, 2005), which considers notions from previous dimension models
(Cabibbo & Torlone, 1998; Hurtado, Mendelzon & Vaisman, 1999; Jagadish et al.,
1999) and formalizes them in terms of basic notions of graph theory. Basic notions
of graph theory used can be consulted in any standard graph theory book (West,
1996). The model allows heterogeneity, among other features not encountered in
traditional dimension models. By ???traditional??? we refer to the most common type
Handling Structural Heterogeneity in OLAP
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of dimensions that appear in textbooks, white papers, and early research papers on
OLAP, first formalized by Cabbibo and Torleone (1998).
Hierarchy Schema
The hierarchy schema is the standard structure for representing the semantics of
OLAP dimensions. This structure is common to different models. The hierarchy
schema is modeled as a directed acyclic graph (DAG) (C, ??°) whose set of nodes
C contains categories, and ??° represents the incidence relation, that is, c1 ??° c2 indicates
that the category c1 is connected to the category c2 in the hierarchy schema. We
will draw the direction of the edges always upward. The DAG has a distinguished
category All reachable by a path from every other category (top category).
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