今天在做练习的时候，遇到了这样的问题：
SQL> create table a1(id int,name varchar2(20));                                                         
create table rmantb1001(id int,name varchar2(20))
*
ERROR at line 1:
ORA-00604: error occurred at recursive SQL level 1
ORA-00376: file 2 cannot be read at this time
ORA-01110: data file 2: '/opt/app/oracle/oradata/orcl/undotbs01.dbf'
同时，在rman里创建catalog时，报：
RMAN> create catalog ;

恢复目录数据库发生 ORACLE 错误: ORA-00604: error occurred at recursive SQL level
 1
ORA-00376: file 2 cannot be read at this time
ORA-01110: data file 2: '/opt/app/oracle/oradata/orcl/undotbs01.dbf'

随后，google了一下ora-00376，发现：
$ oerr ora 376

00376, 00000, "file %s cannot be read at this time"
// *Cause: attempting to read from a file that is not readable. Most likely
// the file is offline.
// *Action: Check the state of the file. Bring it online
$
You might have to recover your datafile, as in following example :

SQL> select ONLINE_STATUS from dba_data_files

2 where file_id = 5;
ONLINE_


RECOVER
select了一下我的dba_data_files，发现：
'/opt/app/oracle/oradata/orcl/undotbs01.dbf'
'/opt/app/oracle/oradata/orcl/sysaux01.dbf'
都是RECOVER的状态。由于sysaux属于系统表空间，不能置为offline，又因为是练习库，所以就：
SQL> shutdown immediate;

SQL> !cp /backup/path/{undotbs01.dbf,sysaux01.dbf} /opt/app/oracle/oradata/orcl/

SQL> startup mount;

SQL> alter database datafile '/opt/app/oracle/oradata/orcl/sysaux01.dbf' offline;                                

SQL> alter database datafile '/opt/app/oracle/oradata/orcl/undotbs01.dbf' offline;                               

SQL> recover datafile '/opt/app/oracle/oradata/orcl/sysaux01.dbf';                                               

SQL> recover datafile '/opt/app/oracle/oradata/orcl/undotbs01.dbf';                                              

SQL> alter database datafile '/opt/app/oracle/oradata/orcl/undotbs01.dbf' online;                                

SQL> alter database datafile '/opt/app/oracle/oradata/orcl/sysaux01.dbf' online;                                 

SQL> alter database open;                                                                                        

Database altered.

此时，再create table，再create catalog ，正常，问题得到解决。