A METHODOLOGY FOR GLOBAL SCHEMA DESIGN.

Abstract

A global schema is an integrated view of heterogeneous databases used to support data sharing among independent, existing databases. Global schema design complexities arise from the volume of details, design choices, potential conflicts, and interdependencies among design choices. The methodology described provides a framework for efficient management of these critical dimensions in generating and evaluating alternative designs. The methodology contains three major steps. First, differences due to the varying local data models are resolved by converting each local schema to an equivalent schema in a unifying data model. Second, the entity types of the local schemas in the unifying model are grouped into clusters called common areas. All the entity types in a common area can possibly be merged via generalization. For each common area, semantic information is defined that drives the merging process. Third, each common area is integrated into the global schema by applying a set of generalization operators. Mapping rules are then defined to resolve differences in the representations of equivalent attributes. Th integration of the local schemas is based on equivalence assertions. Four types of attribute equivalences are defined: two attributes may be locally or globally equivalent, and they can be key or non-key. Strategies for handling each of these cases are proposed and evaluated. The global schema design methodology includes several algorithms which may assist a designer. One algorithm analyzes a set of equivalence assertions for consistency and completeness including resolution of transitively implied assertions. A second algorithm performs an interactive merge of a common area by presenting the possible generalization actions to the designer. It supports the theme that many generalization structures can be possible, and the appropriate structure often depends on designer preferences and application requirements. The methodology is evaluated for several cases involving real databases. The cases demonstrate the utility of the methodology in managing the details, considering many alternatives, and resolving conflicts. In addition, these cases demonstrate the need for a set of computer-aided tools; for even a relatively small case, the number of details and design choices can overwhelm a designer.