As of version 10.3.0, Item::type_handler is defined as:
That is type handler is determined from field_type() by default.
This involves lookup inside get_handler_by_field_type(), which can affect performance negatively.
Also, this is not friendly for pluggable data types, which will define their own functions returning values of non-standard data types.
We'll change the definition of the method to:
and implement it for all classes Item_xxx.