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Designing a qery language for rdf: Marrying open and closed worlds

par Arenas, Marcelo;Ugarte Caraball, Martin Ignacio
Référence ACM transactions on database systems, 42, 4, 21
Publication Publié, 2017-10

Article révisé par les pairs

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Résumé :

When querying an Resource Description Framework (RDF) graph, a prominent feature is the possibility of extending the answer to a query with optional information. However, the defnition 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 con?ict with the underlying open-world semantics of RDF. Many approaches for fxing such problems have been proposed, the most prominent being 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, fnding an e?ective 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 article 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 frst 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 defning 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 infnite RDF graphs are also allowed, so interpolation techniques studied for frst-order logic can be applied. With the use of these techniques, we are able to defne a new operator for SPARQL that gives rise to a query language with the desired properties (over fnite and infnite RDF graphs). It should be noticed that every query in this fragment is weakly monotone if we restrict the semantics to fnite 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 identifed in the article.