Actually, the E/R
model has enough expressivity to represent most concepts necessary for modeling
a DW; on the other hand, in its basic form, it is not able to properly emphasize the
key aspects of the multidimensional model, so that its usage for DWs is expensive
from the point of view of the graphical notation and not intuitive (Golfarelli, Maio,
& Rizzi, 1998).
Some designers claim to use star schemata for conceptual modeling. A star schema
is the standard implementation of the multidimensional model on relational platforms;
it is just a (denormalized) relational schema, so it merely defines a set of
relations and integrity constraints. Using the star schema for conceptual modeling
is like starting to build a complex software by writing the code, without the support
of and static, functional, or dynamic model, which typically leads to very poor
results from the points of view of adherence to user requirements, of maintenance,
and of reuse.
For all these reasons, in the last few years the research literature has proposed several
original approaches for modeling a DW, some based on extensions of E/R, some
on extensions of UML. This chapter focuses on an ad hoc conceptual model, the
dimensional fact model (DFM), that was first proposed in Golfarelli et al. (1998) and
continuously enriched and refined during the following years in order to optimally
suit the variety of modeling situations that may be encountered in real projects of
small to large complexity.
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