Modeling and Querying Semi-Structured data

We extend the model for semi-structured data proposed in (BUN 97), whe re both databases and schemas are represented as graphs, with the possibility o f expressing different types of constraints on the nodes of the graphs. We discuss how the expr essive power of the constraint language may influence the complexity of checking subsumptio n between schemas, and devise a polynomial algorithm for an interesting class of constraints. We then set up a framework for defining queries which are used to select graphs from a da tabase. The proposed query language allows for expressing sophisticated fixpoint properties o graphs and can be regarded as a basic building block of full-featured languages. We show tha t reasoning tasks at the basis of query optimization, such as query-schema comparison, query containment, and query satisfiability, are decidable.

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