The Odyssey Approach for Optimizing Federated SPARQL Queries

Answering queries over a federation of SPARQL endpoints requires combining data from more than one data source. Optimizing queries in such scenarios is particularly challenging not only because of (i) the large variety of possible query execution plans that correctly answer the query but also because (ii) there is only limited access to statistics about schema and instance data of remote sources. To overcome these challenges, most federated query engines rely on heuristics to reduce the space of possible query execution plans or on dynamic programming strategies to produce optimal plans. Nevertheless, these plans may still exhibit a high number of intermediate results or high execution times because of heuristics and inaccurate cost estimations. In this paper, we present Odyssey, an approach that uses statistics that allow for a more accurate cost estimation for federated queries and therefore enables Odyssey to produce better query execution plans. Our experimental results show that Odyssey produces query execution plans that are better in terms of data transfer and execution time than state-of-the-art optimizers. Our experiments using the FedBench benchmark show execution time gains of at least 25 times on average.Comment: 16 pages, 10 figure

Semantics and Optimization of the SPARQL 1.1 Federation Extension

The W3C SPARQL working group is defining the new SPARQL 1.1 query language. The current working draft of SPARQL 1.1 focuses mainly on the description of the language. In this paper, we provide a formalization of the syntax and semantics of the SPARQL 1.1 federation extension, an important fragment of the language that has not yet received much attention. Besides, we propose optimization techniques for this fragment, provide an implementation of the fragment including these techniques, and carry out a series of experiments that show that our optimization procedures could significantly speed up the query evaluation process

Co-evolution of RDF Datasets

Linking Data initiatives have fostered the publication of large number of RDF datasets in the Linked Open Data (LOD) cloud, as well as the development of query processing infrastructures to access these data in a federated fashion. However, different experimental studies have shown that availability of LOD datasets cannot be always ensured, being RDF data replication required for envisioning reliable federated query frameworks. Albeit enhancing data availability, RDF data replication requires synchronization and conflict resolution when replicas and source datasets are allowed to change data over time, i.e., co-evolution management needs to be provided to ensure consistency. In this paper, we tackle the problem of RDF data co-evolution and devise an approach for conflict resolution during co-evolution of RDF datasets. Our proposed approach is property-oriented and allows for exploiting semantics about RDF properties during co-evolution management. The quality of our approach is empirically evaluated in different scenarios on the DBpedia-live dataset. Experimental results suggest that proposed proposed techniques have a positive impact on the quality of data in source datasets and replicas.Comment: 18 pages, 4 figures, Accepted in ICWE, 201

Federating queries in SPARQL 1.1: syntax, semantics and evaluation

Given the sustained growth that we are experiencing in the number of SPARQL endpoints available, the need to be able to send federated SPARQL queries across these has also grown. To address this use case, the W3C SPARQL working group is defining a federation extension for SPARQL 1.1 which allows for combining graph patterns that can be evaluated over several endpoints within a single query. In this paper, we describe the syntax of that extension and formalize its semantics. Additionally, we describe how a query evaluation system can be implemented for that federation extension, describing some static optimization techniques and reusing a query engine used for data-intensive science, so as to deal with large amounts of intermediate and final results. Finally we carry out a series of experiments that show that our optimizations speed up the federated query evaluation process

Opportunistic linked data querying through approximate membership metadata

Between URI dereferencing and the SPARQL protocol lies a largely unexplored axis of possible interfaces to Linked Data, each with its own combination of trade-offs. One of these interfaces is Triple Pattern Fragments, which allows clients to execute SPARQL queries against low-cost servers, at the cost of higher bandwidth. Increasing a client's efficiency means lowering the number of requests, which can among others be achieved through additional metadata in responses. We noted that typical SPARQL query evaluations against Triple Pattern Fragments require a significant portion of membership subqueries, which check the presence of a specific triple, rather than a variable pattern. This paper studies the impact of providing approximate membership functions, i.e., Bloom filters and Golomb-coded sets, as extra metadata. In addition to reducing HTTP requests, such functions allow to achieve full result recall earlier when temporarily allowing lower precision. Half of the tested queries from a WatDiv benchmark test set could be executed with up to a third fewer HTTP requests with only marginally higher server cost. Query times, however, did not improve, likely due to slower metadata generation and transfer. This indicates that approximate membership functions can partly improve the client-side query process with minimal impact on the server and its interface

Reading skills in young adolescents with a history of Specific Language Impairment: The role of early semantic capacity

This study assessed the reading skills of 19 Spanish-Catalan children with Specific Language Impairment (SLI) and 16 age-matched control children. Children with SLI have difficulties with oral language comprehension, which may affect later reading acquisition. We conducted a longitudinal study examining reading acquisition in these children between 8 and 12 years old and we relate this data with early oral language acquisition at 6 years old. Compared to the control group, the SLI group presented impaired decoding and comprehension skills at age 8, as evidenced by poor scores in all the assessed tasks. Nevertheless, only text comprehension abilities appeared to be impaired at age 12. Individual analyses confirmed the presence of comprehension deficits in most of the SLI children. Furthermore, early semantic verbal fluency at age 6 appeared to significantly predict the reading comprehension capacity of SLI participants at age 12. Our results emphasize the importance of semantic capacity at early stages of oral language development over the consolidation of reading acquisition at later stages

Oral morphosyntactic competence as a predictor of reading comprehension in children with specific language impairment

Background Children with a diagnosis of specific language impairment (SLI) present impaired oral comprehension. According to the simple view of reading, general amodal linguistic capacity accounts for both oral and reading comprehension. Considering this, we should expect SLI children to display a reading comprehension deficit. However, previous research regarding the association between reading disorders and SLI has yielded inconsistent results. Aims To study the influence of prior oral comprehension competence over reading comprehension during the first years of reading acquisition of bilingual Catalan-Spanish children with SLI (ages 7-8). Methods & Procedures We assessed groups of bilingual Catalan-Spanish SLI and matched control children at ages 7 and 8 with standardized reading comprehension tasks including grammatical structures, sentence and text comprehension. Early oral competence and prior non‐verbal intelligence were also measured and introduced into regression analyses with the participants' reading results in order to state the relation between the comprehension of oral and written material. Outcomes & Results Although we found no significant differences between the scores of our two participant groups in the reading tasks, data regarding their early oral competence, but not non‐verbal intelligence measures, significantly influence their reading outcome. Conclusions & Implications The results extend our knowledge regarding the course of literacy acquisition of children with SLI and provide evidence in support of the theories that assume common linguistic processes to be responsible for both oral and reading comprehension

Afghan unaccompanied minors in the Netherlands: Far away from home and protected?

This study aims to provide insights into the migration situation of Afghan unaccompanied minors (UAMs) in the Netherlands and the extent to which the UN Convention of the Rights of the Child (CRC) is respected. This paper contributes to the scarce literature on the largest group of UAMs in the Netherlands and in many European countries. On the basis of participant observation in a UAM campus and interviews with Afghan minors and various stakeholders involved with UAMs the following aspects were analysed in relation to the CRC: reception facilities, standard of living and care, possibilities of education, work and leisure, psychological difficulties and the legal situation. The study found that the Netherlands does well on the reception facilities, the standard of living and care, and the work conditions. Improvements can be made regarding educational opportunities, leisure, mental health care, integration and length of asylum procedure. Key words: Afghanistan, unaccompanied minors, refugee, asylum seeker, migratio