The drill down operation has the inverse effect of the rollup operation and
allows users to move to a cube view at a lower granularity.
Dependence
The possible roll-up and drill-down operations can be represented by a structure
called cube dependence graph, which is defined as the direct product of the list of
hierarchy schemas involved. That is, a granularity G??™ is connected to a granularity
G if their corresponding categories are pairwise connected in the corresponding
dimension.
The cube dependence graph is a useful representation for the system and user to
navigate throughout aggregates at different granularities (Harinarayan et al., 1996).
The cube dependence graph also generalizes structures to visualize aggregate views
proposed for statistical databases, such as in the statistical object representation
model by Rafanelli and Shoshani (1990). As an example, Figure 6 shows a cube
dependence graph for the product dimension of Figure 1 and a time dimension with
categories Day, Year, and All.
Handling Structural Heterogeneity in OLAP
Copyright ?© 2007, Idea Group Inc. Copying or distributing in print or electronic forms without written permission
of Idea Group Inc. is prohibited.
A fundamental property of a cube dependence graph in traditional OLAP models
(homogeneous dimensions) is that the rollup operations are correct in the following
sense: if a granularity G reaches G??™, then the following holds: CV[G??™]= ROLLUP
G TO G??™.
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