1,721,135 research outputs found
Materialisation and data partitioning algorithms for distributed RDF systems
Get PDFMany RDF systems support reasoning with Datalog rules via materialisation, where all conclusions of RDF data and the rules are precomputed and explicitly stored in a preprocessing step. As the amount of RDF data used in applications keeps increasing, processing large datasets often requires distributing the data in a cluster of shared-nothing servers. Whereas numerous distributed query answering techniques are known, distributed materialisation is less well understood. In this paper, we present several techniques that facilitate scalable materialisation in distributed RDF systems. First, we present a new distributed materialisation algorithm that aims to minimise communication and synchronisation in the cluster. Second, we present two new algorithms for partitioning RDF data, both of which aim to produce tightly connected partitions, but without loading complete datasets into memory. We evaluate our materialisation algorithm against two state-of-the- art distributed Datalog systems and show that our technique offers competitive performance, particularly when the rules are complex. Moreover, we analyse in depth the effects of data partitioning on reasoning performance and show that our techniques offer performance comparable or superior to the state of the art min-cut partitioning, but computing the partitions requires considerably fewer resources
Scalable reasoning for description logics
Get PDFDescription logics (DLs) are knowledge representation formalisms with well-understood model-theoretic semantics and computational properties. The DL SROIQ provides the logical underpinning for the semantic web language OWL 2, which is quickly becoming the standard for knowledge representation on the web.A central component of most DL applications is an efficient and scalable reasoner, which provides services such as consistency testing and classification. Despite major advances in DL reasoning algorithms over the last decade, however, ontologies are still encountered in practice that cannot be handled by existing DL reasoners. We present a novel reasoning calculus for the description logic SROIQ which addresses two of the major sources of inefficiency present in the tableau-based reasoning calculi used in state-of-the-art reasoners: unnecessary nondeterminism and unnecessarily large model sizes. Further, we describe a new approach to classification which exploits partial information about the subsumption relation between concept names to reduce both the number of individual subsumption tests performed and the cost of working with large ontologies; our algorithm is applicable to the general problem of deducing a quasi-ordering from a sequence of binary comparisons. We also present techniques for extracting partial information about the subsumption relation from the models generated by constructive DL reasoning methods, such as our hypertableau calculus.Empirical results from a prototypical implementation demonstrate substantial performance improvements compared to existing algorithms and implementations
Tractable query answering for description logics via query rewriting
Get PDFWe consider the problem of answering conjunctive queries over description logic knowledge bases via query rewriting. Given a conjunctive query Q and a TBox T, we compute a new query Q′ that incorporates the semantic consequences of T such that, for any ABox A, evaluating Q over T and A can be done by evaluating the new query Q′ over A alone. We present RQR—a novel resolution-based rewriting algorithm for the description logic ELHIO¬ that generalizes and extends existing approaches. RQR not only handles a spectrum of logics ranging from DL-Lite_core up to ELHIO¬, but it is worst-case optimal with respect to data complexity for all of these logics; moreover, given the form of the rewritten queries, their evaluation can be delegated to off-the-shelf (deductive) database systems. We use RQR to derive the novel complexity results that conjunctive query answering for ELHIO¬ and DL-Lite+ are, respectively, PTime and NLogSpace complete with respect to data complexity. In order to show the practicality of our approach, we present the results of an empirical evaluation. Our evaluation suggests that RQR, enhanced with various straightforward optimizations, can be successfully used in conjunction with a (deductive) database system in order to answer queries over knowledge bases in practice. Moreover, in spite of being a more general procedure, RQR will often produce significantly smaller rewritings than the standard query rewriting algorithm for the DL-Lite family of logics
Query answering in distributed RDF databases
Get PDFTo simplify data integration and exchange, modern applications often represent their data using the Resource Description Framework (RDF). As the amount of the available data keeps increasing, many RDF datasets cannot be processed using centralised RDF stores. A common solution is to distribute RDF data in a cluster of shared-nothing servers, and to query the data using a distributed query algorithm. Existing approaches typically use a variant of the data exchange operator to shuffle partial query answers between servers and thus ensure that every query answer is produced. Decisions as to when and where to shuffle the data are usually made statically—that is, at query compile time. In this thesis, we argue that such approaches can miss opportunities for local computation and thus incur considerable overheads. Moreover, we present a novel distributed query evaluation algorithm for RDF based on dynamic data exchange, where all computation that can be done locally is guaranteed to be performed on a single server. Our approach can successfully process any query even if the memory available at each server is bounded, and we argue that this is critical in distributed systems where intermediate results can easily exceed the capacity of each server. We also present a new query planning approach that balances the cost of communication against the cost of local processing at each server, as well as a new approach to partitioning RDF data that aims to increase locality in each server. We have implemented our approach in the well-known RDFox data store, and our empirical evaluation suggests that our techniques can outperform the state of the art by orders of magnitude in terms of query evaluation times, network communication, and memory use
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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