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Please use this identifier to cite or link to this item: http://hdl.handle.net/1942/24360

Title: From relation algebra to semi-join algebra: an approach for graph query optimization
Authors: Hellings, Jelle
Pilachowski, Catherine L.
Van Gucht, Dirk
Gyssens, Marc
Wu, Yuqing
Issue Date: 2017
Publisher: ACM Press
Citation: DBPL '17 Proceedings of The 16th International Symposium on Database Programming Languages, ACM Press,p. 1-10 (Art N° 5)
Series/Report: ICPS: ACM International Conference Proceeding Series
Abstract: Many graph query languages rely on the composition operator to navigate graphs and select nodes of interests, even though evaluating compositions of relations can be costly. Often, this need for composition can be reduced by rewriting towards queries that use semi-joins instead. In this way, the cost of evaluating queries can be significantly reduced. We study techniques to recognize and apply such rewritings. Concretely, we study the relationship between the expressive power of the relation algebras, that heavily rely on composition, and the semi-join algebras, that replace the composition operator in favor of the semi-join operators. As our main result, we show that each fragment of the relation algebras where intersection and/or difference is only used on edges (and not on complex compositions) is expressively equivalent to a fragment of the semi-join algebras. This expressive equivalence holds for node queries that evaluate to sets of nodes. For practical relevance, we exhibit constructive steps for rewriting relation algebra queries to semi-join algebra queries, and prove that these steps lead to only a well-bounded increase in the number of steps needed to evaluate the rewritten queries. In addition, on node-labeled graphs that are sibling-ordered trees, we establish new relationships among the expressive power of Regular XPath, Conditional XPath, FO-logic, and the semi-join algebra augmented with restricted fixpoint operators.
URI: http://hdl.handle.net/1942/24360
Link to publication: http://jhellings.nl/files/dbpl2017_paper.pdf
DOI: 10.1145/3122831.3122833
ISBN: 9781450353540
Category: C1
Type: Proceedings Paper
Appears in Collections: Research publications

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