Part 1 of this article (June 2002 DM Review) described the advantage of a fully normalized database and debunked the fallacy prevailing in the industry that it is the level of full (logical) normalization that determines performance. Part 1 concluded as follows: The only reason you may sometimes achieve better performance when you denormalize your database is because you ignore the integrity implications. If and when those are taken into account, you will lose the performance gain, if any, from denormalization and may even end up with a net performance loss relative to the fully normalized alternative. This article will demonstrate why the notion of "denormalization for performance" is a dangerous illusion.
Consider the assignment of employees to projects and (within-project) to activities in a certain company. The following business rules are in effect:
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