(full version, including appendices, of the PODS’07 paper)

XPath is a simple language for navigating an XML-tree and returning a set of answer nodes. The focus in this paper is on the complexity of the containment problem for various fragments of XPath. We restrict attention to the most common XPath expressions which navigate along the child and/or descendant axis. In addition to basic expressions using only node tests and simple predicates, we also consider disjunction and variables (ranging over nodes). Further, we investigate the containment problem relative to a given DTD. With respect to variables we study two semantics, (1) the original semantics of XPath, where the values of variables are given by an outer context, and (2) an existential semantics introduced by Deutsch and Tannen, in which the values of variables are existentially quantified. In this framework, we establish an exact classification of the complexity of the containment problem for many XPath fragments. 1

Document-centric XML is a mixture of text and structure. With the increased availability of document-centric XML documents comes a need for query facilities in which both structural constraints and constraints on the content of the documents can be expressed. How does the expressiveness of languages for querying XML documents help users to express their information needs? We address this question from both an experimental and a theoretical point of view. Our experimental analysis compares a structure-ignorant with a structure-aware retrieval approach using the test suite of the INEX XML retrieval evaluation initiative. Theoretically, we create two mathematical models of users ’ knowledge of a set of documents and define query languages which exactly fit these models. One of these languages corresponds to an XML version of fielded search, the other to the INEX query language. Our main experimental findings are: First, while structure is used in varying degrees of complexity, two thirds of the queries can be expressed in a fielded-search like format which does not use the hierarchical structure of the documents. Second, three quarters of the queries use constraints on the context of the elements to be returned; these contextual constraints cannot be captured by ordinary keyword queries. Third, structure is used as a search hint, and not as a strict requirement, when judged against the underlying information need. Fourth, the use of structure in queries functions as a precision enhancing device.

We consider the navigational core of XPath, extended with two operators: the Kleene star for taking the transitive closure of path expressions, and a subtree relativisation operator, allowing one to restrict attention to a specific subtree while evaluating a subexpression. We show that the expressive power of this XPath dialect equals that of FO(MTC), first order logic extended with monadic transitive closure. We also give a characterization in terms of nested tree-walking automata. Using the latter we then proceed to show that the language is strictly less expressive than MSO. This solves an open question about the relative expressive power of FO(MTC) and MSO on trees. We also investigate the complexity for our XPath dialect. We show that query evaluation be done in polynomial time (combined complexity), but that satisfiability and query containment (as well as emptiness for our automaton model) are 2ExpTime-complete (it is ExpTime-complete for Core XPath).

Nested words are a structured model of execution paths in procedural programs, reflecting their call and return nesting structure. Finite nested words also capture the structure of parse trees and other tree-structured data, such as XML. We provide new temporal logics for finite and infinite nested words, which are natural extensions of LTL, and prove that these logics are first-order expressivelycomplete. One of them is based on adding a ”within” modality, evaluating a formula on a subword, to a logic CaRet previously studied in the context of verifying properties of recursive state machines. The other logic is based on the notion of a summary path that combines the linear and nesting structures. For that logic, both model-checking and satisfiability are shown to be EXPTIME-complete. Finally, we prove that first-order logic over nested words has the three-variable property, and we present a temporal logic for nested words which is complete for the twovariable fragment of first-order. 1

We survey expressivity results for navigational fragments of XPath 1.0 and 2.0, as well as Regular XPath ≈. We also investigate algebras for these fragments.

We show how Recursive Markov Chains (RMCs) and their restrictions can define probabilistic distributions over XML documents, and study tractability

Meeting notes are documents which contain lots of structure. This structure is often implicit in layout and reserved words. On the other hand, since meetings tend to occur regularly and are repeated for long periods of time, this structure is often (semi-)formalized. This makes these documents suitable for automatic semantic annotation efforts. We describe the annotation we performed on the notes of more than 20 years of Dutch parliamentary debates. We annotated every word spoken in parliament with 1) the speaker, 2) her party at the time of speaking, 3) her role/function in parliament and 4) the iso-date. These annotations yield numerous new ways of searching, browsing, mining and summarizing these documents. Meetings are always too long, whence so are their verbatim notes. But of course they contain valuable information and notes have to be consulted from time to time. In this paper we show that semantic annotation can make finding things easier, and more fun. 1.

We study fragments of first-order logic and of least fixed point logic that allow only unary negation: negation of formulas with at most one free variable. These logics generalize many interesting known formalisms, including modal logic and the µ-calculus, as well as conjunctive queries and monadic Datalog. We show that satisfiability and finite satisfiability are decidable for both fragments, and we pinpoint the complexity of satisfiability, finite satisfiability, and model checking. We also show that the unary negation fragment of first-order logic is model-theoretically very well behaved. In particular, it enjoys Craig interpolation and the Beth property.

We study the expressiveness of Positive XPath with parent/child navigation, denoted XPath +, from two angles. First, we establish that XPath + is equivalent in expressive power to some of its sub-fragments as well as to the class of tree queries, a sub-class of the first-order conjunctive queries defined over label, parent, and child predicates. The translation algorithm from tree queries to XPath + yields a simple normal form for XPath + queries. Using this normal form, we can effectively partition an XPath + query into subqueries that can be expressed in a very small sub-fragment of XPath + for which efficient evaluation strategies are available. Second, we characterize the expressiveness of XPath + in terms of its ability to distinguish nodes in a document. We show that two such nodes cannot be distinguished if and only if the paths from the root of the documents to these nodes have equal length and corresponding nodes on these paths are bisimilar.