This is a cleanup task to simplify future development of SQL function and operator overloading.

The only user visible side affect will be the change in error reporting in some cases, e.g.:

-ERROR 21000: Operand should contain 1 column(s)

+ERROR HY000: Illegal parameter data types row and int for operation '+'

To implement easier overloading for SQL operators and functions for user defined data type arguments, we should:

• Gradually turn all Item_func descendants (for which we want overloading) into Item_handled_func
• Add infrastructure to search and choose best overload candidates. Overload candidates will be searched and chosen during fix_length_and_dec() time.
• The default implementation of Item_func::check_arguments(), which disallows ROW type arguments, should be removed for all Item_func_xxx classes that we want overloading for.

As a first step, let's cleanup Item_num_op and Item_func_min_max. Namely, let's do the following:
1. Move Type_handler_row into its own Type_collection_row (separate from Type_collection_std, which after this change will hold only scalar built-in types).
2. Overload Item_num_op::check_arguments() and Item_func_min_max::check_arguments() from the default implementation to:

bool check_arguments() const

{

  return false;

}

Arguments for these classes are checked during aggregate_for_num_op() and aggregate_for_min_max(), which after change #1, will return an error for a combination of ROW and scalar arguments, e.g.:

SELECT ROW(1,1)+2;

SELECT 2+ROW(1,1);

SELECT LEAST(ROW(1,1), 2);

SELECT LEAST(2, ROW(1,1));

and for two ROW arguments:

SELECT ROW(1,1)+ROW(2,2);

SELECT LEAST(ROW(1,1),ROW(2,2));

so the second test for ROW type arguments in check_arguments() won't be needed.

As an additional benefit:

• The virtual method Type_handler::is_traditional_type() will be removed
• A non-virtual method Type_handller::is_traditional_scalar_type() will be added, with this implementation

  bool Type_handler::is_traditional_scalar_type() const

  {

  return type_collection() == &type_collection_std;

  }