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The Art of SQL by Peter Robson, Stephane Faroult

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Defining Good Performance

Load is one thing, performance another. Good performance proves an elusive notion to define. Using CPU or performing a large number of I/O operations is not wrong in itself; your company, presumably, didn't buy powerful hardware with the idea of keeping it idle.

When the time comes to assess performance, there is a striking similarity between the world of databases and the world of corporate finance. You find in both worlds some longing for "key performance indicators" and magical ratio—and in both worlds, global indicators and ratios can be extremely misleading. A good average can hide distressing results during the peaks, and a significant part of the load may perhaps be traced back to a batch program that is far from optimal but that runs at a time of night when no one cares what the load is. To get a true appreciation of the real state of affairs, you must drill down to a lower level of detail.

To a large extent, getting down to the details is an exercise similar to that which is known in managerial circles as "activity-based costing." In a company, knowing in some detail how much you spend is relatively easy. However, relating costs to benefits is an exercise fraught with difficulties, notoriously for transverse operations such as information technology. Determining if you spend the right amount on hardware, software, and staff, as well as the rubber bands and duct tape required to hold everything together is extremely difficult, particularly when the ...

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