An approach to querying multiple object databases

In a multidatabase system which consists of object databases, a global schema created by integrating schemas of the component databases provides a uniform interface and high level location transparency to help users retrieve data. The mapping between the global and component object schemas is complicated due to schema restructuring conducted to resolve various conflicts among component schemas before conducting schema integration. This mapping information is important for global query processing. In this paper, a mapping strategy is presented. A mapping equation is defined to denote the mappings for attributes and object instances between a virtual class and its constituent classes. In addition, a mapping graph is used to describe the mapping equation. Based on the mapping information, a mechanism for processing global queries in parallel is introduced. One processing unit is responsible for decomposing the global query into subqueries against the component databases. To handle the effects of schema restructuring, preprocessing and postprocessing units are also provided for each local DBMS. The results returned from component databases need to be integrated. The concept of object isomerism, where a real-world entity is represented by more than one object in different component databases, is considered for integrating query results.