You may have noticed I have not mentioned the number of waits. This is because it is not an issue for our analysis. We are focusing on what affects our users, and that's time, not the number of waits. Look closely at the average wait times in Figure 2-8. You'll see that some wait events have an average wait time of less then 0.1 ms, while the largest average wait time is 110 ms. It is the combination of the number of waits and the individual wait time that affects our users, and therefore what's important. It is very common for the wait event with the largest number of waits to not be associated with the most time. Focus on the statistics that primarily impact the user experience. In this report, what impacts our users the most is the percentage of time waited.

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.