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The steps in the normal transform and subsumption testing processes are not particularly onerous. However, queries require thousands or millions of such tests to be carried out. It is therefore likely that most practical implementations will require some type of optimization to support tests for subsumption between expressions .
The advantages of this include the following:
Neither of these approaches appears to be scalable over time, as record volumes increase. In contrast the proposed optimization is not affected by the total number or records but only by the total number of distinct expressions encountered.
The approach described in the following section is only one way of implementing the central idea of optimization using an expression repository. There several ways to harness the same general technique and some of these may be better suited to particular requirements or technical environments.