数据库静默模式（quiesce）实验

Occasionally you might want to put a database in a state that allows only DBA transactions, queries, fetches, or PL/SQL statements. 
Such a state is referred to as a quiesced state, in the sense that no ongoing non-DBA transactions, queries, fetches, or PL/SQL statements are running in the system.

--Oracle® Database Administrator's Guide 11g Release 2 (11.2) p.138

--实验--切换到静默状态。SYS@PROD1> ALTER SYSTEM QUIESCE RESTRICTED;  System altered.SYS@PROD1> select active_state from v$instance;ACTIVE_ST---------QUIESCEDYou can query the ACTIVE_STATE column of the V$INSTANCE view to see the currentstate of an instance. The column values has one of these values:■ NORMAL: Normal unquiesced state.■ QUIESCING: Being quiesced, but some non-DBA sessions are still active.■ QUIESCED: Quiesced; no non-DBA sessions are active or allowed.--Oracle® Database Administrator's Guide 11g Release 2 (11.2) p.140--另起一个SCOTT的session进行DML操作，结果hang住。SCOTT@PROD1> create table t as select * from emp;--在SYS成功DML操作。SYS@PROD1> create table scott.t2 as select * from scott.emp;Table created.--切换回正常状态。SYS@PROD1> ALTER SYSTEM UNQUIESCE; System altered.SYS@PROD1> select active_state from v$instance;ACTIVE_ST---------NORMAL--查看另一session。SCOTT@PROD1> create table t as select * from emp;create table t as select * from emp             *ERROR at line 1:ORA-00955: name is already used by an existing objectSCOTT@PROD1> Non-DBA active sessions will continue until they become inactive.