8 research outputs found
The combined approach to ontology-based data access
The use of ontologies for accessing data is one of
the most exciting new applications of description
logics in databases and other information systems.
A realistic way of realising sufficiently scalable ontology-
based data access in practice is by reduction
to querying relational databases. In this paper,
we describe the combined approach, which incorporates
the information given by the ontology into
the data and employs query rewriting to eliminate
spurious answers. We illustrate this approach for
ontologies given in the DL-Lite family of description
logics and briefly discuss the results obtained
for the EL family
On the Succinctness of Query Rewriting over OWL 2 QL Ontologies with Shallow Chases
We investigate the size of first-order rewritings of conjunctive queries over
OWL 2 QL ontologies of depth 1 and 2 by means of hypergraph programs computing
Boolean functions. Both positive and negative results are obtained. Conjunctive
queries over ontologies of depth 1 have polynomial-size nonrecursive datalog
rewritings; tree-shaped queries have polynomial positive existential
rewritings; however, in the worst case, positive existential rewritings can
only be of superpolynomial size. Positive existential and nonrecursive datalog
rewritings of queries over ontologies of depth 2 suffer an exponential blowup
in the worst case, while first-order rewritings are superpolynomial unless
. We also analyse rewritings of
tree-shaped queries over arbitrary ontologies and observe that the query
entailment problem for such queries is fixed-parameter tractable
Agregando Semántica a MongoDB
Bajo la premisa de satisfacer las necesidades de flexibilidad y diversidad en el almacenamiento de datos, MongoDB ha ganado popularidad en los últimos años, permitiendo almacenar datos sin la necesidad de definir un
esquema previo. Diversas metodologías han sido desarrolladas para la implementación de tecnologías semánticas a partir de bases de datos relacionales. Por el contrario, el uso de tecnologías semánticas partiendo de bases de datos no relacionales es aún un área novedosa. Este trabajo presenta una comparación cualitativa de 3 herramientas (Morph-xR2RML, Tripod y MongoGraph) para la extensión de la capacidad de semántica de MongoDB
SUMA: A Partial Materialization-Based Scalable Query Answering in OWL 2 DL
AbstractOntology-mediated querying (OMQ) provides a paradigm for query answering according to which users not only query records at the database but also query implicit information inferred from ontology. A key challenge in OMQ is that the implicit information may be infinite, which cannot be stored at the database and queried by off -the -shelf query engine. The commonly adopted technique to deal with infinite entailments is query rewriting, which, however, comes at the cost of query rewriting at runtime. In this work, the partial materialization method is proposed to ensure that the extension is always finite. The partial materialization technology does not rewrite query but instead computes partial consequences entailed by ontology before the online query. Besides, a query analysis algorithm is designed to ensure the completeness of querying rooted and Boolean conjunctive queries over partial materialization. We also soundly and incompletely expand our method to support highly expressive ontology language, OWL 2 DL. Finally, we further optimize the materialization efficiency by role rewriting algorithm and implement our approach as a prototype system SUMA by integrating off-the-shelf efficient SPARQL query engine. The experiments show that SUMA is complete on each test ontology and each test query, which is the same as Pellet and outperforms PAGOdA. Besides, SUMA is highly scalable on large datasets
Semantic Query Reasoning in Distributed Environment
Master's thesis in Computer scienceSemantic Web aims to elevate simple data in WWW to semantic layer, so that knowledge, processed by machine, can be shared more easily. Ontology is one of the key technologies to realize Semantic Web. Semantic reasoning is an important step in Semantic technology. For Ontology developers, semantic reasoning finds out collisions in Ontology definition, and optimizes it; for Ontology users, semantic reasoning retrieves implicit knowledge from known knowledge.
The main research of this thesis is reasoning of semantic data querying in distributed environment, which tries to get correct results of semantic data querying, given Ontology definition and data. This research studied two methods: data materialization and query rewriting. Using Amazon cloud computing service and LUBM, we compared these two methods, and have concluded that when size of data to be queried scales up, query rewriting is more feasible than data materialization. Also, based on the conclusion, we developed an application, which manages and queries semantic data in a distributed environment. This application can be used as a prototype of similar applications, and a tool for other Semantic Web researches as well
Tree-like Queries in OWL 2 QL: Succinctness and Complexity Results
This paper investigates the impact of query topology on the difficulty of
answering conjunctive queries in the presence of OWL 2 QL ontologies. Our first
contribution is to clarify the worst-case size of positive existential (PE),
non-recursive Datalog (NDL), and first-order (FO) rewritings for various
classes of tree-like conjunctive queries, ranging from linear queries to
bounded treewidth queries. Perhaps our most surprising result is a
superpolynomial lower bound on the size of PE-rewritings that holds already for
linear queries and ontologies of depth 2. More positively, we show that
polynomial-size NDL-rewritings always exist for tree-shaped queries with a
bounded number of leaves (and arbitrary ontologies), and for bounded treewidth
queries paired with bounded depth ontologies. For FO-rewritings, we equate the
existence of polysize rewritings with well-known problems in Boolean circuit
complexity. As our second contribution, we analyze the computational complexity
of query answering and establish tractability results (either NL- or
LOGCFL-completeness) for a range of query-ontology pairs. Combining our new
results with those from the literature yields a complete picture of the
succinctness and complexity landscapes for the considered classes of queries
and ontologies.Comment: This is an extended version of a paper accepted at LICS'15. It
contains both succinctness and complexity results and adopts FOL notation.
The appendix contains proofs that had to be omitted from the conference
version for lack of space. The previous arxiv version (a long version of our
DL'14 workshop paper) only contained the succinctness results and used
description logic notatio
A Comparison of Query Rewriting Techniques for DL-Lite
An incomplete database is defined by a set of constraints and a partial database instance [1]. Answering conjunctive queries over incomplete databases is an important computational task that lies at the core of many problems, such as information integration [12], data exchange [9], and data warehousing [17]. Give