- Containment of Nested XML Queries

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This chapter discusses the theoretical aspects of the problem of query containment for XML queries with nesting. Query containment is the most fundamental relationship between a pair of database queries. It is an important problem in a wide variety of data management applications, including verification of integrity constraints, reasoning about contents of data sources in data integration, semantic caching, verification of knowledge bases, determining queries independent of updates, and most recently, in query reformulation for peer data management systems. The chapter presents a conjunctive XML queries (c-XQueries), and shows that containment is in polynomial time if the fanout is restricted to be one. For arbitrary fanout, containment is coNP-hard already for queries with nesting depth two, even if the query does not include variables in the return clauses. The analysis exhibits that for queries with fixed nesting depth, containment is coNP-complete.

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