Abstract
We study languages for manipulating partially ordered structures with duplicates (e.g. trees, lists). As a general framework, we consider the pomset (partially ordered multiset) datatype. We introduce an algebra for pomsets, which generalizes traditional algebras for (nested) sets, bags and lists. This paper is motivated by the study of the impact of different language primitives on the expressive power. We show that the use of partially ordered types increases the expressive power significantly. Surprisingly, it turns out that the algebra when restricted to both unordered (bags) and totally ordered (lists) intermediate types, yields the same expressive power as fixpoint logic with counting on relational databases. It therefore constitutes a rather robust class of relational queries. On the other hand, we obtain a characterization of PTIME queries on lists by considering only totally ordered types.
Work supported in part by the Esprit Project BRA FIDE 2. Work was done while the second author was visiting University of Toronto, and supported by the Institute for Robotics and Intelligent Systems.
The authors wish to thank Vassos Hadzilacos for technical remarks.
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Grumbach, S., Milo, T. (1995). An algebra for pomsets. In: Gottlob, G., Vardi, M.Y. (eds) Database Theory — ICDT '95. ICDT 1995. Lecture Notes in Computer Science, vol 893. Springer, Berlin, Heidelberg. https://doi.org/10.1007/3-540-58907-4_16
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DOI: https://doi.org/10.1007/3-540-58907-4_16
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