OPT+: A Monotonic Alternative to OPTIONAL in SPARQL

  • Sijin Cheng Department of Computer and Information Science (IDA), Linköping University, Sweden
  • Olaf Hartig Department of Computer and Information Science (IDA), Linköping University, Sweden
Keywords: OPT, Monotonic Alternative to OPTIONAL


Due to the OPTIONAL operator, the core fragment of the SPARQL query language is non-monotonic. That is, some solutions of a query result can be returned to the user only after having consulted all relevant parts of the queried dataset(s). This property presents an obstacle when developing query execution approaches that aim to reduce responses times rather than the overall query execution times. Reducing the response times–i.e., returning as many solutions as early as possible– is important in particular in Web-based client-server query processing scenarios in which network latencies dominate query execution times. Such scenarios are typical in the context of integration of Web data sources where a data integration component executes queries over a decentralized federation of such data sources. In this paper we introduce an alternative operator that is similar in spirit to OPTIONAL but without causing non-monotonicity. We show fundamental properties of this operator and observe that the downside of achieving the desired monotonicity property is a potentially significant increase in query result sizes. We study the extend of this trade-off in practice. Thereafter, we introduce different algorithms to implement the new operator and evaluate them regarding their potential to reduce response times. Keywords: Semantic web, linked data, query language, optimization.


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Author Biographies

Sijin Cheng, Department of Computer and Information Science (IDA), Linköping University, Sweden

Sijin Cheng is a Ph.D. student at Linköping University since Spring 2019. She holds a double Master’s degree in Computer Science and Software Engineering from Linköping University, Sweden, and the Harbin Institute of Technology, China. Sijin’s Ph.D. work focuses on problems related to querying federations of graph data sources with heterogeneous forms of data access interfaces.

Olaf Hartig, Department of Computer and Information Science (IDA), Linköping University, Sweden

Olaf Hartig is an Associate Professor in Computer Science at Linköping University. He holds a Ph.D. in Computer Science from the Humboldt-Universität zu Berlin and has been awarded the academic qualification Docent in Computer Science from Linköping University.

His research focuses on data on the Web and on graph data, as well as on problems in which the data is distributed over multiple, autonomous and/or heterogeneous sources. Regarding these topics, Olaf’s interests range from systems-building related research to theoret-ical foundations. For his Ph.D. dissertation “Querying a Web of Linked Data: Foundations and Query Execution” he was honored with the 2015 Distinguished Dissertation Award of the Semantic Web Science Foundation (SWSA), and he has received several best research paper awards (ESWC2009, ESWC2015, ISWC2017, Semantics2018).


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