In this chapter, we
examine the problems encountered when handling structural heterogeneity in OLAP
and survey techniques that have been proposed to solve them. We show how to incorporate
structural heterogeneity in the design of OLAP models. We explain why
structural heterogeneity weakens aggregate navigation, the framework that guides
users to formulate correct OLAP operations and systems to efficiently process them.
We survey different techniques to deal with heterogeneity, including the modeling
of heterogeneity by unbalanced dimensions, the solution proposed by Kimball, and
the use of null elements to fix heterogeneity. Finally, we present a class of integrity
constraints to model structural heterogeneity, called dimension constraints, introduced
in previous work of the authors. We show the practical application of dimension
constraints to support aggregate navigation and some of the aforementioned
techniques for dealing with the problem.
28 Hurtado & Gutierrez
Copyright ?© 2007, Idea Group Inc. Copying or distributing in print or electronic forms without written permission of
Idea Group Inc. is prohibited.
Introduction
Much of the success of OLAP can be attributed to the intuitive approach to data visualization
provided by the multidimensional data model. Nowadays, the notions of
facts and dimensions have been largely disseminated among database practitioners
and researchers and have been proved to be useful metaphors to support querying
data for decision support.
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