如何删除使用过的undo表空间
1、新建立UNDOTBS1表空间  
create undo tablespace orcl_undo
datafile 'C:\DATABASE\disk3\orcl\dog_undo.dbf'
size 50m
extent management local
autoextend on maxsize 12G;
2、切换回orcl_undo
alter system set undo_tablespace=orcl_undo scope=both;  
3、等待UNDO表空间dog_undo is OFFLINE；  
alter tablespace dog_undo offline;
4、删除dog_undo表空间
drop tablespace dog_undo including contents and datafiles;

到此完毕！