Item::get_temporal_with_sql_mode() has a test for field_type(). This is not friendly for new data types.

Under terms of this task we'll do the following:

• Remove Item::get_temporal_with_sql_mode()
• Remove these methods from Item:

  my_decimal *val_decimal_from_date(my_decimal *decimal_value);

  my_decimal *val_decimal_from_time(my_decimal *decimal_value);

  longlong val_int_from_date();

  double val_real_from_date();

  String *val_string_from_date(String *str);

• Introduce to_double(), to_longlong(), to_decimal(), to_string() methods in Time, Date, Datetime
• The code that previously used val_xxx_from_date() will be changed to instantiate Time, Date or Datetime, then call these new methods to_xxx().
• Remove Item_temporal_func and derive Item_timefunc, Item_datefunc, Item_datetimefunc directly from Item_func, with simultaneous adding val_xxx() methods like this (in case of Item_datetimefunc):

  longlong val_int() { return Datetime(this).to_longlong(); }

  double val_real() { return Datetime(this).to_double(); }

  String *val_str(String *to) { return Datetime(this).to_string(to, decimals); }

  my_decimal *val_decimal(my_decimal *to) { return Datetime(this).to_decimal(to); }

• Remove Item_temporal_typecast and derive Item_time_typecast, Item_date_typecast, Item_datetime_typecast directly from the corresponding Item_(timefunc|datefunc|datetimefunc). This is to avoid code duplication in val_xxx().
• Remove this code from Item_temporal_literal:

  bool get_date_with_sql_mode(MYSQL_TIME *to);

  String *val_str(String *str)

  { return val_string_from_date(str); }

  longlong val_int()

  { return val_int_from_date(); }

  double val_real()

  { return val_real_from_date(); }

  my_decimal *val_decimal(my_decimal *decimal_value)

  { return val_decimal_from_date(decimal_value); }

  and add a Time, Date, Datetime based code into Item_(time|date|datetime)_literal instead.

• Do the same changes in Item_cache_(time|date|datetime)
• Add classes Handler_temporal, Handler_temporal_string, Handler_date, Handler_time, Handler_datetime into Item_handled_func and reorganize Func_handler_xxx in item_timefunc.h to derive from these new classes.
• Add class Temporal_hybrid - a MYSQL_TIME based class, but (unlike Time,Date,Datetime) without automatic timestamp type conversion. Use it in Handler_temporal_string, which will be the parent class for Func_handler_add_time_xxx and Func_handler_date_add_interval_xxx.

After this change there will be four classes to get MYSQL_TIME from an Item:

• Time - with automatic conversion to MYSQL_TIMESTAMP_TIME
• Date - with automatic conversion to MYSQL_TIMESTAMP_DATE
• Datetime - with automatic conversion to MYSQL_TIMESTAMP_DATE
• Temporal_hybrid - without automatic timestamp type conversion

All these four classes will have methods:

• to_longlong
• to_double
• to_string
• to_decimal