mysql-test/r/group_min_max.result:
  Test for BUG#12672.
mysql-test/t/group_min_max.test:
  Test for BUG#12672.
sql/opt_range.cc:
  Some storage managers like InnoDB always retrieve the value of
  the primary key (if any) with any other key. For such storage
  managers the optimizer marks an index as covering if that index
  and the primary key contain all query fields. This is good for
  reading data from an index, but not for lookup. The patch detects
  such cases and tests whether an index is really a covering index
  by itself, without the primary key. If yes, then that index can
  be used for the index skip algorithm of the GROUP_MIN_MAX
  access method.