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Subject Item
n2:91264
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bibo:AcademicArticle n7:EPrint bibo:Article n7:ConferenceItemEPrint
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n14:978-3-030-00671-6_21
dcterms:title
Efficient Handling of SPARQL OPTIONAL for OBDA
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n9:export_kar_RDFN3.n3 n11:export_kar_RDFN3.n3
dcterms:date
2018-10-08
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n5:ext-7b3500f16780a70524ab5fe827343547 n5:ext-71a8a69aa55165735ed911b1305c06a1 n5:ext-e.botoeva@kent.ac.uk n5:ext-5a0bb5735eb2f40e98a8bf66ea27f899 n5:ext-08fc32d35a562fbbd46568513d1fa244
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n16:peerReviewed n16:published
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n19:ext-1c5ddec173ca8cdfba8b274309638579
bibo:abstract
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. We tackle this problem in the Ontology-Based Data Access (OBDA) setting, where the data is stored in a SQL relational database and exposed as a virtual RDF graph by means of an R2RML mapping. We start with a succinct translation of a SPARQL fragment into SQL. It fully respects bag semantics and three-valued logic and relies on the extensive use of the LEFT JOIN operator and COALESCE function. We then propose optimisation techniques for reducing the size and improving the structure of generated SQL queries. Our optimisations capture interactions between JOIN, LEFT JOIN, COALESCE and integrity constraints such as attribute nullability, uniqueness and foreign key constraints. Finally, we empirically verify effectiveness of our techniques on the BSBM OBDA benchmark.
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n15:ext-fb5fea9af81777d227554f8c09862bb3
bibo:volume
11136