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41
XPath satisfiability in the presence of DTDs
 In PODS ’05: Proceedings of the twentyfourth ACM Symposium on Principles of Database Systems
, 2005
"... We study the satisfiability problem associated with XPath in the presence of DTDs. This is the problem of determining, given a query p in an XPath fragment and a DTD D, whether or not there exists an XML document T such that T conforms to D and the answer of p on T is nonempty. We consider a variety ..."
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Cited by 107 (6 self)
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We study the satisfiability problem associated with XPath in the presence of DTDs. This is the problem of determining, given a query p in an XPath fragment and a DTD D, whether or not there exists an XML document T such that T conforms to D and the answer of p on T is nonempty. We consider a variety of XPath fragments widely used in practice, and investigate the impact of different XPath operators on satisfiability analysis. We first study the problem for negationfree XPath fragments with and without upward axes, recursion and datavalue joins, identifying which factors lead to tractability and which to NPcompleteness. We then turn to fragments with negation but without data values, establishing lower and upper bounds in the absence and in the presence of upward modalities and recursion. We show that with negation the complexity ranges from PSPACE to EXPTIME. Moreover, when both data values and negation are in place, we find that the complexity ranges from NEXPTIME to undecidable. Finally, we give a finer analysis of the problem for particular classes of DTDs, exploring the impact of various DTD constructs, identifying tractable cases, as well as providing the complexity in the query size alone. 1.
Conditional XPath, the first order complete XPath dialect
, 2004
"... XPath is the W3Cstandard node addressing language for XML documents. XPath is still under development and its technical aspects are intensively studied. What is missing at present is a clear characterization of the expressive power of XPath, be it either semantical or with reference to some well e ..."
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Cited by 52 (5 self)
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XPath is the W3Cstandard node addressing language for XML documents. XPath is still under development and its technical aspects are intensively studied. What is missing at present is a clear characterization of the expressive power of XPath, be it either semantical or with reference to some well established existing (logical) formalism. Core XPath (the logical core of XPath 1.0 defined by Gottlob et al.) cannot express queries with conditional paths as exemplified by "do a child step, while test is true at the resulting node." In a firstorder complete extension of Core XPath, such queries are expressible. We add conditional axis relations to Core XPath and show that the resulting language, called conditional XPath, is equally expressive as firstorder logic when interpreted on ordered trees. Both the result, the extended XPath language, and the proof are closely related to temporal logic. Specifically, while Core XPath may be viewed as a simple temporal logic, conditional XPath extends this with (counterparts of) the since and until operators.
Conditional XPath
 ACM Trans. Database Syst
, 2005
"... Abstract. XPath 1.0 is a variable free language designed to specify paths between nodes in XML documents. Such paths can alternatively be specified in firstorder logic. The logical abstraction of XPath 1.0, usually called Navigational or Core XPath, is not powerful enough to express every firstord ..."
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Cited by 45 (4 self)
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Abstract. XPath 1.0 is a variable free language designed to specify paths between nodes in XML documents. Such paths can alternatively be specified in firstorder logic. The logical abstraction of XPath 1.0, usually called Navigational or Core XPath, is not powerful enough to express every firstorder definable path. In this paper we show that there exists a natural expansion of Core XPath in which every firstorder definable path in XML document trees is expressible. This expansion is called Conditional XPath. It contains additional axis relations of the form (child::n[F])+, denoting the transitive closure of the path expressed by child::n[F]. The difference with XPath’s descendant::n[F] is that the path (child::n[F])+ is conditional on the fact that all nodes in between should be labeled by n and should make the predicate F true. This result can be viewed as the XPath analogue of the expressive completeness of the relational algebra with respect to firstorder logic. 1
Specifying Access Control Policies for XML Documents with XPath
, 2004
"... Access control for XML documents is a nontrivial topic, as can be witnessed from the number of approaches presented in the literature. Trying to compare these, we discovered the need for a simple, clear and unambiguous language to state the declarative semantics of an access control policy. All cur ..."
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Cited by 38 (5 self)
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Access control for XML documents is a nontrivial topic, as can be witnessed from the number of approaches presented in the literature. Trying to compare these, we discovered the need for a simple, clear and unambiguous language to state the declarative semantics of an access control policy. All current approaches state the semantics in natural language, which has none of the above properties. This makes it hard to assess whether the proposed algorithms are correct (i.e., really implement the described semantics). It is also hard to assess the proposed policy on its merits, and to compare it to others (for file systems for instance).
