Now suppose we want to gain a better understanding of the other sessions waiting for multiblock reads. Once again, we can reference v$session_wait. But instead of selecting a specific session, we list all the sessions waiting on a given wait event-in this case, db file scattered read. Figure 2-13 shows such a query, which simply uses the wait event like a command-line parameter. At the specific moment of the query, there were three sessions patiently waiting for the blocks to be retrieved from the operating system. Sampling the file and block numbers while running a script like objfb.sql (shown in Figure 2-12) will help build a strong connection between all three circles in a 3-circle analysis.

Figure 2-13. The v$session_wait view can also be used to list wait event details about all sessions waiting on a specific event. In this example, all four sessions are waiting for the operating system to return 16 blocks.

If you understand the instrumentation concept, are familiar with the three core wait event views, and use a methodical diagnosis framework, you are prepared to jump to the next level, which is performing an ORTA. Obviously, you need an understanding of what the wait events are telling you. Otherwise, you are like a doctor who hears what the patient is saying, but doesn't know enough to do anything about it. The upcoming chapters will provide you with the knowledge you need to take a stellar diagnosis and derive solutions that will directly attack the problem.