Whereas the system event view focuses on the overall system wait situation, the session event view focuses on the individual session wait situation. This view is helpful for answering questions like "What sessions have been waiting on db file scattered reads?" and "What is wait situation for session 205?" It's the natural next drill-down level.

The v$session_event view does not report in real time. In fact, the information could be as old as 3 seconds! Even though it has a column named time_waited_micro, the contents could have been updated seconds ago. So don't be fooled. If you want real-time details, you will need to sample from the v$session_wait view. The only significant difference between the system event view columns, listed in Table 2-1, and the session event view column is the session identifier column (sid) in the latter.

Figure 2-9 is a report based on v$session_event. Notice the command-line parameter is the session identifier. The report was run twice with around a 30-second gap. If session 205 were active, we would expect some increased wait-event activity. Notice that a couple of the wait times did increase. We would never expect the wait time to decrease unless session 205 disconnected and then a new session 205 was initialized upon connection.