Querying Schema Information

Schema queries can play an important role while retrieving information from multiple sources, for example, in query formulation and in query optimization. We identify four classes of schema queries that we have found useful while designing an application programming interface for frame representation systems (FRSs): taxonomic, frame structure, constraint and class comparison queries. We propose a scheme for direct support for these queries in a mediator language such as Object Query Language (OQL). 1 Introduction A system that answers queries by accessing information residing in multiple sources has to perform the following tasks. Given a query, it has to determine which data items should be retrieved from which source, retrieve those data items, process the results, and present them to the user. Many of these tasks are performed by a software component called Mediator [Wid92]. Mediator usually issues queries to several data sources. The implementation of a mediator is considerably ..