快速了解Oracle9i中的一个特殊等待事件

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