The Bag Semantics of Ontology-Based Data Access [article]

Charalampos Nikolaou and Egor V. Kostylev and George Konstantinidis and Mark Kaminski and Bernardo Cuenca Grau and Ian Horrocks
2017 arXiv   pre-print
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 views over the data to ontology predicates. Motivated by the need for OBDA systems supporting database-style aggregate queries, we propose a bag semantics for OBDA, where duplicate tuples in the views defined by the mappings are retained, as is the case in standard databases. We show that bag
more » ... emantics makes conjunctive query answering in OBDA coNP-hard in data complexity. To regain tractability, we consider a rather general class of queries and show its rewritability to a generalisation of the relational calculus to bags.
arXiv:1705.07105v1 fatcat:gcnfenumdrf5jn5x53ddqbciz4