Towards Reconciling SPARQL and Certain Answers

Abstract

SPARQL entailment regimes are strongly influenced by the big body of works on ontology-based query answering, notably in the area of Description Logics (DLs). However, the semantics of query answering under SPARQL entailment regimes is defined in a more naive and much less expressive way than the certain answer semantics usually adopted in DLs. The goal of this work is to introduce an intuitive certain answer semantics also for SPARQL and to show the feasibility of this approach. For OWL 2 QL entailment, we present algorithms for the evaluation of an interesting fragment of SPARQL (the so-called well-designed SPARQL). Moreover, we show that the complexity of the most fundamental query analysis tasks (such as query containment and equivalence testing) is not negatively affected by the presence of OWL 2 QL entailment under the proposed semantics.

Complexity and Expressive Power of Weakly Well-Designed SPARQL

“…Moreover, since u is contained in T (P μ ), there is a mapping μ 1 μ such that μ 1 ∈ T G . Since v is not contained in T (P μ ), we have μ 1 μ| v and, since T (P μ ) is the largest r-subtree witnessing μ, μ 1 is not compatible with any mapping in T v G . On the other hand, μ satisfies the label of v, and thus, since T v contains no top-level filters, μ | vars(v) can be extended to a mapping of μ ∈ T v G .…”

“…Following [5,33], a set of mappings 1 is subsumed by a set of mappings 2 (written 1 2 ) if for every μ 1 ∈ 1 there exists a mapping μ 2 ∈ 2 such that μ 1 μ 2 . A query Q is weakly monotone if Q G 1 Q G 2 for any two graphs G 1 and G 2 with G 1 ⊆ G 2 , and a fragment L is weakly monotone if it contains only weakly monotone queries.…”

“…On graph G in Fig. 1a the result is exactly the same as for query (1), shown in Fig. 1b, simply because the IRI v card:name is not present in G, and so cannot be matched against the second optional part of the query.…”

“…In particular, we now know much about evaluation [1,3,4,6,8,20,22,23,25,28,34,38], optimisation [8,9,12,13,24,27,35], federation [10,11], expressive power [2,20,21,25,36,39], and provenance tracking [15,16] for queries from various fragments and extensions of SPARQL. These studies have had a great impact in the community, in fact influencing the evolution of SPARQL as a standard.…”

“…Well-designed queries can be efficiently transformed to an intuitive normal form allowing for a transparent graphical representation of queries as trees [27,35]. Hence, many recent studies concentrate partially [23,25,27,40,41] or entirely [1,8,35] on well-designed queries.…”