Future new data types might want the function ROUND:
- to do something different than INT,DECIMAL,DOUBLE rounding
- or even return an error (if this operation is not applicable)
We'll split the following switch in Item_func_round::fix_length_and_dec
to virtial methods in Type_handler.
Also, we'll disallow ROUND for the GEOMETRY type, as it's useless. So GEOMETRY will be an example of a type whose ROUND operation is not applicable.