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SQL> create tablespace test datafile '/u01/app/oracle/oradata/orcl/test01.dbf' size 10M autoextend on next 1M;
Tablespace created.
SQL> create table test(id number,name varchar2(12)) tablespace test;
Table created.
SQL> insert into test values (1,'colin');
1 row created.
SQL> insert into test values (2,'janey');
1 row created.
SQL> insert into test values (3,'tom');
1 row created.
SQL> insert into test values (4,'adele');
1 row created.
SQL> commit;
Commit complete.
SQL> select * from test;
ID NAME
---------- ------------
1 colin
2 janey
3 tom
4 adele
SQL> alter database datafile '/u01/app/oracle/oradata/orcl/test01.dbf' offline drop;
Database altered.
SQL> commit;
Commit complete.
SQL> select * from test;
select * from test
*
ERROR at line 1:
ORA-00376: file 2 cannot be read at this time
ORA-01110: data file 2: '/u01/app/oracle/oradata/orcl/test01.dbf'
SQL> select LOG_MODE,OPEN_MODE from v$database;
LOG_MODE OPEN_MODE
------------ ----------
NOARCHIVELOG READ WRITE
SQL> alter database datafile '/u01/app/oracle/oradata/orcl/test01.dbf' online;
alter database datafile '/u01/app/oracle/oradata/orcl/test01.dbf' online
*
ERROR at line 1:
ORA-01113: file 2 needs media recovery
ORA-01110: data file 2: '/u01/app/oracle/oradata/orcl/test01.dbf'
SQL> recover datafile '/u01/app/oracle/oradata/orcl/test01.dbf';
Media recovery complete.
SQL> alter database datafile '/u01/app/oracle/oradata/orcl/test01.dbf' online;
Database altered.
SQL> select * from test;
ID NAME
---------- ------------
1 colin
2 janey
3 tom
4 adele
恢复成功。那是因为日志还没有切换，使用了online日志。
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切换日志来测试：
SQL> alter database datafile '/u01/app/oracle/oradata/orcl/test01.dbf' offline drop;
Database altered.
SQL> select * from test;
select * from test
*
ERROR at line 1:
ORA-00376: file 2 cannot be read at this time
ORA-01110: data file 2: '/u01/app/oracle/oradata/orcl/test01.dbf'
SQL> alter system switch logfile;
System altered.
SQL> /
System altered.
SQL> /
/
System altered.
SQL>
System altered.
SQL> recover datafile '/u01/app/oracle/oradata/orcl/test01.dbf';
ORA-00279: change 654004 generated at 07/09/2012 21:07:18 needed for thread 1
ORA-00289: suggestion : /u01/app/oracle/arch/1_32_777559117.arc
ORA-00280: change 654004 for thread 1 is in sequence #32
Specify log: {=suggested | filename | AUTO | CANCEL}
cancel
Media recovery cancelled.
SQL> /
System altered.
SQL> recover datafile '/u01/app/oracle/oradata/orcl/test01.dbf';
ORA-00279: change 654004 generated at 07/09/2012 21:07:18 needed for thread 1
ORA-00289: suggestion : /u01/app/oracle/arch/1_32_777559117.arc
ORA-00280: change 654004 for thread 1 is in sequence #32
Specify log: {=suggested | filename | AUTO | CANCEL}
auto
ORA-00308: cannot open archived log '/u01/app/oracle/arch/1_32_777559117.arc'
ORA-27037: unable to obtain file status
Linux Error: 2: No such file or directory
Additional information: 3
ORA-00308: cannot open archived log '/u01/app/oracle/arch/1_32_777559117.arc'
ORA-27037: unable to obtain file status
Linux Error: 2: No such file or directory
Additional information: 3
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通过以上测试表明，在非归档模式下使用了alter database datafile 'XXX' offline drop命令是不能被恢复的，虽然我上面的第一次实验成功了，那是因为online日志还没有切换，在日志中还能够找到相应的反操作。在第二个实验中，手工切换日志，会提示不能打开归档日志，因为我根本就没有归档，肯定找不到啊，所以不能够恢复的。切记，在非归档模型下不要随意使用一些drop命令，在实际生产环境中日志的切换是很频繁且快的，否则只有掏钱找oracle了。当然，如果在业务不繁忙的系统可以及时的通过online日志进行恢复，这种几率比较小。
