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Relative expressiveness of nested regular expressions
 Proceedings 6th Alberto Mendelzon International Workshop on Foundations of a b c d Management, volume 866 of CEUR Workshop Proceedings
, 2012
"... Abstract. Nested regular expressions (NREs) have been proposed as a powerful formalism for querying RDFS graphs, but not too much investigation on NREs has been pursued in a more general graph database context. In this paper we study the relative expressiveness of NREs by comparing it with the langu ..."
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Abstract. Nested regular expressions (NREs) have been proposed as a powerful formalism for querying RDFS graphs, but not too much investigation on NREs has been pursued in a more general graph database context. In this paper we study the relative expressiveness of NREs by comparing it with the language of conjunctive twoway regular path queries (C2RPQs), which is one of the most widely studied query languages for graph databases. Among other results, we show that NREs and C2RPQs are incomparable in terms of expressive power, but NREs properly extend the language of unions of acyclic C2RPQs. Even more, there is a natural fragment of NREs that coincide in expressive power with the class of unions of acyclic C2RPQs. Our results, plus previous results that show that NREs can be evaluated in linear time in combined complexity, put forward NREs as a query language for graphstructured data that deserves further attention. 1
Relative Expressive Power of Navigational Querying on Graphs
"... Motivated by both established and new applications, we study navigational query languages for graphs (binary relations). The simplest language has only the two operators union and composition, together with the identity relation. We make more powerful languages by adding any of the following operato ..."
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Motivated by both established and new applications, we study navigational query languages for graphs (binary relations). The simplest language has only the two operators union and composition, together with the identity relation. We make more powerful languages by adding any of the following operators: intersection; set difference; projection; coprojection; converse; transitive closure; and the diversity relation. All these operators map binary relations to binary relations. We compare the expressive power of all resulting languages. We do this not only for general path queries (queries where the result may be any binary relation) but also for boolean or yes/no queries (expressed by the nonemptiness of an expression). For both cases, we present the complete Hasse diagram of relative expressiveness. In particular, the Hasse diagram for boolean queries contains nontrivial separations and a few surprising collapses.
Viewbased query answering over Description Logic Ontologies
, 2008
"... Viewbased query answering is the problem of answering a query based only on the answers precomputed for a set of views. While this problem has been widely investigated in databases, it is largely unexplored in the context of Description Logic ontologies. Differently from traditional databases, Desc ..."
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Viewbased query answering is the problem of answering a query based only on the answers precomputed for a set of views. While this problem has been widely investigated in databases, it is largely unexplored in the context of Description Logic ontologies. Differently from traditional databases, Description Logics may express several forms of incomplete information, and this poses challenging problems in characterizing the semantics of views. In this paper, we first present a general framework for viewbased query answering, where we address the above semantical problems by defining a spectrum of notions of viewbased query answering over ontologies, all based on the idea that the precomputed answers to views are the certain answers to the corresponding queries. We also relate such notions to relevant issues in ontology management, in particular ontology access authorization. Then, we provide decidability results, algorithms, and data complexity characterizations for viewbased query answering in several Description Logics, ranging from the DLLite family to very expressive Description Logics.
Querying Graph Patterns
"... Graph data appears in a variety of application domains, and many uses of it, such as querying, matching, and transforming data, naturally result in incompletely specified graph data, i.e., graph patterns. While queries need to be posed against such data, techniques for querying patterns are generall ..."
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Graph data appears in a variety of application domains, and many uses of it, such as querying, matching, and transforming data, naturally result in incompletely specified graph data, i.e., graph patterns. While queries need to be posed against such data, techniques for querying patterns are generally lacking, and properties of such queries are not well understood. Our goal is to study the basics of querying graph patterns. We first identify key features of patterns, such as node and label variables and edges specified by regular expressions, and define a classification of patterns based on them. We then study standard graph queries on graph patterns, and give precise characterizations of both data and combined complexity for each class of patterns. If complexity is high, we do further analysis of features that lead to intractability, as well as lowercomplexity restrictions. We introduce a new automata model for query answering with two modes of acceptance: one captures queries returning nodes, and the other queries returning paths. We study properties of such automata, and the key computational tasks associated with them. Finally, we provide additional restrictions for tractability, and show that some intractable cases can be naturally cast as instances of constraint satisfaction problem.
Graph Logics with Rational Relations and the Generalized Intersection Problem
"... Abstract—We investigate some basic questions about the interaction of regular and rational relations on words. The primary motivation comes from the study of logics for querying graph topology, which have recently found numerous applications. Such logics use conditions on paths expressed by regular ..."
