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Given a massive data graph G = (V, E) and a small pattern graph Q, the goal of subgraph enumeration is to list all the subgraphs of G isomorphic to Q. In the external memory (EM) model, it is well-known that every indivisible algorithm must perform Ω({|E|^ρ}/{M^{ρ-1} B}) I/Os in the worst case, where M represents the number of words in (internal) memory, B denotes the number of words in a disk block, and ρ is the fractional edge covering number of Q. It has been a longstanding open problem to design an algorithm to match this lower bound. The state of the art is an algorithm in ICDT'23 that achieves an I/O complexity of O({|E|^ρ}/{M^{ρ-1} B} log_{M/B} |E|/B) with high probability. In this paper, we remove the log_{M/B} |E|/B factor, thereby settling the open problem when randomization is permitted.
@InProceedings{deng_et_al:LIPIcs.ICDT.2024.21,
author = {Deng, Shiyuan and Tao, Yufei},
title = {{Subgraph Enumeration in Optimal I/O Complexity}},
booktitle = {27th International Conference on Database Theory (ICDT 2024)},
pages = {21:1--21:20},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-312-6},
ISSN = {1868-8969},
year = {2024},
volume = {290},
editor = {Cormode, Graham and Shekelyan, Michael},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ICDT.2024.21},
URN = {urn:nbn:de:0030-drops-198033},
doi = {10.4230/LIPIcs.ICDT.2024.21},
annote = {Keywords: Subgraph Enumeration, Conjunctive Queries, External Memory, Algorithms}
}