ORA-16003: standby database is restricted to read-only access
    dg失败导致，从库一直只能read only开启。

    1. shutdown immediate
    2.  startup nomount
    3.  alter database mount standby database;
    4.  alter database activate standby database;
    5.   shutdown immediate;
    6.   startup open;
    10.   SQL> select open_mode from v$database;
    13. OPEN_MODE
    14. --------------------
    15. READ WRITE