执行以下语句报错：
SQL> alter tablespace system begin backup; ERROR at line 1: ORA-01123: cannot start online backup; media recovery not enabled
通过查看资料发现时归档未开启，开启归档操作步骤：
SQL> alter database close normal; Database altered. SQL> select open_mode from v$database; OPEN_MODE ------------------------------------------------------------ MOUNTED SQL> alter database archivelog; Database altered.
发现这时候要重启数据库，否则无法直接切换到读写模式：
SQL> alter database open; alter database open * ERROR at line 1: ORA-16196: database has been previously opened and closed SQL> shutdown immediate; ORA-01109: database not open Database dismounted. ORACLE instance shut down. SQL> startup; ORACLE instance started. Total System Global Area 6680915968 bytes Fixed Size 2213936 bytes Variable Size 3556771792 bytes Database Buffers 3087007744 bytes Redo Buffers 34922496 bytes Database mounted. Database opened.
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