UNDO表空间会在大表导入数据时变的很大，有时候会霸占着大量的磁盘空间。为了有效利用系统资源，可以重建UNDO表空间：
SQL>conn sys/sms@smsdb as sysdba
SQL>create undo tablespace UNDOTBS2 datafile 'C:oracleoradatasmsdbUNDOTBS02.DBF'size 1M extent management local;
SQL>alter system set undo_tablespace=UNDOTBS2;
SQL>drop tablespace UNDOTBS1 including contents;
SQL>shutdown immediate
SQL>host del C:oracleoradatasmsdbUNDOTBS01.DBF;
SQL>startup
SQL>create undo tablespace UNDOTBS1 datafile 'C:oracleoradatasmsdbUNDOTBS01.DBF'size 2048M extent management local;
SQL>alter system set undo_tablespace=UNDOTBS1;
SQL>drop tablespace UNDOTBS2 including contents;
SQL>host del C:oracleoradatasmsdbUNDOTBS02.DBF;
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