Citation Context

...an be also applied in the more strict ‘virtual’ OBDA context, in which data sources are independent and data can only be accessed ‘on-the-fly’. An initial exploration of this problem can be found in [=-=Rodríguez-Muro and Calvanese, 2011-=-]. Moreover, although TBox pre-processing is the best choice to start optimizing w.r.t. completeness, it is also a necessary step during query reformulation. With respect to evaluation, our experiment...

Citation Context

...arly for properties) then the problem (H) does not exist. Second, we can make the virtual ABoxes H-complete by taking the composition of T andM as a new mapping. This composition, called a T -mapping =-=[24]-=-, can be simplified with the help of Σ and the features of the target query language before being used in the unfolding. As the simplifications use Σ, they preserve correct answers only over database ...

Citation Context

...e the problem of arriving at an SQL expression that can be effectively evaluated on the database. In this scenario, a very interesting way to limit the size of the rewritten UCQ has been presented in =-=[13]-=-. This approach proposes the use of the so-called ABox dependencies to optimize query rewriting in DL-LiteA. ABox dependencies are inclusions between concepts and roles which are interpreted as integr...

Citation Context

...sing the mapping assertions; we call this step the mapping rewriting of the query. While in recent years we have seen many approaches to the ontology rewriting step and its optimizations, (see, e.g., =-=[5, 22, 23, 9]-=-), very little has been done towards the optimization the mapping rewriting step. Note that mapping rewriting is also relevant in data integration. However, the literature on data integration has main...

Citation Context

... containment checks performed. Second, Blackout implements an optimization technique known as data oracle. This optimization is related to the so-called extensional constraints technique presented in =-=[13]-=-. If a derived query contains an atom which is empty with respect to a given ABox, the query is discarded as it would obviously produce empty 7 http://www.dlvsystem.com/ 8 http://stardog.com/8 result...

Citation Context

...ppings and analyse the database metadata, extracting constraints (e.g., Primary Keys, etc.) and using this information, together with query containment checks to optimise the mappings (see T-mappings =-=[5]-=-). The TBox will also be optimised, eliminating redundant vocabulary and detecting any redundancy w.r.t. the mappings (see [6]). At query time, the system will go through 4 key steps: (i) SPARQLto-Dat...

Citation Context

...es and replace the recursive predicates by fresh predicates; these fresh predicates are defined by cached CTEs. 4.2 T-Mappings for SWRL Rules We introduce now an extension of the notion of T-mappings =-=[20]-=- to cope with SWRL rules. T-mappings have been introduced in the context of OBDA systems in which queries posed over an OWL 2 QL ontology that is mapped to a relational database, are rewritten in term...

Citation Context

...and 2, this optimization would generate at least the following additional mapping: SELECT id FROM condition WHERE cond = 22; {<":patient/{$id}"> rdf:type :Patient.} This technique, called T -Mappings =-=[8]-=-, makes most reasoning w.r.t. T redundant, similarly to the Semantic Index technique in classic ABox mode. (iv) Mapping program optimization. In this step, Quest transforms the resulting OBDA model in...

Citation Context

...able improvements in execution times. Considering the previous observations, in this paper we extend the results aiming at improving query answering performance in OBDA systems that were developed in =-=[9]-=- for DL-LiteF, to the case where also role inclusions are present in the TBox. Specifically, we first show that we can characterize completeness of an ABox by means of dependencies, and that we can us...