清理回滚段表空间

创建一个新的回滚段（如果不行，则先将tablespace=''设置为空，删除掉旧的回滚段）
create undo tablespace undotbs1 datafile '/opt/oracle/app/oradata/undotbs1.dbf' size 10m reuse autoextend on;

切换undo表空间
alter system set undo_tablespace=undotbs1 scope=both;

删除旧的表空间
drop tablespace undotbs2 including contents and datafiles;