GROUP# THREAD# SEQUENCE# BYTES BLOCKSIZE MEMBERS ARC STATUS FIRST_CHANGE# FIRST_TIM NEXT_CHANGE# NEXT_TIME
---------- ---------- ---------- ---------- ---------- ---------- --- ---------------- ------------- --------- ------------ ---------
1 1 133 52428800 512 1 YES INACTIVE 3155684 15-FEB-15 3155687 15-FEB-15
2 1 134 52428800 512 1 NO CURRENT 3155687 15-FEB-15 2.8147E+14
3 1 132 52428800 512 1 YES INACTIVE 3155681 15-FEB-15 3155684 15-FEB-15
SQL> select file#,status from v$datafile where file#=9;
FILE# STATUS
---------- -------
9 OFFLINE
SQL> select file#,status from v$datafile_header where file#=9;
FILE# STATUS
---------- -------
9 OFFLINE
SQL> alter database datafile 9 online;
Database altered.
SQL> select file#,status from v$datafile where file#=9;
FILE# STATUS
---------- -------
9 ONLINE
SQL>
知识点：
1.The only case in which the offline dropped datafile can not be online is
when you have added to many datafiles in the database after offline drop
2.在非归档模式下，为了让一个datafile 变成offline，必须带drop关键字。
drop关键字不会把datafile从database 中 remove掉。
To do that, you must drop the tablespace in which the datafile resides. Until you
do so, the datafile remains in the data dictionary with the status RECOVER or OFFLINE.
若是database处于归档模式，Oracle会忽略掉drop 关键字.