Here the atom ??©Product,MusicCat,All??? expresses that the product element go the
element all in All passing through an element in MusicCat. The other atom is interpreted
similarly.
In previous work (Hurtado & Gutierrez, 2003), we provide an algorithm to compute
the set of correct rollup operations from the dimension schema. As the dimension
schema is fixed, the correct operations can be computed once before the data cube
is queried. In addition, dimension constraints allow identifying consistent paths of
aggregation in the hierarchy schema. As an example, from a schema D that models
the dimension of Figure 7, one may want to check whether the path Product??°Bran
d??°Company??°All is consistent, which reduces to testing the implication of following
constraint ??©Product,Brand,Company,All??? from the schema D.
Conclusion
When the homogeneity condition is dropped the framework for aggregate navigation
that underlies OLAP systems cannot be applied. In this chapter we have surveyed
Handling Structural Heterogeneity in OLAP
Copyright ?© 2007, Idea Group Inc. Copying or distributing in print or electronic forms without written permission
of Idea Group Inc. is prohibited.
different methods to deal with this problem. The situation can be handled by adding
null elements in a way such that we obtain homogeneous dimensions with all the standard
properties.
Pages:
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130