First order paths in ordered trees
 In ICDT’05
, 2005
"... Abstract. We give two sufficient conditions on XPath like languages for having first order expressivity, meaning that every first order definable set of paths in an ordered nodelabeled tree is definable in that XPath language. They are phrased in terms of expansions of navigational (sometimes calle ..."
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Cited by 36 (5 self)
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Abstract. We give two sufficient conditions on XPath like languages for having first order expressivity, meaning that every first order definable set of paths in an ordered nodelabeled tree is definable in that XPath language. They are phrased in terms of expansions of navigational (sometimes called “Core”) XPath. Adding either complementation, or the more elegant conditional paths is sufficient. A conditional path is an axis relation of the form (one step axis::n[F]) +, denoting the transitive closure of the relation expressed by one step axis::n[F]. As neither is expressible in navigational XPath we also give characterizations in terms of first order logic of the answer sets and the sets of paths navigational XPath can define. The first in terms of a suitable two variable fragment, the second in terms of unions of conjunctive queries. 1
Xpath leashed
 IN ACM COMPUTING SURVEYS
, 2007
"... This survey gives an overview of formal results on the XML query language XPath. We identify several important fragments of XPath, focusing on subsets of XPath 1.0. We then give results on the expressiveness of XPath and its fragments compared to other formalisms for querying trees, algorithms and c ..."
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Cited by 36 (3 self)
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This survey gives an overview of formal results on the XML query language XPath. We identify several important fragments of XPath, focusing on subsets of XPath 1.0. We then give results on the expressiveness of XPath and its fragments compared to other formalisms for querying trees, algorithms and complexity bounds for evaluation of XPath queries, and static analysis of XPath queries.
On the complexity of XPath containment in the presence of disjunction, DTDs, and variables
 Logical Methods in Computer Science
"... Vol. 2 (3:1) 2006, pp. 1–1–30 www.lmcsonline.org ..."
Rewriting Regular XPath Queries on XML Views
 In Proc. ICDE
, 2007
"... We study the problem of answering queries posed on virtual views of XML documents, a problem commonly encountered when enforcing XML access control and integrating data. We approach the problem by rewriting queries on views into equivalent queries on the underlying document, and thus avoid the overh ..."
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Cited by 29 (3 self)
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We study the problem of answering queries posed on virtual views of XML documents, a problem commonly encountered when enforcing XML access control and integrating data. We approach the problem by rewriting queries on views into equivalent queries on the underlying document, and thus avoid the overhead of view materialization and maintenance. We consider possibly recursively defined XML views and study the rewriting of both XPath and regular XPath queries. We show that while rewriting is not always possible for XPath over recursive views, it is for regular XPath; however, the rewritten query may be of exponential size. To avoid this prohibitive cost we propose a rewriting algorithm that characterizes rewritten queries as a new form of automata, and an efficient algorithm to evaluate the automatonrepresented queries. These allow us to answer queries on views in linear time. We have fully implemented a prototype system, SMOQE, which yields the first regular XPath engine and a practical solution for answering queries over possibly recursively defined XML views. 1.
Complexity of decision problems for simple regular expressions
 IN PROCEEDINGS OF THE 29TH INTERNATIONAL SYMPOSIUM ON MATHEMATICAL FOUNDATIONS OF COMPUTER SCIENCE (MFCS 2004
, 2004
"... We study the complexity of the inclusion, equivalence, and intersection problem for simple regular expressions arising in practical XML schemas. These basically consist of the concatenation of factors where each factor is a disjunction of strings possibly extended with ‘∗ ’ or ‘?’. We obtain lower ..."
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Cited by 27 (12 self)
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We study the complexity of the inclusion, equivalence, and intersection problem for simple regular expressions arising in practical XML schemas. These basically consist of the concatenation of factors where each factor is a disjunction of strings possibly extended with ‘∗ ’ or ‘?’. We obtain lower and upper bounds for various fragments of simple regular expressions. Although we show that inclusion and intersection are already intractable for very weak expressions, we also identify some tractable cases. For equivalence, we only prove an initial tractability result leaving the complexity of more general cases open. The main motivation for this research comes from database theory, or more specifically XML and semistructured data. We namely show that all lower and upper bounds for inclusion and equivalence, carry over to the corresponding decision problems for extended contextfree grammars and singletype tree grammars, which are abstractions of DTDs and XML Schemas, respectively. For intersection, we show that the complexity only carries over for DTDs.
The complexity of query containment in expressive fragments of XPath 2.0
 In Proc. PODS’07
, 2007
"... (full version, including appendices, of the PODS’07 paper) ..."
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Cited by 25 (5 self)
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(full version, including appendices, of the PODS’07 paper)