Figure 9-36. Shown is a logical IO-focused response-time curve highlighting and contrasting the initial performance situation (point A) to the final performance situation (point B). This response-time curve indicates a very successful performance effort because fewer resources are required for a single logical IO (service time decreased), users are putting less of a load on the system (not shown: while their work productivity has increased), and the database server's CPU subsystem can now accommodate a much larger future growth.

When adding the status column index, we anticipated only a 3% decrease in CPU utilization, but in reality, there was a 13% drop! Always try to be conservative, but in this case, the anticipated performance impact was simply wrong. We got lucky because many other SQL statements were impacted (for the better) in addition to the four we targeted. Because we did not analyze all possible affected SQL statements, there could have easily been other statements negatively impacted, eliminating any performance gain achieved from our targeted efforts.

I could have simply left the index addition section out of the book, but I included it for two reasons. First, to provide another example of how performance change can be anticipated. Second, so you can observe how easy it is to be wrong by not thinking through a change.