1,720,974 research outputs found
The semijoin algebra
No full textIn the 1970s Codd introduced the now standard relational data model, in which a database is a finite collection of relations, where a relation is a finite set of tuples. To express queries in the relational model, Codd introduced the relational algebra (RA) with operators selection (called restriction by Codd), projection, union, difference and join [14]. Since then the relational algebra has been extensively studied [1]. A very important result is that its expressive power is equivalent to the expressive power of f irst-order logic, called relational calculus in database theory [15]. The “semijoin” operator, which is non-primitive in Codd’s relational algebra, selects a set of tuples in one relation that have a joining tuple in another relation. The semijoin operator has also been extensively studied in the past. For example, while computing project-join queries in general is NP-complete in the size of the query and the database, this can be done in polynomial time when the database schema is acyclic [61], a property known to be equivalent to the existence of a semijoin program [11, 13, 12]. Semijoins are often used as part of a query pre-processing phase where dangling tuples are eliminated, i.e., the database is resized to the part that is relevant for answering the query. Another interesting property is that the size of a relation resulting from a semijoin is always linear in the size of the input. Therefore, a query processor will try to use semijoins as often as possible when generating a query plan for a given query (a technique known as “pushing projections” [19]). Also in distributed query processing, semijoins have great importance, because when a database is distributed across several sites, they can help avoid the shipment of many unneeded tuples
The semijoin algebra
No full textIn the 1970s Codd introduced the now standard relational data model, in which a database is a finite collection of relations, where a relation is a finite set of tuples. To express queries in the relational model, Codd introduced the relational algebra (RA) with operators selection (called restriction by Codd), projection, union, difference and join [14]. Since then the relational algebra has been extensively studied [1]. A very important result is that its expressive power is equivalent to the expressive power of f irst-order logic, called relational calculus in database theory [15]. The “semijoin” operator, which is non-primitive in Codd’s relational algebra, selects a set of tuples in one relation that have a joining tuple in another relation. The semijoin operator has also been extensively studied in the past. For example, while computing project-join queries in general is NP-complete in the size of the query and the database, this can be done in polynomial time when the database schema is acyclic [61], a property known to be equivalent to the existence of a semijoin program [11, 13, 12]. Semijoins are often used as part of a query pre-processing phase where dangling tuples are eliminated, i.e., the database is resized to the part that is relevant for answering the query. Another interesting property is that the size of a relation resulting from a semijoin is always linear in the size of the input. Therefore, a query processor will try to use semijoins as often as possible when generating a query plan for a given query (a technique known as “pushing projections” [19]). Also in distributed query processing, semijoins have great importance, because when a database is distributed across several sites, they can help avoid the shipment of many unneeded tuples
Implementing Widely-used Vocabularies to Produce Linked Open Data in the Context of Open Repositories
Full text linkPresentation at Open Repositories 2014, Helsinki, Finland, June 9-13, 2014EPrints Interest Group PresentationsThe Food and Agriculture Organization (FAO) of the United Nations works to provide support to agricultural information communities to build and maintain open repositories that meet recommended metadata standards and controlled vocabularies. To this end, FAO set up partnerships with organizations working in this field. The most recent work focus on the implementation of an authority tool in the submission process of Eprints Software . Its purpose is to facilitate the use of controlled vocabularies published as Linked Open Data, and the exposure of data on the Semantic Web . This project has been technically implemented by the University of Southampton in partnership with FAO, UNESCO-IOC/IODE and Hasselt University Library .Halabi, Nawar (University of Southampton)Leinders, Dirk (Hasselt University Library)Goovaerts, Marc (Hasselt University Library)Well, Andrew (University of Southampton)Subirats Coll, Imma (FAO of the United Nations, Italy
AOD 2.0: A category based submission add-on for DSpace 4.0
Full text linkPresentation at Open Repositories 2014, Helsinki, Finland, June 9-13, 2014DSpace Interest Group PresentationsThe Institute of Biology of the Southern Seas, Ukraine (IBSS) and Hasselt University (Belgium) are developing with the support of FAO of the United Nations a new version of AgriOcean DSpace named AOD 2.0, as an add-on for DSpace 4.0 and later. It works further on the specifications of AgriOcean DSpace 1.x including the type-based submission module, the use of controlled vocabularies (ASFA, AGROVOC) and an easy-to-install version for Windows. While AgriOcean DSpace 1.x was created as a fully customized version of DSpace 1.7.2, AOD 2.0 is developed with a modular approach. Different stand-alone modules are developed and integrated with DSpace 4.0: Ontology Plug-in, Category-based Submission, Batch Import for Submitter. They are made available as AOD 2.0, an easy-to-install package for Linux and Windows.Goovaerts, Marc (Hasselt University, Belgium)Slipetskyy, Denys (Institute of Biology of the Southern Seas)Subirats, Imma (Food and Agriculture Organizations of the United Nations)Keizer, Johannes (Food and Agriculture Organizations of the United Nations)Leinders, Dirk (Hasselt University, Belgium)Dupriez, Christophe (Destin)Surinx, Dimitri (Hasselt University, Belgium)Gybels, Michelle (Hasselt University, Belgium)Vaelen, Jeroen (Hasselt University, Belgium)Vandesbosch, Niki (Hasselt University, Belgium
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Full text link“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
Full text linkWe 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
Full text linkWe 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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