今天在试图处理一个数据库(11.2.0.1)的性能问题时，发现无法运行awr相关的查询、生成操作，继而发现后台进程mmon/mmnl缺失。
重启mmon/mmnl的方法，是在业务闲时启用restricted模式，再马上禁用：
alter system enable restricted session;
alter system disable restricted session;
为了尽可能的减少设置restricted模式带来的影响，需两个语句一起执行。
启用restricted模式后，应用新连接数据库的会话将直接报错ORA-12526无法连接，直到禁用restricted模式后才可以恢复。
操作的后台日志如下:
Tue Dec 08 17:54:05 2020 Restarting dead background process QMNC Tue Dec 08 17:54:05 2020 QMNC started with pid=167, OS id=54919192 Tue Dec 08 17:58:08 2020 Restarting dead background process QMNC Tue Dec 08 17:58:08 2020 QMNC started with pid=240, OS id=46399744 Tue Dec 08 17:58:39 2020 Starting background process MMON Tue Dec 08 17:58:39 2020 MMON started with pid=182, OS id=13697714 Starting background process MMNL Tue Dec 08 17:58:39 2020 MMNL started with pid=325, OS id=28574400 ALTER SYSTEM enable restricted session; ALTER SYSTEM disable restricted session;
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