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
