some comments added

    a constant expression

  */

  virtual bool basic_const_item() const { return 0; }

  virtual Item *new_item() { return 0; }	/* Only for const items */

  /* cloning of constant items (0 if it is not const) */

  virtual Item *new_item() { return 0; }

  virtual cond_result eq_cmp_result() const { return COND_OK; }

  inline uint float_length(uint decimals_par) const

  { return decimals != NOT_FIXED_DEC ? (DBL_DIG+2+decimals_par) : DBL_DIG+8;}

  virtual bool get_date_result(TIME *ltime,uint fuzzydate)

  { return get_date(ltime,fuzzydate); }

  virtual bool is_null() { return 0; }

  /*

    it is "top level" item of WHERE clause and we do not need correct NULL

    handling

  */

  virtual void top_level_item() {}

  /*

    set field of temporary table for Item which can be switched on temporary

    table during query processing (groupping and so on)

  */

  virtual void set_result_field(Field *field) {}

  virtual bool is_result_field() { return 0; }

  virtual bool is_bool_func() { return 0; }

  virtual void save_in_result_field(bool no_conversions) {}

  /*

    set value of aggegate function in case of no rows for groupping were found

  */

  virtual void no_rows_in_result() {}

  virtual Item *copy_or_same(THD *thd) { return this; }

  virtual Item *copy_andor_structure(THD *thd) { return this; }