is involved.

The Arg_comparator::compare_datetime() comparator caches its arguments if
they are constants i.e. const_item() returns true. The
Item_func_get_user_var::const_item() returns true or false based on
the current query_id and the query_id where the variable was created.
Thus even if a query can change its value its const_item() still will return
true. All this leads to a wrong comparison result when an object of the
Item_func_get_user_var class is involved.

Now the Arg_comparator::can_compare_as_dates() and the
get_datetime_value() functions never cache result of the GET_USER_VAR()
function (the Item_func_get_user_var class).


mysql-test/t/type_datetime.test:
  A test case is added for the bug#28261: Wrong DATETIME comparison result when the GET_USER_VAR function
  is involved.
mysql-test/r/type_datetime.result:
  A test case is added for the bug#28261: Wrong DATETIME comparison result when the GET_USER_VAR function
  is involved.
sql/item_cmpfunc.cc:
  Bug#28261: Wrong DATETIME comparison result when the GET_USER_VAR function
  is involved.
  Now the Arg_comparator::can_compare_as_dates() and the
  get_datetime_value() functions never cache result of the GET_USER_VAR()
  function (the Item_func_get_user_var class).