The standard approach for optimization of XPath queries by rewriting queries using views techniques consists in navigating inside a view's output, thus allowing the usage of only one view in the rewritten query. Richer classes of rewritings, using intersection or joins on node identifiers, have been proposed, but they either lack completeness guarantees, or require additional information about the data. Our work is the first to characterize the problem of rewriting XPath queries using an intersection of views, by identifying the tightest restrictions under which the problem can be solved in polynomial time. As an additional contribution, we characterize the related problem of deciding whether an intersection of XPath expressions can be equivalently expressed as only one XPath.

Pre-2018 CSE ID: CS2008-0920

We study the problem of querying data sources that accept only a limited set of queries, such as sources accessible by Web services which can implement very large (potentially infinite) families of queries. We revisit a classical setting in which the application queries are conjunctive queries and the source accepts families of (possibly parameterized) conjunctive queries specified as the expansions of a (potentially recursive) Datalog program with parameters. We say that query Q is expressible by the program P if it is equivalent to some expansion of P. Q is supported by P if it has an equivalent rewriting using some finite set of P's expansions. We present the first study of expressibility and support for sources that satisfy integrity constraints, which is generally the case in practice. We start by closing a problem left open by prior work even while ignoring constraints, namely the precise relationship between expressibility and support: surprisingly, we show them to be inter-reducible in PTIME in both the absence and the presence of constraints. This enables us to employ the same algorithm for solving both problems. We identify practically relevant restrictions on the program specifying the views that ensure decidability under a mix of key and weakly acyclic foreign key constraints, and beyond. We show that these restrictions are as permissive as possible, since their slightest relaxation leads to undecidability. We present an algorithm that is guaranteed to be sound when applied to unrestricted input and in addition complete under the restrictions. As a side-effect of our investigation, we improve the previously best known upper bound for deciding support in the constraint-free case.

Pre-2018 CSE ID: CS2007-0886

We study the problem of querying XML data sources that accept only a limited set of queries, such as sources accessible by Web services which can implement very large (potentially infinite) families of XPath queries. To compactly specify such families of queries we adopt the Query Set Specifications~\cite{PetropoulosDP03}, a formalism close to context-free grammars. We say that query $Q$ is {\em expressible} by the specification \calP\ if it is equivalent to some expansion of \calP. $Q$ is {\em supported} by \calP\ if it has an equivalent rewriting using some finite set of \calP's expansions. We study the complexity of expressibility and support and identify large classes of XPath queries for which there are efficient (PTIME) algorithms. Our study considers both the case in which the XML nodes in the results of the queries lose their original identity and the one in which the source exposes persistent node ids.

Pre-2018 CSE ID: CS2009-0941