid orderingOp, SortSupport ssup)
{
	Oid			opfamily;
	Oid			opcintype;
	int16		strategy;

	Assert(ssup->comparator == NULL);

	/* Find the operator in pg_amop */
	if (!get_ordering_op_properties(orderingOp, &opfamily, &opcintype,
									&strategy))
		elog(ERROR, "operator %u is not a valid ordering operator",
			 orderingOp);
	ssup->ssup_reverse = (strategy == BTGreaterStrategyNumber);

	FinishSortSupportFunction(opfamily, opcintype, ssup);
}

/*
 * Fill in SortSupport given an index relation, attribute, and strategy.