Function PrepareSortSupportFromIndexRel

src/backend/utils/sort/sortsupport.c:161–178

* Fill in SortSupport given an index relation, attribute, and strategy. * * Caller must previously have zeroed the SortSupportData structure and then * filled in ssup_cxt, ssup_attno, ssup_collation, and ssup_nulls_first. This * will fill in ssup_reverse (based on the supplied strategy), as well as the * comparator function pointer. */

 * comparator function pointer.
 */
void
PrepareSortSupportFromIndexRel(Relation indexRel, int16 strategy,
							   SortSupport ssup)
{
	Oid			opfamily = indexRel->rd_opfamily[ssup->ssup_attno - 1];
	Oid			opcintype = indexRel->rd_opcintype[ssup->ssup_attno - 1];

	Assert(ssup->comparator == NULL);

	if (!IsIndexAccessMethod(indexRel->rd_rel->relam, BTREE_AM_OID))
		elog(ERROR, "unexpected non-btree AM: %u", indexRel->rd_rel->relam);
	if (strategy != BTGreaterStrategyNumber &&
		strategy != BTLessStrategyNumber)
		elog(ERROR, "unexpected sort support strategy: %d", strategy);
	ssup->ssup_reverse = (strategy == BTGreaterStrategyNumber);

	FinishSortSupportFunction(opfamily, opcintype, ssup);
}

/*
 * Fill in SortSupport given a GiST index relation