Convergences are represented by letting two
or more arcs converge on the same dimension attribute.
The existence of apparently equal attributes does not always determine a convergence.
If in the invoice fact we had a brand city attribute on the product hierarchy,
representing the city where a brand is manufactured, there would be no convergence
with attribute (customer) city, since a product manufactured in a city can obviously
be sold to customers of other cities as well.
Figure 4. The complete fact schema for the INVOICE LINE fact
2 Rizzi
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Optional.Arcs
Definition 11: An optional arc models the fact that an association represented
within the fact scheme is undefined for a subset of the events. An
optional arc is graphically denoted by marking it with a dash.
For instance, attribute diet takes a value only for food products; for the other products,
it is undefined.
In the presence of a set of optional arcs exiting from the same dimension attribute,
their coverage can be denoted in order to pose a constraint on the optionalities involved.
Like for IS-A hierarchies in the E/R model, the coverage of a set of optional
arcs is characterized by two independent coordinates. Let a be a dimension attribute,
and b1,.
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