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    Rewriting XQL queries on XML repositories

    Wood, Peter (2000) Rewriting XQL queries on XML repositories. In: Lings, B. and Jeffery, K.G. (eds.) BNCOD 17: Advances in Databases. Lecture Notes in Computer Science 1832. Springer, pp. 209-226. ISBN 9783540677437.

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    XQL is one of the query languages proposed for querying XML documents on the world wide web. In this paper, we consider the logical rewriting of XQL query expressions in order to improve the efficiency of query evaluation when XML document type definitions (DTDs) are present. We first define three classes of constraint which can be derived from a given DTD. With a suitable representation of an XML repository R as a relational database D, it turns out that these DTD constraints correspond to tuple- and equality-generating dependencies which must hold on D. Next, we identify a subset of XQL queries on R which is equivalent to a class of conjunctive queries on D. Given a conjunctive query C equivalent to an XQL query Q, we then apply techniques from relational dependency theory to reduce the number of conjuncts in C, yielding query C′. Conjunctive query C′ can then be mapped back to an XQL query Q′ in which redundant filter subexpressions and unnecessary selections have been removed. Whether Q′ can be evaluated more efficiently than Q depends on whether or not appropriate indices exist in R. However, the techniques presented in this paper can provide a query optimizer with a set of equivalent XQL expressions from which to select the best. The representation of queries in relational form and of constraints as dependencies should also permit the application of these results to other XML query languages.


    Item Type: Book Section
    School: Birkbeck Faculties and Schools > Faculty of Science > School of Computing and Mathematical Sciences
    Depositing User: Sarah Hall
    Date Deposited: 18 Oct 2021 15:10
    Last Modified: 09 Aug 2023 12:52


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