The problem was that get_best_group_min_max() did not check if fields used
by the "group_min_max optimization" where used in sub queries.
Because of this, it did not detect that a key (b,a) was used in the WHERE
clause for the statement:
SELECT DISTINCT b FROM t1 WHERE EXISTS ( SELECT 1 FROM DUAL WHERE a > 1 ).

Fixed by also traversing the sub queries when checking if a field is used.
This disables group_min_max_optimization for the above query.

Reviewer: Sergei Petrunia