SQL Index Performance - ASC vs DESC

Question

I\'ve got a user table keyed on an auto-increment int column that looks something like this:

CREATE TABLE `user_def` (
  `user_id` int(11) NOT NULL AUTO_INC         


        

Answer 1

Someday I've been given by simple yet brilliant trick, how to make a descending index for mysql: Just by adding another column with negative (mirror value). Say, for the unsigned int it would be just value*-1 - so, it works for the unix timestamps.
For varchars the idea is similar but implementation is a bit more complex.