We study the expressiveness and complexity of Shape Expression Schema (ShEx), a novel schema formalism for RDF currently under development by W3C. A ShEx assigns types to the nodes of an RDF graph and allows to constrain the admissible neighborhoods of nodes of a given type with regular bag expressions (RBEs). We formalize and investigate two alternative semantics, multi- and single-type, depending on whether or not a node may have more than one type. We study the expressive power of ShEx and study the complexity of the validation problem. We show that the single-type semantics is strictly more expressive than the multi-type semantics, single-type validation is generally intractable and multi-type validation is feasible for a small (yet practical) subclass of RBEs. To curb the high computational complexity of validation, we propose a natural notion of determinism and show that multi-type validation for the class of deterministic schemas using single-occurrence regular bag expressions (SORBEs) is tractable.
@InProceedings{staworko_et_al:LIPIcs.ICDT.2015.195, author = {Staworko, Slawek and Boneva, Iovka and Labra Gayo, Jose E. and Hym, Samuel and Prud'hommeaux, Eric G. and Solbrig, Harold}, title = {{Complexity and Expressiveness of ShEx for RDF}}, booktitle = {18th International Conference on Database Theory (ICDT 2015)}, pages = {195--211}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-939897-79-8}, ISSN = {1868-8969}, year = {2015}, volume = {31}, editor = {Arenas, Marcelo and Ugarte, Mart{\'\i}n}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ICDT.2015.195}, URN = {urn:nbn:de:0030-drops-49856}, doi = {10.4230/LIPIcs.ICDT.2015.195}, annote = {Keywords: RDF, Schema, Graph topology, Validation, Complexity, Expressiveness} }
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