5,046 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
Combined FO rewritability for conjunctive query answering in DL-Lite
Standard description logic (DL) reasoning services such as satisfiability and subsumption mainly aim to support TBox design. When the design stage is over and the TBox is used in an actual application, it is usually combined with instance data stored in an ABox, and therefore query answering becomes the most importan
Horn rewritability vs PTime query evaluation for description logic TBoxes
We study the following question: if τ is a TBox that is formulated in an expressive DL L and all CQs can be evaluated in PTime w.r.t. τ, can τ be replaced by a TBox τ' that is formulated in the Horn-fragment of L and such that for all CQs and ABoxes, the answers w.r.t. τ and τ' coincide? Our main results are that this is indeed the case when L is the set of ALCHI or ALCIF TBoxes of quantifier depth 1 (which covers the majority of such TBoxes), but not for ALCHIF and ALCQ TBoxes of depth 1
Fusions of Description Logics and Abstract Description Systems
Fusions are a simple way of combining logics. For normal modal logics,
fusions have been investigated in detail. In particular, it is known that,
under certain conditions, decidability transfers from the component logics to
their fusion. Though description logics are closely related to modal logics,
they are not necessarily normal. In addition, ABox reasoning in description
logics is not covered by the results from modal logics. In this paper, we
extend the decidability transfer results from normal modal logics to a large
class of description logics. To cover different description logics in a uniform
way, we introduce abstract description systems, which can be seen as a common
generalization of description and modal logics, and show the transfer results
in this general setting
First order-rewritability and containment of conjunctive queries in horn description logics
International audienceWe study FO-rewritability of conjunctive queries in the presence of ontologies formulated in a description logic between EL and Horn-SHIF, along with related query containment problems. Apart from providing characterizations, we establish complexity results ranging from EXPTIME via NEXPTIME to 2EXPTIME, pointing out several interesting effects. In particular, FO-rewriting is more complex for conjunctive queries than for atomic queries when inverse roles are present, but not otherwise
Nitrogen and phosphorus budgets for Iowa and Iowa watersheds
https://ir.uiowa.edu/igs_tis/1046/thumbnail.jp
Query-based entailment and inseparability for ALC ontologies
We investigate the problem whether two ALC knowledge bases are indistinguishable by queries over a given vocabulary. We give model-theoretic criteria and prove that this problem is undecid- able for conjunctive queries (CQs) but decidable in 2EXPTIME for unions of rooted CQs. We also consider the problem whether two ALC TBoxes give the same answers to any query in a given vocabulary over all ABoxes, and show that for CQs this problem is undecidable, too, but becomes decidable and 2EXPTIME-complete in Horn-ALC, and even EXPTIME-complete in Horn-ALC when restricted to (unions of) rooted CQs
Query inseparability for ALC ontologies
We investigate the problem whether two ALC ontologies are indistinguishable (or inseparable) by means of queries in
a given signature, which is fundamental for ontology engineering tasks such as ontology versioning, modularisation,
update, and forgetting. We consider both knowledge base (KB) and TBox inseparability. For KBs, we give model-theoretic
criteria in terms of (finite partial) homomorphisms and products and prove that this problem is undecidable
for conjunctive queries (CQs), but 2ExpTime-complete for unions of CQs (UCQs). The same results hold if (U)CQs are
replaced by rooted (U)CQs, where every variable is connected to an answer variable. We also show that inseparability
by CQs is still undecidable if one KB is given in the lightweight DL EL and if no restrictions are imposed on the
signature of the CQs. We also consider the problem whether two ALC TBoxes give the same answers to any query
over any ABox in a given signature and show that, for CQs, this problem is undecidable, too. We then develop
model-theoretic criteria for HornALC TBoxes and show using tree automata that, in contrast, inseparability becomes
decidable and 2ExpTime-complete, even ExpTime-complete when restricted to (unions of) rooted CQs
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