Efficient Query Processing in an Ontology-Based Mediation System

João Carlos PinheiroVânia Maria Ponte Vidal

Ontologies have been extensively used to model domain­specific knowledge. Recent research has used ontologies for specifying the mediated schema in the context of data integration. In this work we propose an ontology-based approach for integration of XML data sources. In our approach, the mediated schema is represented by a domain ontology, which provides a conceptual representation of the application. Each local source is described by an application ontology, whose vocabulary is restricted to be a subset of the vocabulary of domain ontology. Each application ontology is translated into an XML schema, which constitutes the schema of the XML view exported by the application ontology's local source. Furthermore, mediated mappings define concepts and properties of the domain ontology in terms of concepts and properties of the application ontologies. Local mappings specify the correspondences between each XML view schema and its local source schema. In our approach, the process of answering a query posed on the mediated view consists of four steps: (i) Semantic rewriting. Based on the mediated mappings, the initial query is decomposed into a set of elementary sub­queries over the application ontologies, and it is generated the semantic execution plan (SEP), which specifies how the results for the sub­ queries are combined to the final result. (ii) XML translation. The sub­queries resulting from the previous step are translated into queries over the XML views, and the SEP is translated to an XML algebra expression. (iii) Optimization. This step tries to find a near to optimal final execution plan (FEP). (iv) Evaluation. The sub­queries over the XML views are rewritten in terms of their local source schemas, with the help of the local mappings. The results of the XML views sub­queries are returned to the mediator, where the final result is built according to the final execution plan.

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