The Bag Semantics of Ontology-Based Data Access
This addresses the need for OBDA systems to handle aggregate queries, which is important for data integration applications, but it is incremental as it extends existing OBDA frameworks with bag semantics.
The paper tackles the problem of supporting database-style aggregate queries in ontology-based data access (OBDA) by proposing a bag semantics that retains duplicate tuples in mappings, and shows that this makes conjunctive query answering coNP-hard in data complexity, but identifies a general class of queries that is rewritable to a relational calculus for bags to regain tractability.
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 semantics 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.