Guideline.9:.Ragged hierarchies may lead to summarizability problems.
A way for avoiding them is to fragment a fact into two or more facts, each
including a subset of the hierarchies characterized by uniform interlevel
relationships.
Thus, in the invoice example, fragmenting INVOICE LINE into U.S. INVOICE LINE
and E.U. INVOICE LINE (the first with the state attribute, the second without state)
restores the completeness of the geographic hierarchy.
Unbalanced Hierarchies
Definition 14: An unbalanced (or recursive) hierarchy is a hierarchy
where, though interattribute relationships are consistent, the instances
may have different length. Graphically, it is represented by introducing
a cycle within the hierarchy.
A typical example of unbalanced hierarchy is the one that models the dependence
interrelationships between working persons. Figure 4 includes an unbalanced hierarchy
on sale agents: there are no fixed roles for the different agents, and the different
???leaf??? agents have a variable number of supervisor agents above them.
Guideline.10:.Recursive hierarchies lead to complex solutions during
ROLAP logical design and to poor querying performance. A way for
avoiding them is to ???unroll??? them for a given number of times.
For instance, in the agent example, if the user states that two is the maximum number
of interesting levels for the dependence relationship, the customer hierarchy could
be transformed as in Figure 7.
Pages:
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69