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Abstract—We investigate some basic questions about the interaction of regular and rational relations on words. The primary motivation comes from the study of logics for querying graph topology, which have recently found numerous applications. Such logics use conditions on paths expressed by regular languages and relations, but they often need to be extended by rational relations such as subword (factor) or subsequence. Evaluating formulae in such extended graph logics boils down to checking nonemptiness of the intersection of rational relations with regular or recognizable relations (or, more generally, to the generalized intersection problem, asking whether some projections of a regular relation have a nonempty intersection with a given rational relation). We prove that for several basic and commonly used rational relations, the intersection problem with regular relations is either undecidable (e.g., for subword or suffix, and some generalizations), or decidable with nonmultiplyrecursive complexity (e.g., for subsequence and its generalizations). These results are used to rule out many classes of graph logics that freely combine regular and rational relations, as well as to provide the simplest problem related to verifying lossy channel systems that has nonmultiplyrecursive complexity. We then prove a dichotomy result for logics combining regular conditions on individual paths and rational relations on paths, by showing that the syntactic form of formulae classifies them into either efficiently checkable or undecidable cases. We also give examples of rational relations for which such logics are decidable even without syntactic restrictions. I.
Determining Relevance of Accesses at Runtime
"... Consider the situation where a query is to be answered using Web sources that restrict the accesses that can be made on backend relational data by requiring some attributes to be given as input of the service. The accesses provide lookups on the collection of attributes values that match the binding ..."
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Cited by 13 (8 self)
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Consider the situation where a query is to be answered using Web sources that restrict the accesses that can be made on backend relational data by requiring some attributes to be given as input of the service. The accesses provide lookups on the collection of attributes values that match the binding. They can differ in whether or not they require arguments to be generated from prior accesses. Prior work has focused on the question of whether a query can be answered using a set of data sources, and in developing static access plans (e.g., Datalog programs) that implement query answering. We are interested in dynamic aspects of the query answering problem: given partial information about the data, which accesses could provide relevant data for answering a given query? We consider immediate and longterm notions of “relevant accesses”, and ascertain the complexity of query relevance, for both conjunctive queries and arbitrary positive queries. In the process, we relate dynamic relevance of an access to query containment under access limitations and characterize the complexity of this problem; we produce several complexity results about containment that are of interest by themselves.
Monadic Datalog Containment
"... Abstract. We reconsider the problem of containment of monadic datalog (MDL) queries in unions of conjunctive queries (UCQs). Prior work has dealt with special cases, but has left the precise complexity characterization open. We begin by establishing a 2EXPTIME lower bound on the MDL/UCQ containment ..."
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Cited by 11 (5 self)
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Abstract. We reconsider the problem of containment of monadic datalog (MDL) queries in unions of conjunctive queries (UCQs). Prior work has dealt with special cases, but has left the precise complexity characterization open. We begin by establishing a 2EXPTIME lower bound on the MDL/UCQ containment problem, resolving an open problem from the early 90’s. We then present a general approach for getting tighter bounds on the complexity, based on analysis of the number of mappings of queries into treelike instances. We use the machinery to present an important case of the MDL/UCQ containment problem that is in coNEXPTIME, and a case that is in EXPTIME. We then show that the technique can be used to get a new tight upper bound for containment of tree automata in UCQs. We show that the new MDL/UCQ upper bounds are tight. 1
Simplifying Schema Mappings
"... A schema mapping is a formal specification of the relationship holding between the databases conforming to two given schemas, called source and target, respectively. While in the general case a schema mapping is specified in terms of assertions relating two queries in some given language, various si ..."
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A schema mapping is a formal specification of the relationship holding between the databases conforming to two given schemas, called source and target, respectively. While in the general case a schema mapping is specified in terms of assertions relating two queries in some given language, various simplified forms of mappings, in particular LAV and GAV, have been considered, based on desirable properties that these forms enjoy. Recent works propose methods for transforming schema mappings to logically equivalent ones of a simplified form. In many cases, this transformation is impossible, and one might be interested in finding simplifications based on a weaker notion, namely logical implication, rather than equivalence. More precisely, given a schema mapping M, find a simplified (LAV, or GAV) schema mapping M ′ such that M ′ logically implies M. In this paper we formally introduce this problem, and study it in a variety of cases, providing techniques and complexity bounds. The various cases we consider depend on three parameters: the simplified form to achieve (LAV, or GAV), the type of schema mapping considered (sound, or exact), and the query language used in the schema mapping specification (conjunctive queries and variants over relational databases, or regular path queries and variants over graph databases). Notably, this is the first work on comparing schema mappings for graph databases. Categories and Subject Descriptors D.2.12 [Software Engineering]: Interoperability—data mapping; H.2.3 [Database Management]: Languages—
Nested regular path queries in description logics
 In Proc. of KR 2014
, 2014
"... Both in knowledge representation and in databases, there has been great interest recently in expressive mechanisms for querying data, while taking into account complex domain knowledge [9]. Description Logics (DLs), which on the one hand underlie the W3C standard Web Ontology Language (OWL), and on ..."
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Cited by 9 (2 self)
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Both in knowledge representation and in databases, there has been great interest recently in expressive mechanisms for querying data, while taking into account complex domain knowledge [9]. Description Logics (DLs), which on the one hand underlie the W3C standard Web Ontology Language (OWL), and on the other hand are able to capture at
Reasoning and Query Answering in Description Logics”, Reasoning Web 2012, Volume 7487
 of Lecture Notes in Computer Science
, 2012
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