Backward scans and joins

If two or more tables are being joined, and the order by clause specifies descending order for index keys on the joined tables, any of the tables and indexes involved can be scanned with a backward scan to avoid the worktable and sort costs. If all the columns for one table are in ascending order, and the columns for the other tables are in descending order, the first table is scanned in ascending order and the others in descending order.