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Foundations of ontology-based data access under bag semantics

Foundations of ontology-based data access under bag semantics
Foundations of ontology-based data access under bag semantics

Ontology-based data access (OBDA) is a popular approach for integrating and querying multiple data sources by means of a shared ontology. The ontology is linked to the sources using mappings, which assign to ontology predicates views over the data. The conventional semantics of OBDA is set-based—that is, the extension of the views defined by the mappings does not contain duplicate tuples. This treatment is, however, in disagreement with the standard semantics of database views and database management systems in general, which is based on bags and where duplicate tuples are retained by default. The distinction between set and bag semantics in databases is very significant in practice, and it influences the evaluation of aggregate queries. In this article, we propose and study a bag semantics for OBDA which provides a solid foundation for the future study of aggregate and analytic queries. Our semantics is compatible with both the bag semantics of database views and the set-based conventional semantics of OBDA. Furthermore, it is compatible with existing bag-based semantics for data exchange recently proposed in the literature. We show that adopting a bag semantics makes conjunctive query answering in OBDA CONP-hard in data complexity. To regain tractability of query answering, we consider suitable restrictions along three dimensions, namely, the query language, the ontology language, and the adoption of the unique name assumption. Our investigation shows a complete picture of the computational properties of query answering under bag semantics over ontologies in the DL-Lite family.

Bag semantics, Description logics, Ontology-based data access, Query answering, Query rewriting
0004-3702
91-132
Nikolaou, Charalampos
8f136351-0f3f-479c-b709-311be8138b03
Kostylev, Egor V.
68747f03-563d-4ae1-9471-e7561770dae4
Konstantinidis, George
f174fb99-8434-4485-a7e4-bee0fef39b42
Kaminski, Mark
ab75eafc-19b9-426a-8006-3cc47c8ea853
Cuenca Grau, Bernardo
acaa565c-dec8-4445-bc4c-0ee1419d5600
Horrocks, Ian
b8e97f51-6af4-4351-a05c-10b23413652f
Nikolaou, Charalampos
8f136351-0f3f-479c-b709-311be8138b03
Kostylev, Egor V.
68747f03-563d-4ae1-9471-e7561770dae4
Konstantinidis, George
f174fb99-8434-4485-a7e4-bee0fef39b42
Kaminski, Mark
ab75eafc-19b9-426a-8006-3cc47c8ea853
Cuenca Grau, Bernardo
acaa565c-dec8-4445-bc4c-0ee1419d5600
Horrocks, Ian
b8e97f51-6af4-4351-a05c-10b23413652f

Nikolaou, Charalampos, Kostylev, Egor V., Konstantinidis, George, Kaminski, Mark, Cuenca Grau, Bernardo and Horrocks, Ian (2019) Foundations of ontology-based data access under bag semantics. Artificial Intelligence, 274, 91-132. (doi:10.1016/j.artint.2019.02.003).

Record type: Article

Abstract

Ontology-based data access (OBDA) is a popular approach for integrating and querying multiple data sources by means of a shared ontology. The ontology is linked to the sources using mappings, which assign to ontology predicates views over the data. The conventional semantics of OBDA is set-based—that is, the extension of the views defined by the mappings does not contain duplicate tuples. This treatment is, however, in disagreement with the standard semantics of database views and database management systems in general, which is based on bags and where duplicate tuples are retained by default. The distinction between set and bag semantics in databases is very significant in practice, and it influences the evaluation of aggregate queries. In this article, we propose and study a bag semantics for OBDA which provides a solid foundation for the future study of aggregate and analytic queries. Our semantics is compatible with both the bag semantics of database views and the set-based conventional semantics of OBDA. Furthermore, it is compatible with existing bag-based semantics for data exchange recently proposed in the literature. We show that adopting a bag semantics makes conjunctive query answering in OBDA CONP-hard in data complexity. To regain tractability of query answering, we consider suitable restrictions along three dimensions, namely, the query language, the ontology language, and the adoption of the unique name assumption. Our investigation shows a complete picture of the computational properties of query answering under bag semantics over ontologies in the DL-Lite family.

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Accepted/In Press date: 12 February 2019
e-pub ahead of print date: 15 February 2019
Published date: 1 September 2019
Keywords: Bag semantics, Description logics, Ontology-based data access, Query answering, Query rewriting

Identifiers

Local EPrints ID: 428615
URI: https://eprints.soton.ac.uk/id/eprint/428615
ISSN: 0004-3702
PURE UUID: 6bfd73f4-8908-44b1-8874-2002f6a09e8f

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Date deposited: 05 Mar 2019 17:30
Last modified: 30 Apr 2019 16:30

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Contributors

Author: Charalampos Nikolaou
Author: Egor V. Kostylev
Author: George Konstantinidis
Author: Mark Kaminski
Author: Bernardo Cuenca Grau
Author: Ian Horrocks

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