Depending on what is granted or revoked, a grant or revoke takes effect at different times:
All grants and revokes of system and object privileges to anything (users, roles, and PUBLIC) take immediate effect.
All grants and revokes of roles to anything (users, other roles, PUBLIC) take effect only when a current user session issues a SET ROLE statement to reenable the role after the grant and revoke, or when a new user session is created after the grant or revoke.
测试grant DBA权限
从上面的测试中可以总结出，grant和revoke系统权限和对象权限时会立即生效，而grant或revoke角色时对当前会话不会立即生效，除非使用set role语句启用角色或重新连接会话后设置才会生效。与官方文档的描述一致。
但是有一个问题是如果查看已经连接的其他会话所拥有的role呢？
官方文档：http://docs.oracle.com/cd/E11882_01/network.112/e36292/authorization.htm#DBSEG99974
system privilege:http://docs.oracle.com/cd/E11882_01/server.112/e41084/statements_9013.htm#BABEFFEE
object privilege:http://docs.oracle.com/cd/E11882_01/server.112/e41084/statements_9013.htm#BGBCIIEG
set role:http://docs.oracle.com/cd/E11882_01/server.112/e41084/statements_10004.htm#SQLRF01704