null event不是空事件，不是没有等待事件,而是Oracle忘记了刚才等待的事件的名字是什么了，但保留了该事件的真实情况。Oracle 的开发人员直到9.2.0.3才更新了这些事件名称。
那是不是我们每当遇到性能问题的时候都要patch到9.2.0.3呢？
我们已经知道他的真实情况依然在v$session_wait的P1、P2、P3参数里体现，所以，我们可以从这些参数顺藤摸瓜，找出这个null event 到底是什么等待事件。
下面是ITPUB上COPY过来的一些信息：
1. LCK processes to pick up extra or missed 'posts' (messages).
In all cases that can be identified the wait time is zero,
although on most ports this is changed to 1/100th second in
order to force the process to be rescheduled.
2. Multiple DB writers.
Two cases:
- When the master has sent i/o requests to the slaves, it
waits on this event for up to 6 seconds until the slaves
signal that the i/o is complete.
- The slaves wait on this event for up to 3 seconds whilst
waiting for requests from the master db writer.
目前已经被证实的会涉及到null event 的等待事件还有有SQL*Net message to client、 db file scattered (or sequential) read
