The bug report revealed two problems related to min/max optimization:
1. If the length of a constant key used in a SARGable condition for
for the MIN/MAX fields is greater than the length of the field an 
unwanted warning on key truncation is issued;
2. If MIN/MAX optimization is applied to a partial index, like INDEX(b(4))
than can lead to returning a wrong result set.


mysql-test/r/func_group.result:
  Added test cases for bug #18206.
mysql-test/t/func_group.test:
  Added test cases for bug #18206.
sql/opt_sum.cc:
  Fixed bug #18206.
  Suppressed the warning about data truncation when store_val_in_field
  was used to store keys for the field used in MIN/MAX optimization.
  Blocked MIN/MAX optimization for partial keys, such as in INDEX(b(4)).
sql/sql_select.cc:
  Fixed bug #18206.
  Added a parameter for the function store_val_in_field allowing to
  control setting warnings about data truncation in the function.
sql/sql_select.h:
  Fixed bug #18206.
  Added a parameter for the function store_val_in_field allowing to
  control setting warnings about data truncation in the function.