过去发生的等待事件，我们可以通过什么视图查出来呢？

1.自实例启动以来

select * from ( select event,total_waits,time_waited,wait_class from v$system_event where wait_class<>'Idle' order by 3 desc )where rownum<=50; 9i select * from ( select event,total_waits,time_waited from v$system_event where event not in ('rdbms ipc message','pmon timer','smon timer','SQL*Net message from client','rdbms ipc reply','SQL*Net more data from client') order by 3 desc )where rownum<=50;

2.当前已连接的各会话的历史等待事件信息

select * from (select event, wait_class, sum(total_waits), sum(time_waited) from v$session_event where wait_class <> 'Idle' group by event, wait_class order by sum(time_waited) desc) where rownum < 20;
9i select * from (select event, sum(total_waits), sum(time_waited) from v$session_event where event not in ('rdbms ipc message','pmon timer','smon timer','SQL*Net message from client','rdbms ipc reply','SQL*Net more data from client') group by event order by sum(time_waited) desc) where rownum < 20;

3.当前已连接的某SESSION的历史等待事件信息

select * from (select sid, event, total_waits, time_waited from v$session_event where sid=&1 order by sid asc,time_waited desc) where event not like '%rdbms ipc%' and event not like '%SQL*Net message%';
9i: select sid, event, total_waits, time_waited from v$session_event where sid in (&1) order by sid asc,time_waited desc

备注： total_waits代表总共等待的次数，比较重要的是time_waited的，这反映了用户的等待体验，time_waited是百分之一秒。