# The Bag Semantics of Ontology-Based Data Access

## TL;DR

This paper introduces a bag semantics for ontology-based data access (OBDA) that preserves duplicates in query views, aligning OBDA more closely with traditional database semantics, but increases computational complexity.

## Contribution

It proposes a bag semantics model for OBDA, analyzes its computational complexity, and identifies conditions for query rewriting to maintain tractability.

## Key findings

- Bag semantics makes conjunctive query answering coNP-hard in data complexity.
- A class of queries can be rewritten to a generalized relational calculus for bags.
- Supports database-style aggregate queries in OBDA.

## 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 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.