管理undo tablespace

0.创建undo tablespace ：
CREATE UNDO TABLESPACE undotbs_02
     DATAFILE '/u01/oracle/rbdb1/undo0201.dbf' SIZE 2M REUSE AUTOEXTEND ON;

1.添加一个数据文件：
ALTER TABLESPACE undotbs_01
    ADD DATAFILE '/u01/oracle/rbdb1/undo0102.dbf' AUTOEXTEND ON NEXT 1M
         MAXSIZE UNLIMITED;

2.删除undo tablespace：
DROP TABLESPACE undotbs_01;

3.却换undo tablespace：
ALTER SYSTEM SET UNDO_TABLESPACE = undotbs_02;

ALTER SYSTEM SET UNDO_TABLESPACE = '';
-------------系统会自动切换到下一个可用的undo tablespace