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Title
Abstract
Overview of This Deliverable
Introduction 1.1 Motivation
Normal Forms and Basic Results
Related Work
Conclusion
Summary
References

I 4-D 14 Exploring and Taming Existence in Rule-based RDFQueries

Profile image of Clemens LeyClemens Ley

2007

Abstract

RDF is an emerging knowledge representation formalism proposed by the W3C. A central feature of RDF are blank nodes, which allow to assert the existence of an entity without naming for it. Despite the importance of blank nodes for RDF, many existing RDF query language have only insu cient support for blank nodes. We propose a query language for RDF, called RDFLog, with extensive blank node support. ¿e evaluation of RDFLog may be reduced to the evaluation of Datalog. ¿is allows to apply standard database technology to querying RDF. Our Experimental evaluation shows that our implementation scales well, even for large data sets. ¿e core feature of the reduction is Skolemisation and an new form of unSkolemisation. In contrast to previous de nitions of un-Skolemisation our un-Skolemisation has desirable symmetric properties to those of the Skolemisation. We de ne a hierarchy of syntactical restrictions of RDFLog with lower expressivity but better complexity, thereby showing the computati...

Related papers

Designing a Query Language for RDF
Marcelo Arenas

Proceedings of the 35th ACM SIGMOD-SIGACT-SIGAI Symposium on Principles of Database Systems - PODS '16, 2016

When querying an RDF graph, a prominent feature is the possibility of extending the answer to a query with optional information. However, the definition of this feature in SPARQL --the standard RDF query language-- has raised some important issues. Most notably, the use of this feature increases the complexity of the evaluation problem, and its closed-world semantics is in conflict with the underlying open-world semantics of RDF. Many approaches for fixing such problems have been proposed, being the most prominent the introduction of the semantic notion of weakly-monotone SPARQL query. Weakly-monotone SPARQL queries have shaped the class of queries that conform to the open-world semantics of RDF. Unfortunately, finding an effective way of restricting SPARQL to the fragment of weakly-monotone queries has proven to be an elusive problem. In practice, the most widely adopted fragment for writing SPARQL queries is based on the syntactic notion of well designedness. This notion has proven to be a good approach for writing SPARQL queries, but its expressive power has yet to be fully understood. The starting point of this paper is to understand the relation between well-designed queries and the semantic notion of weak monotonicity. It is known that every well-designed SPARQL query is weakly monotone; as our first contribution we prove that the converse does not hold, even if an extension of this notion based on the use of disjunction is considered. Given this negative result, we embark on the task of defining syntactic fragments that are weakly-monotone, and have higher expressive power than the fragment of well-designed queries. To this end, we move to a more general scenario where infinite RDF graphs are also allowed, so that interpolation techniques studied for first-order logic can be applied. With the use of these techniques, we are able to define a new operator for SPARQL that gives rise to a query language with the desired properties (over finite and infinite RDF graphs). It should be noticed that every query in this fragment is weakly monotone if we restrict to the case of finite RDF graphs. Moreover, we use this result to provide a simple characterization of the class of monotone CONSTRUCT queries, that is, the class of SPARQL queries that produce RDF graphs as output. Finally, we pinpoint the complexity of the evaluation problem for the query languages identified in the paper.

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Some experiments on the usage of a deductive database for RDFS querying and reasoning
G. Ianni

Ontologies are pervading many areas of knowledge representation and management. To date, most research efforts have been spent on the development of sufficiently expressive languages for the representation and querying of ontologies; however, querying efficiency has received attention only recently, especially for ontologies referring to large amounts of data. In fact, it is still uncertain how reasoning tasks will scale when applied on massive amounts of data. This work is a first step toward this setting: based on a previous result showing that the SPARQL query language can be mapped to a Datalog, we show how efficient querying of big ontologies can be accomplished with a database oriented extension of the well known system DLV, recently developed. We report our initial results and we discuss about benefits of possible alternative data structures for representing RDF graphs in our architecture.

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RDFLog: It’s like Datalog for RDF
François Bry

2008

Abstract. RDF data is set apart from relational or XML data by its support of rich existential information in the form of blank nodes. Where SQL null values are scoped over a single statement, blank nodes in RDF can span over any number of statements and thus can be seen as existentially quantified variables.

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Minimal Deductive Systems for RDF
Claudio Gutierrez

The Semantic Web: Research and …, 2007

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DR, DEVICE: A Defeasible Logic RDF Rule Language
Ioannis Vlahavas

lpis.csd.auth.gr

Defeasible reasoning is a rule4based approach for efficient reasoning with incomplete and inconsistent information. Such reasoning is, among others, useful for ontology integration, where conflicting information arises naturally; and for the modeling of business rules and policies, where rules with exceptions are often used. In this demonstration we pre4 sent a prototype system for defeasible reasoning on the Web. The system is called DR4DEVICE [4] and is capable of reasoning about RDF metadata over multiple Web sources using ...

