4,015 research outputs found
Preliminary results on Ontology-based Open Data Publishing
Despite the current interest in Open Data publishing, a formal and
comprehensive methodology supporting an organization in deciding which data to
publish and carrying out precise procedures for publishing high-quality data,
is still missing. In this paper we argue that the Ontology-based Data
Management paradigm can provide a formal basis for a principled approach to
publish high quality, semantically annotated Open Data. We describe two main
approaches to using an ontology for this endeavor, and then we present some
technical results on one of the approaches, called bottom-up, where the
specification of the data to be published is given in terms of the sources, and
specific techniques allow deriving suitable annotations for interpreting the
published data under the light of the ontology
Query Rewriting and Optimization for Ontological Databases
Ontological queries are evaluated against a knowledge base consisting of an
extensional database and an ontology (i.e., a set of logical assertions and
constraints which derive new intensional knowledge from the extensional
database), rather than directly on the extensional database. The evaluation and
optimization of such queries is an intriguing new problem for database
research. In this paper, we discuss two important aspects of this problem:
query rewriting and query optimization. Query rewriting consists of the
compilation of an ontological query into an equivalent first-order query
against the underlying extensional database. We present a novel query rewriting
algorithm for rather general types of ontological constraints which is
well-suited for practical implementations. In particular, we show how a
conjunctive query against a knowledge base, expressed using linear and sticky
existential rules, that is, members of the recently introduced Datalog+/-
family of ontology languages, can be compiled into a union of conjunctive
queries (UCQ) against the underlying database. Ontological query optimization,
in this context, attempts to improve this rewriting process so to produce
possibly small and cost-effective UCQ rewritings for an input query.Comment: arXiv admin note: text overlap with arXiv:1312.5914 by other author
Composition and Inversion of Schema Mappings
In the recent years, a lot of attention has been paid to the development of
solid foundations for the composition and inversion of schema mappings. In this
paper, we review the proposals for the semantics of these crucial operators.
For each of these proposals, we concentrate on the three following problems:
the definition of the semantics of the operator, the language needed to express
the operator, and the algorithmic issues associated to the problem of computing
the operator. It should be pointed out that we primarily consider the
formalization of schema mappings introduced in the work on data exchange. In
particular, when studying the problem of computing the composition and inverse
of a schema mapping, we will be mostly interested in computing these operators
for mappings specified by source-to-target tuple-generating dependencies
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