The problem was that when there was no MIN or MAX function, after finding the
group prefix based on the DISTINCT or GROUP BY attributes we did not search further
for a key in the group that satisfies the equi-join conditions on attributes that
follow the group attributes. Thus we ended up with the wrong rows, and subsequent
calls to select_cond->val_int() in evaluate_join_record() were filtering those
rows. Hence - the query result set was empty.

The problem occured both for GROUP BY queries without MIN/MAX and for queries
with DISTINCT (which were internally executed as GROUP BY queries).


mysql-test/r/group_min_max.result:
  Added test result for BUG#11044. Notice that the group by query is
  equivalent to the distinct query and both are executed via the same
  algorithm.
mysql-test/t/group_min_max.test:
  Added test for BUG#11044. Notice that the group by query is
  equivalent to the distinct query and both are executed via the
  same algorithm.
sql/opt_range.cc:
  * Use the extended prefix in QUICK_GROUP_MIN_MAX_SELECT::get_next()
    to find keys that satisfy equality conditions in the case when there is
    no MIN or MAX function.
  * Corrected some method comments.
  * Corrected debug printout of cost information.