[MDEV-20331] Add class Type_numeric_attributes - Jira

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We should eventually remove DTCollation from non-string items, as it's always initialized to DTCollation_numeric() and therefore wastes memory.

As a step to this, let's move the following members from Type_std_attributes:

into a new class Type_numeric_attributes and derive Type_std_attributes from it.

Let's also add a new virtual method in Field:

virtual Type_numeric_attributes type_numeric_attributes() const;

and turn Field::type_std_attributes() from a virtual to a non-virtual method like this:

  Type_std_attributes type_std_attributes() const

    return Type_std_attributes(type_numeric_attributes(), dtcollation());

All former Type_std_attributes methods that operate on only the moved members should also move to class Type_numeric_attributes.

As a result, the classes will look about like this:

class Type_numeric_attributes

public:

  static uint count_unsigned(Item **item, uint nitems);

  static uint32 find_max_char_length(Item **item, uint nitems);

  static uint32 find_max_octet_length(Item **item, uint nitems);

  static int find_max_decimal_int_part(Item **item, uint nitems);

  static uint find_max_decimals(Item **item, uint nitems);

public:

/*

    The maximum value length in characters multiplied by collation->mbmaxlen.

    Almost always it's the maximum value length in bytes.

*/

  uint32 max_length;

  uint decimals;

  bool unsigned_flag;

public:

  Type_numeric_attributes()

   :max_length(0), decimals(0), unsigned_flag(false)

{ }

  Type_numeric_attributes(uint32 max_length_arg, uint decimals_arg,

                          bool unsigned_flag_arg)

   :max_length(max_length_arg),

    decimals(decimals_arg),

    unsigned_flag(unsigned_flag_arg)

{ }

protected:

  void aggregate_numeric_attributes_real(Item **item, uint nitems);

  void aggregate_numeric_attributes_decimal(Item **item, uint nitems,

                                            bool unsigned_arg);

};

class Type_std_attributes: public Type_numeric_attributes

public:

  DTCollation collation;

  Type_std_attributes()

   :collation(&my_charset_bin, DERIVATION_COERCIBLE)

{ }

  Type_std_attributes(const Type_numeric_attributes &nattr,

                      const DTCollation &dtc)

   :Type_numeric_attributes(nattr), collation(dtc)

{ }

...

};

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