EVALUATION AND SUMMARY OF THE LOGICAL MODEL

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EVALUATION AND SUMMARY OF THE LOGICAL MODEL

This chapter has focused on the implementation of the solutions that were introduced into the conceptual model in Chapter 5.

Principally, Chapter 6 has focused on the implementation of retrospection. We have seen how true retrospection can be implemented by the use of existence attributes and how the use of such attributes enables queries to be expressed that are otherwise very difficult to write and impossible to generalize using the approach adopted generally by practitioners . Existence attributes can be described as selective attribute timestamps that are applied only where needed. In the pursuit of accuracy, the implementation of true retrospection has enabled a very much greater level of accuracy to be achieved in queries involving circumstances and dimensions as well as maintaining accuracy in queries involving facts.

As a result of attempting to simplify the expression of queries involving time, we have seen that the use of the time dimension can be greatly extended to enable joins to circumstances and dimensions rather than just to facts. This approach challenges standard practices and is not recommended by some authors, but the advantages that accrue, in terms of the simplification of queries, are significant enough to justify its adoption.

In introducing this new idea, another problem has emerged which is that the new relationships tend to obliterate the essential star shape of the dimensional model because the time dimension is now related not only to the fact table but also to one or more of the dimensions and even some attributes. Upon reflection, however, in recognition of the fact that data warehouses are temporal databases, it seems entirely appropriate to change the status of the time dimension from being an explicit component of the data model to becoming an implicit component that does not, therefore, need to exist on the diagram. Far from being simply a convenient way of removing the problem, this is regarded as a benefit of the method and a natural consequence of accepting the fact that data warehouses are temporal databases.

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Designing a Data Warehouse . Supporting Customer Relationship Management
Designing A Data Warehouse: Supporting Customer Relationship Management
ISBN: 0130897124
EAN: 2147483647
Year: 2000
Pages: 96
Authors: Chris Todman

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