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Compression of Unordered XML Trees

Authors Markus Lohrey, Sebastian Maneth, Carl Philipp Reh

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Markus Lohrey
Sebastian Maneth
Carl Philipp Reh

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Markus Lohrey, Sebastian Maneth, and Carl Philipp Reh. Compression of Unordered XML Trees. In 20th International Conference on Database Theory (ICDT 2017). Leibniz International Proceedings in Informatics (LIPIcs), Volume 68, pp. 18:1-18:17, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2017)
https://doi.org/10.4230/LIPIcs.ICDT.2017.18

Abstract

Many XML documents are data-centric and do not make use of the inherent document order. Can we provide stronger compression for such documents through giving up order? We first consider compression via minimal dags (directed acyclic graphs) and study the worst case ratio of the size of the ordered dag divided by the size of the unordered dag, where the worst case is taken for all trees of size n. We prove that this worst case ratio is n / log n for the edge size and n log log n / log n for the node size. In experiments we compare several known compressors on the original document tree versus on a canonical version obtained by length-lexicographical sorting of subtrees. For some documents this difference is surprisingly large: reverse binary dags can be smaller by a factor of 3.7 and other compressors can be smaller by factors of up to 190.
Keywords
  • tree compression
  • directed acyclic graphs
  • XML

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