Function build_index_pathkeys

src/backend/optimizer/path/pathkeys.c:851–938

* build_index_pathkeys * Build a pathkeys list that describes the ordering induced by an index * scan using the given index. (Note that an unordered index doesn't * induce any ordering, so we return NIL.) * * If 'scandir' is BackwardScanDirection, build pathkeys representing a * backwards scan of the index. * * We iterate only key columns of covering indexes, since non-key columns

 * truncate_useless_pathkeys() to possibly remove more pathkeys.
 */
List *
build_index_pathkeys(PlannerInfo *root,
					 IndexOptInfo *index,
					 ScanDirection scandir)
{
	List	   *retval = NIL;
	ListCell   *lc;
	int			i;

	if (index->sortopfamily == NULL)
		return NIL;				/* non-orderable index */

	i = 0;
	foreach(lc, index->indextlist)
	{
		TargetEntry *indextle = (TargetEntry *) lfirst(lc);
		Expr	   *indexkey;
		bool		reverse_sort;
		bool		nulls_first;
		PathKey    *cpathkey;

		/*
		 * INCLUDE columns are stored in index unordered, so they don't
		 * support ordered index scan.
		 */
		if (i >= index->nkeycolumns)
			break;

		/* We assume we don't need to make a copy of the tlist item */
		indexkey = indextle->expr;

		if (ScanDirectionIsBackward(scandir))
		{
			reverse_sort = !index->reverse_sort[i];
			nulls_first = !index->nulls_first[i];
		}
		else
		{
			reverse_sort = index->reverse_sort[i];
			nulls_first = index->nulls_first[i];
		}

		/*
		 * OK, try to make a canonical pathkey for this sort key.  Note we're
		 * underneath any outer joins, so nullable_relids should be NULL.
		 */
		cpathkey = make_pathkey_from_sortinfo(root,
											  indexkey,
											  NULL,
											  index->sortopfamily[i],
											  index->opcintype[i],
											  index->indexcollations[i],
											  reverse_sort,
											  nulls_first,
											  0,
											  index->rel->relids,
											  false);