Add Type_handler::type_handler_for_tmp_table() and Type_handler::type_handler_for_union()

The code in the following methods:

Item::make_string_field()

Item::tmp_table_field_from_field_type()

Item_type_holder::make_field_by_type()

is not friendly to new data types, because it has conditions on field_type.

To make the code more friendly, we'll add two new virtual methods in Type_handler:

virtual const Type_handler *type_handler_for_tmp_table(const Item *) const;

virtual const Type_handler *type_handler_for_union(const Item *) const;

This will simplify the code a lot:

• Item::make_string_field() will be gone
• Item_func_group_concat::make_string_field() will be gone
• Item::tmp_table_field_from_field_type() will simplify to:

  Field *tmp_table_field_from_field_type(TABLE *table)

  {

  const Type_handler *h= type_handler()->type_handler_for_tmp_table(this);

  return h->make_and_init_table_field(&name, Record_addr(maybe_null),

  *this, table);

  }

• Item_type_holder::make_field_by_type() will be renamed to create_tmp_field} and simplify to:

  Field *create_tmp_field(bool group, TABLE *table)

  {

  return Item_type_holder::type_handler()->

  make_and_init_table_field(&name, Record_addr(maybe_null),

  *this, table);

  }

• The TIME_RESULT, STRING_RESULT, DECIMAL_RESULT branches in Item::create_tmp_field() will be simplified to:

  case TIME_RESULT:

  case DECIMAL_RESULT:

  case STRING_RESULT:

  new_field= tmp_table_field_from_field_type(table);