在函式中執行DDL語句失敗

測試函式中執行DDL語句，看piner的FAQ裡面說可以，但是我在執行的時候報錯，難道它說的PL/SQL不是指這些物件？

Create Or Replace Function Create_Cols(Ow In String, Tab_Name In String)
  Return Varchar2 Is
  Cols Varchar2(4000);
  Num  Number(8);
  --  Len  Number(8);
  Cursor All_Cols Is
    Select Column_Name
      From All_Tab_Cols
     Where Owner = Ow
       And Table_Name = Tab_Name
     Order By Internal_Column_Id;
  Cursor Get_Sent Is
    Select 'insert into len_cols Select ''' || Column_Name ||
           ''', Max(length(' || Column_Name || ')) From ' || Owner || '.' ||
           Table_Name || ''
      From All_Tab_Cols
     Where Owner = Ow
       And Table_Name = Tab_Name;
  Type Studydate Is Table Of Varchar2(400) Index By Binary_Integer;
  v_Studydate Studydate;
  v_Get_Sent  Studydate;
Begin
  Select Count(Column_Name)
    Into Num
    From All_Tab_Cols
   Where Owner = Ow
     And Table_Name = Tab_Name;
  Open All_Cols;
  Open Get_Sent;
  For i In 1 .. Num Loop
    Fetch All_Cols
      Into v_Studydate(i);
    Fetch Get_Sent
      Into v_Get_Sent(i);
  End Loop;
  For i In 1 .. Num Loop
    /*    Execute Immediate 'comment on column BUDGET_FILE.B_FILE_ID  is ''eee''';*/
    Dbms_Utility.Exec_Ddl_Statement('comment on column BUDGET_FILE.B_FILE_ID  is ''eee''');
    Dbms_Output.Put_Line(v_Get_Sent(i));
  End Loop;
  Select Substr(Cols, 1, Length(Cols) - 1) Into Cols From Dual;
  Close All_Cols;
  Close Get_Sent;
  Return Cols;
End Create_Cols;

報錯：
ORA-14552在查詢或DML中無法執行DDL，提交或回退