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A query and inference service for RDF
Jürgen Angele

1998

The creation of RDF raises the prospect of a widely accepted standard for representing expressive declarative knowledge on the Web. However, just representing knowledge and information is not enough: users as well as information agents have to query and use the data in several ways. An RDF Query specification would allow a range of applicat ions to exploit any information source which can be represented in terms of the RDF data model. In this paper we describe some existing techniques used in logic program evaluation and see how RDF descri ptions fit into this framework.

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Understanding the structure of RDF data: a practical study
Alejandro Mallea

2009

In spite of being the standard query language for RDF, SPARQL is not efficient. In fact, the problem of evaluating SPARQL patterns is PSPACEcomplete. In practice, however, the features that make this problem so hard appear to be used in very restricted ways. Establishing these restrictions should lead to low complexity bounds for the average case. In this article we propose a detailed study of real-world RDF databases, which will provide us with the background to extend the semantics of SPARQL to handle the aforementioned features. From there, we expect to be able to develop and test (efficient) query processing algorithms in a pilot implementation of an RDF data management system.

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B.: RDFLog: It’s like Datalog for RDF
Clemens Ley

2008

Abstract. RDF data is set apart from relational or XML data by its support of rich existential information in the form of blank nodes. Where in SQL databases null values are scoped over a single tuple, blank nodes in RDF can span over any number of statements and thus can be seen as existentially quantified variables. Blank node querying is considered in most RDF query languages, but blank node construction, i.e., the introduction of new blank nodes has been mostly ignored (e.g., in Triple) or treated in a very limited form (e.g., in SPARQL). In this paper, we classify three kinds of blank nodes in RDF query languages and introduce the recursive, rule-based RDF query language RDFLog. RDFLog is the first RDF query language with full arbitrary quantifier alternation: blank nodes may occur in the scope of all, some, or none of the universal variables of a rule. RDFLog is also aware of important RDF features such as the distinction between blank nodes, literals and URIs or the RDFS voca...

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Formal aspects of querying RDF databases
Claudio Gutierrez

2003

Abstract. We study formal aspects of querying databases containing RDF data. We present a formal definition of a query language for RDF and compare it with other proposals. Our language is intended to make it easy to formalize and prove results about its properties. We study novel features of query languages derived from the presence of blank nodes and reification. Finally we provide complexity results for query processing, static optimization of queries, and redundancy elimination in answers.

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A Datalog-Based Language for Querying RDF Graphs
Georg Gottlob

2016

Citation for published version: Arenas, M, Gottlob, G & Pieris, A 2016, A Datalog-Based Language for Querying RDF Graphs. in Proceedings of the 10th Alberto Mendelzon International Workshop on Foundations of Data Management, Panama City, Panama, May 8-10, 2016. CEUR Workshop Proceedings, CEUR Workshop Proceedings (CEUR-WS.org), 10th Alberto Mendelzon International Workshop on Foundations of Data Management, Panama City, Panama, 6/06/16.

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R-device: A deductive rdf rule language
Ioannis Vlahavas, Nick Bassiliades

Rules and Rule Markup Languages for the …, 2004

In this paper we present R-DEVICE, a deductive rule language for reasoning about RDF metadata. R-DEVICE includes features such as normal and generalized path expressions, stratified negation, aggregate, grouping, and sorting, functions. The rule language supports a second-order syntax, where variables can range over classes and properties. Users can define views which are materialized and, optionally, incrementally maintained by translating deductive rules into CLIPS production rules. Users can choose between an OPS5/CLIPS-like or a RuleML-like syntax. R-DEVICE is based on a OO RDF data model, different than the established graph model, which maps resources to objects and encapsulates properties inside resource objects, as traditional OO attributes. In this way, less joins are required to access the properties of a single resource resulting in better inferencing/querying performance. The descriptive semantics of RDF may call for dynamic re-definitions of resource classes and objects, which are handled by R-DEVICE effectively.

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SPARQLog: SPARQL with rules and quantification
François Bry

2010

Abstract SPARQL has become the gold-standard for RDF query languages. Nevertheless, we believe there is further room for improving RDF query languages. In this chapter, we investigate the addition of rules and quantifier alternation to SPARQL. That extension, called SPARQLog, extends previous RDF query languages by arbitrary quantifier alternation: blank nodes may occur in the scope of all, some, or none of the universal variables of a rule.

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