This use of the term slightly differs from
the use given in this chapter.
The unbalancedness of the hierarchy schema is not allowed in traditional models, as
in the early model of Cabbibo and Torleone (1998), along with the snowflake and
the star schemas. The restriction of having a bottom category in these models is due
Figure 9. An unbalanced dimension that models the Product dimension of Figure 2:
(a) hierarchy schema; (b) hierarchy domain
(a)
(b)
48 Hurtado & Gutierrez
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to the simplicity of having a unique bottom granularity at which the base facts are
addressed (the attribute associated to such category participates in the composed
key of the fact table in star and snowflake schemas). By allowing many bottom categories,
the heterogeneous dimension can be broken up into the different structures
that are mixed in the original dimension. In this form a homogeneous dimension
having consistent paths for aggregate navigation is obtained.
Several commercial OLAP systems allow storing this sort of structure. Basically
the user should enter separately each hierarchy path in a dimension set to allow
multiple hierarchies.
As an example, Figure 9 shows an unbalanced homogeneous dimension that models
the product dimension of Figure 2.
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