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We study the problem of finding the worst-case size of the result Q(D) of a fixed conjunctive query Q applied to a database D satisfying given functional dependencies. We provide a characterization of this bound in terms of entropy vectors, and in terms of finite groups. In particular, we show that an upper bound provided by [Gottlob, Lee, Valiant and Valiant, J.ACM, 2012] is tight, and that a correspondence of [Chan and Yeung, ACM TOIT, 2002] is preserved in the presence of functional dependencies. However, tightness of a weaker upper bound provided by Gottlob et al., which would have immediate applications to evaluation of join queries ([Khamis, Ngo, and Suciu, PODS, 2016]) remains open. Our result shows that the problem of computing the worst-case size bound, in the general case, is closely related to difficult problems from information theory.
@InProceedings{gogacz_et_al:LIPIcs.ICDT.2017.15,
author = {Gogacz, Tomasz and Torunczyk, Szymon},
title = {{Entropy Bounds for Conjunctive Queries with Functional Dependencies}},
booktitle = {20th International Conference on Database Theory (ICDT 2017)},
pages = {15:1--15:17},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-024-8},
ISSN = {1868-8969},
year = {2017},
volume = {68},
editor = {Benedikt, Michael and Orsi, Giorgio},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ICDT.2017.15},
URN = {urn:nbn:de:0030-drops-70479},
doi = {10.4230/LIPIcs.ICDT.2017.15},
annote = {Keywords: database theory, conjunctive queries, size bounds, entropy, finite groups, entropy cone}
}