Suppose an Oracle instance has been running for two months and you wanted to know the cache hit ratio for the buffer cache. Querying v$sysstat once would give you the average cache hit ratio since the instance has started. If you wanted to know the cache hit ratio over a specific period, you would need to have beginning and ending values. Think in terms of Statspack and AWR. Before you run the report, you must set both the start and end points. This provides an interval, or period of time, so your calculations are relevant to the time period desired. I call the interval calculations the blue line, and the simple v$sysstat query the red line, as illustrated in Figure 1-5.

Figure 1-5. Blue line statistics are created from an interval of time, whereas red line statistics contain all activity (accumulated) since the instance started.

Figure 1-5 startles many DBAs at first because both lines represent the same underlying statistics. Suppose you wanted to know the statistic value at time 8. If you ran a red line query, the beginning value would be 0 (zero), since that is when the instance started, and the ending value would be queried at time 8. Now suppose you wanted to know the statistic within the last hour. The blue line query's initial value would be queried at time 7 and the ending value at time 8. In Figure 1-5, while the ending statistic values are the same, the performance statistic value for the red line is 1.5, yet for the blue line, it is 2.0. That is a significant difference. And notice the longer the instance is running, the flatter the red line is and the less likely that it is somewhat close to the blue line.