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We address the problem of generating a mediated schema from a set of relational data source schemas and conjunctive queries that specify where those schemas overlap. Unlike past approaches that generate only the mediated schema, our algorithm also generates view definitions, i.e., source-to-mediated schema mappings.
Our main goal is to understand the requirements that a mediated schema and views should satisfy, such as completeness, preservation of overlapping information, normalization, and minimality. We show how these requirements influence the detailed structure of schemas and view definitions that are produced. We introduce a normal form for mediated schemas and view definitions, show how to generate them, and prove that schemas and views in this normal form satisfy our requirements.
The view definitions in our normal form use stylized GLAV mappings, for which query rewriting is easier than general GLAV mappings. We demonstrate the efficiency of query rewriting in a prototype implementation.