Entire portions of hierarchies are frequently replicated two or more times in fact schemata. Temporal and geographical hierarchies are classic examples.
|In a shared hierarchy, an abbreviated graphic notation (a double circle) emphasizes that all or part of a hierarchy is used twice or more in the same fact schema. A role can be added to entering arcs to clarify ambiguities.|
Differently from a convergence, in a shared hierarchy there is no constraint requiring that each alternative roll-up path functionally determines the same value of the shared attribute.