Dynamic.Hierarchies
Time is a key factor in data warehousing systems, since the decision process is often
based on the evaluation of historical series and on the comparison between snapshots
of the enterprise taken at different moments. The multidimensional models implicitly
assume that the only dynamic components described in a cube are the events that
6 Rizzi
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instantiate it; hierarchies are traditionally considered to be static. Of course this is
not correct: sales manager alternate, though slowly, on different departments; new
products are added every week to those already being sold; the product categories
change, and their relationship with products change; sales districts can be modified,
and a customer may be moved from one district to another.1
The conceptual representation of hierarchy dynamicity is strictly related to its impact
on user queries. In fact, in presence of a dynamic hierarchy we may picture three
different temporal scenarios for analyzing events (SAP, 1998):
???. Today. for. yesterday: All events are referred to the current configuration
of hierarchies. Thus, assuming on January 1, 2005 the responsible agent for
customer Smith has changed from Mr. Black to Mr.
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