As DBAs, we have many ways to classify time, which gives us many options when communicating concepts to a variety of audiences. I personally use a very simplistic classification scheme for both service time and queue time. Oracle began classifying its wait events in Oracle Database 10g, which uses 12 classifications; 11g uses 13. For performance firefighting, I find it much more useful to keep the number of classifications to around 4.

Figure 2-14. Various levels of Oracle-focused time classification. The contents could be the result of a one-hour snapshot of all Oracle activity or the result of profiling a specific Oracle transaction. There are numerous classification schemes. The schemes shown here are exceptional for firefighting purposes. When combined with a response-time curve, the communication impact is phenomenal.

Here, I'll introduce queue time classification and service time classification, discussing how these classifications can be used and their data sources. You'll find more details about these topics in Chapter 5.