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OPTIONAL is a key feature in SPARQL for dealing with missing information.While this operator is used extensively, it is also known for its complexity,which can make efficient evaluation of queries with OPTIONAL challenging. Wetackle this problem in the Ontology-Based Data Access (OBDA) setting, where thedata is stored in a SQL relational database and exposed as a virtual RDF graphby means of an R2RML mapping. We start with a succinct translation of a SPARQLfragment into SQL. It fully respects bag semantics and three-valued logic andrelies on the extensive use of the LEFT JOIN operator and COALESCE function. Wethen propose optimisation techniques for reducing the size and improving thestructure of generated SQL queries. Our optimisations capture interactionsbetween JOIN, LEFT JOIN, COALESCE and integrity constraints such as attributenullability, uniqueness and foreign key constraints. Finally, we empiricallyverify effectiveness of our techniques on the BSBM OBDA benchmark.

Efficient Handling of SPARQL OPTIONAL for OBDA (Extended Version)