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.

What is dangerous is that most DBAs have never thought about this. Yet when in a firefighting situation and running interactive scripts, usually very simple queries that gather data once from the underlying performance view are run. The result is a red line query, which can be very misleading.