Oracle出现WARNING: datafile #1 was not in online backup mode 出现的原因

今天帮朋友看个数据库,数据库日志中出现以下信息：
alter database end backup
WARNING: datafile #1 was not in online backup mode
WARNING: datafile #2 was not in online backup mode
WARNING: datafile #3 was not in online backup mode
WARNING: datafile #4 was not in online backup mode
ORA-1142 signalled during: alter database end backup...

这个经过测试，发现如果对数据库设置为结束热备份时日志中才会报以上错误日志信息
,因为没有把数据库设置为开始热备份的状态
SQL> alter database end backup;
alter database end backup
*
第 1 行出现错误:
ORA-01142: 无法结束联机备份 - 没有文件在备份中
SQL> alter database begin backup;
数据库已更改。
SQL> alter database end backup;
数据库已更改。
则数据库日志报以下信息，不再出现警告信息
alter database begin backup
Completed: alter database begin backup
alter database end backup
Completed: alter database end backup