Oracle Deref函数：错误的数字或参数类型

我正在尝试在Oracle中创建一个对象方法,如下所示：

CREATE OR REPLACE TYPE BODY TheType AS
    MEMBER FUNCTION getAtt RETURN VARCHAR2 IS
    BEGIN
        RETURN DEREF(SELF.Att).Att2;
    END;
END;
/

但是我收到以下错误：

PLS-00306: wrong number or types of arguments in call to 'DEREF'

类型TheType也是这样声明的：

CREATE OR REPLACE TYPE TheType UNDER SuperType ();
/

...

ALTER TYPE TheType ADD ATTRIBUTE ( Att REF TheType ) CASCADE;

...

ALTER TYPE TheType 
    ADD MEMBER FUNCTION getAtt RETURN VARCHAR2
CASCADE;

和Supertype的定义：

CREATE OR REPLACE TYPE SuperType AS OBJECT ( Att2 VARCHAR2(50) )
NOT FINAL NOT INSTANTIABLE;
/

我给DEREF函数一个具有正确类型的var,为什么会出现这个错误？

如果我信任Oracle doc,它应该工作

谢谢.

解决方法

SQL> create or replace type supertype as object ( att2 varchar2(50) )
  2  not final not instantiable;
  3  /

Type created.

SQL> create or replace type thetype under supertype (
  2  att ref thetype,3  member function getatt return varchar2);
  4  /

Type created.

SQL> show errors type thetype
No errors.
SQL> create or replace type body thetype as
  2    member function getatt return varchar2 is
  3      v_t thetype;
  4    begin
  5             select deref(self.att) into v_t from dual;
  6             return v_t.att2;
  7    end;
  8  end;
  9  /

Type body created.

SQL> show errors type body thetype
No errors.
SQL> create table thetypes of thetype;

Table created.

SQL> insert into thetypes values ('hi there',null);

1 row created.

SQL> set serverout on
SQL> declare
  2    v_t thetype;
  3  begin
  4    select thetype(null,ref(a)) into v_t from thetypes a;
  5    dbms_output.put_line(v_t.getatt);
  6  end;
  7  /
hi there

PL/SQL procedure successfully completed.

SQL>

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