The report shown in Figure 2-8 is based on the system event view. Over the years, Oracle developers have added more columns, and I'm sure they will continue to do so, but for our work, the most important columns are shown in Table 2-1. The wait class columns will be discussed in the "Time Classification" section a little later in this chapter.

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.