Mapping strategy for querying multiple object databases with a global object schema

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 for the users to retrieve data. The mapping information between the global and component schemas is important for the 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 among a virtual class and its constituent classes. In addition, a mapping graph is used to describe the mapping equation. According to the mapping information, the mechanism to process global queries is introduced. One processing unit is responsible to decompose the global query into the subqueries against the component databases. Moreover, to handle the effects of schema restructuring, preprocessing and postprocessing units are provided for each local DBMS. Finally, the results returned from component databases need to be integrated. The certain result and maybe result for a global query are discussed. Also, the situation where a real-world entity is represented in different component databases as different objects is considered for the result integration.