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$system_event where wait_class'idle' order by 3 desc )where rownum
2.当前已连接的各会话的历史等待事件信息
select * from (select event, wait_class, sum(total_waits), sum(time_waited) from v$session_event where wait_class 'idle' group by event, wait_class order by sum(time_waited) desc) where rownum 9i select * from (select event, sum(total_waits), sum(time_waited) from v$session_event where event not in ('rdbms ipc message','pmon timer','smon timer','sql*net message from client','rdbms ipc reply','sql*net more data from client') group by event order by sum(time_waited) desc) where rownum
3.当前已连接的某session的历史等待事件信息
select * from (select sid, event, total_waits, time_waited from v$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。