The IN function was comparing DATE/DATETIME values either as ints or as
strings. Both methods have their disadvantages and may lead to a wrong
result.

Now IN function checks whether all of its arguments has the STRING result
types and at least one of them is a DATE/DATETIME item. If so it uses either
an object of the in_datetime class or an object of the cmp_item_datetime
class to perform its work. If the IN() function arguments are rows then
row columns are checked whether the DATE/DATETIME comparator should be used
to compare them.
The in_datetime class is used to find occurence of the item to be checked
in the vector of the constant DATE/DATETIME values. The cmp_item_datetime
class is used to compare items one by one in the DATE/DATETIME context.
Both classes obtain values from items with help of the get_datetime_value()
function and cache the left item if it is a constant one.



mysql-test/t/type_datetime.test:
  Added a test case for the bug#28133: Wrong DATE/DATETIME comparison in IN() function.
mysql-test/r/type_datetime.result:
  Added a test case for the bug#28133: Wrong DATE/DATETIME comparison in IN() function.
mysql-test/r/func_in.result:
  A test case result is corrected after the fix for the bug#28133.
sql/item_cmpfunc.h:
  Bug#28133: Wrong DATE/DATETIME comparison in IN() function.
  Two DATE/DATETIME comparison classes are added.
  The in_datetime class is used to find occurence of the item to be checked
  in the vector of the constant DATE/DATETIME values. The cmp_item_datetime
  class is used to compare items one by one in the DATE/DATETIME context.
  Both classes obtain values from items with help of the get_datetime_value()
  function and cache the left item if it is a constant one.
sql/item_cmpfunc.cc:
  Bug#28133: Wrong DATE/DATETIME comparison in IN() function.
  Now IN function checks whether all of its arguments has the STRING result
  types and at least one of them is a DATE/DATETIME item. If so it uses either
  an object of the in_datetime class or an object of the cmp_item_datetime
  class to perform its work. If the IN() function arguments are rows then
  row columns are checked whether the DATE/DATETIME comparator should be used
  to compare them.