多项选择题 Media recovery can sometimes be stopped by the inability to read past a certain point in the redo stream. This is often referred to as “stuck recovery”. Before Oracle9i， the  Database Administrator had few options to deal with stuck recovery. If the corrupt redo could not be recovered from some other source， then all transactions that committed after the corrupt point in the redo steam would be lost.  Oracle9i changes that with the Trial Recovery feature. Trial Recover is used to test the application of the redo logs to the database.  What are three other characteristics of Trial Recovery（）

多项选择题 Examine the code：   CREATE ROLE readonly IDENTIFIES USING app.chk_readwrite ； CREATE ROLE readwrite IDENTIFIED USING app.chk_readwrite ；   CREATE OR REPLACE PROCEDURE app.chk_readwrite  AUTHID CURRENT_USER IS  ipchk STRING（30）；  BEGIN  IF sys_context（‘USERENV’，’ISDBA’）=’TRUE’  THEN DBMS_SESSION.SET_ROLE‘READWRITE’） ；  ELSE DBMS_SESSION.SET_ROLE（‘READONLY’） ；  END； /   Which three statements correctly describe the Secure Application role definition？（）

单项选择题 Consider this RMAN command：   RMAN> CONFIGURE RETENTION POLICY CLEAR；   What is the effect of this command？（）