Computing FO-Rewritings in EL in Practice: from Atomic to Conjunctive Queries

A prominent approach to implementing ontology-mediated queries (OMQs) is to rewrite into a first-order query, which is then executed using a conventional SQL database system. We consider the case where the ontology is formulated in the description logic EL and the actual query is a conjunctive query and show that rewritings of such OMQs can be efficiently computed in practice, in a sound and complete way. Our approach combines a reduction with a decomposed backwards chaining algorithm for OMQs that are based on the simpler atomic queries, also illuminating the relationship between first-order rewritings of OMQs based on conjunctive and on atomic queries. Experiments with real-world ontologies show promising results

Query Rewriting for Existential Rules with Compiled Preorder

International audienceWe address the issue of Ontology-Based Query Answering (OBQA), which seeks to exploit knowledge expressed in ontologies when querying data. Ontologies are represented in the framework of ex-istential rules (aka Datalog±). A commonly used technique consists in rewriting queries into unions of conjunctive queries (UCQs). However, the obtained queries can be prohibitively large in practice. A well-known source of combinatorial explosion are very simple rules, typically expressing taxonomies and relation signatures. We propose a rewriting technique, which consists in compiling these rules into a preorder on atoms and embedding this preorder into the rewriting process. This allows to compute compact rewritings that can be considered as " pivotal " representations, in the sense that they can be evaluated by different kinds of database systems. The provided algorithm computes a sound, complete and minimal UCQ rewriting , if one exists. Experiments show that this technique leads to substantial gains, in terms of size and runtime, and scales on very large ontologies. We also compare to other tools for OBQA with existen-tial rules and related lightweight description logics