Query Reformulation/Rewriting

Using the source descriptions, user query written in terms of the mediated schema is reformulated into a query that refers directly to the schemas of the sources (but still in the data model of the mediated system). Goals to achieve:

• Semantic Correctness of the Reformulation: The answers obtained from the sources will be correct answers to the original query.
• Minimizing the Source Access: Sources that can not contribute any answer or partial answer to the query should not be accessed. In addition to avoiding access to redundant sources, we should reformulate the queries as specific as possible to each of the accessed sources to avoid redundant query evaluation.