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Documents authored by Spinrath, Christopher

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DOI: 10.4230/LIPIcs.ICDT.2023.16
Work-Efficient Query Evaluation with PRAMs

Authors: Jens Keppeler, Thomas Schwentick, and Christopher Spinrath

Published in: LIPIcs, Volume 255, 26th International Conference on Database Theory (ICDT 2023)

Abstract
The paper studies query evaluation in parallel constant time in the PRAM model. While it is well-known that all relational algebra queries can be evaluated in constant time on an appropriate CRCW-PRAM, this paper is interested in the efficiency of evaluation algorithms, that is, in the number of processors or, asymptotically equivalent, in the work. Naive evaluation in the parallel setting results in huge (polynomial) bounds on the work of such algorithms and in presentations of the result sets that can be extremely scattered in memory. The paper first discusses some obstacles for constant time PRAM query evaluation. It presents algorithms for relational operators that are considerably more efficient than the naive approaches. Further it explores three settings, in which efficient sequential query evaluation algorithms exist: acyclic queries, semi-join algebra queries, and join queries - the latter in the worst-case optimal framework. Under natural assumptions on the representation of the database, the work of the given algorithms matches the best sequential algorithms in the case of semi-join queries, and it comes close in the other two settings. An important tool is the compaction technique from Hagerup (1992).
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Jens Keppeler, Thomas Schwentick, and Christopher Spinrath. Work-Efficient Query Evaluation with PRAMs. In 26th International Conference on Database Theory (ICDT 2023). Leibniz International Proceedings in Informatics (LIPIcs), Volume 255, pp. 16:1-16:20, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2023)

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@InProceedings{keppeler_et_al:LIPIcs.ICDT.2023.16,
  author =	{Keppeler, Jens and Schwentick, Thomas and Spinrath, Christopher},
  title =	{{Work-Efficient Query Evaluation with PRAMs}},
  booktitle =	{26th International Conference on Database Theory (ICDT 2023)},
  pages =	{16:1--16:20},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-270-9},
  ISSN =	{1868-8969},
  year =	{2023},
  volume =	{255},
  editor =	{Geerts, Floris and Vandevoort, Brecht},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops-dev.dagstuhl.de/entities/document/10.4230/LIPIcs.ICDT.2023.16},
  URN =		{urn:nbn:de:0030-drops-177589},
  doi =		{10.4230/LIPIcs.ICDT.2023.16},
  annote =	{Keywords: PRAM, query evaluation, work-efficient, parallel, acyclic queries, free-connex queries}
}
Document
DOI: 10.4230/LIPIcs.ICDT.2022.8
Rewriting with Acyclic Queries: Mind Your Head

Authors: Gaetano Geck, Jens Keppeler, Thomas Schwentick, and Christopher Spinrath

Published in: LIPIcs, Volume 220, 25th International Conference on Database Theory (ICDT 2022)

Abstract
The paper studies the rewriting problem, that is, the decision problem whether, for a given conjunctive query Q and a set 𝒱 of views, there is a conjunctive query Q' over 𝒱 that is equivalent to Q, for cases where the query, the views, and/or the desired rewriting are acyclic or even more restricted. It shows that, if Q itself is acyclic, an acyclic rewriting exists if there is any rewriting. An analogous statement also holds for free-connex acyclic, hierarchical, and q-hierarchical queries. Regarding the complexity of the rewriting problem, the paper identifies a border between tractable and (presumably) intractable variants of the rewriting problem: for schemas of bounded arity, the acyclic rewriting problem is NP-hard, even if both Q and the views in 𝒱 are acyclic or hierarchical. However, it becomes tractable, if the views are free-connex acyclic (i.e., in a nutshell, their body is (i) acyclic and (ii) remains acyclic if their head is added as an additional atom).
Cite as

Gaetano Geck, Jens Keppeler, Thomas Schwentick, and Christopher Spinrath. Rewriting with Acyclic Queries: Mind Your Head. In 25th International Conference on Database Theory (ICDT 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 220, pp. 8:1-8:20, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)

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@InProceedings{geck_et_al:LIPIcs.ICDT.2022.8,
  author =	{Geck, Gaetano and Keppeler, Jens and Schwentick, Thomas and Spinrath, Christopher},
  title =	{{Rewriting with Acyclic Queries: Mind Your Head}},
  booktitle =	{25th International Conference on Database Theory (ICDT 2022)},
  pages =	{8:1--8:20},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-223-5},
  ISSN =	{1868-8969},
  year =	{2022},
  volume =	{220},
  editor =	{Olteanu, Dan and Vortmeier, Nils},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops-dev.dagstuhl.de/entities/document/10.4230/LIPIcs.ICDT.2022.8},
  URN =		{urn:nbn:de:0030-drops-158829},
  doi =		{10.4230/LIPIcs.ICDT.2022.8},
  annote =	{Keywords: rewriting, acyclic rewriting, acyclic conjunctive queries, free-connex queries, hierarchical queries, NP-hardness}
}
Document
DOI: 10.4230/LIPIcs.ICDT.2019.14
Parallel-Correctness and Parallel-Boundedness for Datalog Programs

Authors: Frank Neven, Thomas Schwentick, Christopher Spinrath, and Brecht Vandevoort

Published in: LIPIcs, Volume 127, 22nd International Conference on Database Theory (ICDT 2019)

Abstract
Recently, Ketsman et al. started the investigation of the parallel evaluation of recursive queries in the Massively Parallel Communication (MPC) model. Among other things, it was shown that parallel-correctness and parallel-boundedness for general Datalog programs is undecidable, by a reduction from the undecidable containment problem for Datalog. Furthermore, economic policies were introduced as a means to specify data distribution in a recursive setting. In this paper, we extend the latter framework to account for more general distributed evaluation strategies in terms of communication policies. We then show that the undecidability of parallel-correctness runs deeper: it already holds for fragments of Datalog, e.g., monadic and frontier-guarded Datalog, with a decidable containment problem, under relatively simple evaluation strategies. These simple evaluation strategies are defined w.r.t. data-moving distribution constraints. We then investigate restrictions of economic policies that yield decidability. In particular, we show that parallel-correctness is 2EXPTIME-complete for monadic and frontier-guarded Datalog under hash-based economic policies. Next, we consider restrictions of data-moving constraints and show that parallel-correctness and parallel-boundedness are 2EXPTIME-complete for frontier-guarded Datalog. Interestingly, distributed evaluation no longer preserves the usual containment relationships between fragments of Datalog. Indeed, not every monadic Datalog program is equivalent to a frontier-guarded one in the distributed setting. We illustrate the latter by considering two alternative settings where in one of these parallel-correctness is decidable for frontier-guarded Datalog but undecidable for monadic Datalog.
Cite as

Frank Neven, Thomas Schwentick, Christopher Spinrath, and Brecht Vandevoort. Parallel-Correctness and Parallel-Boundedness for Datalog Programs. In 22nd International Conference on Database Theory (ICDT 2019). Leibniz International Proceedings in Informatics (LIPIcs), Volume 127, pp. 14:1-14:19, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2019)

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@InProceedings{neven_et_al:LIPIcs.ICDT.2019.14,
  author =	{Neven, Frank and Schwentick, Thomas and Spinrath, Christopher and Vandevoort, Brecht},
  title =	{{Parallel-Correctness and Parallel-Boundedness for Datalog Programs}},
  booktitle =	{22nd International Conference on Database Theory (ICDT 2019)},
  pages =	{14:1--14:19},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-101-6},
  ISSN =	{1868-8969},
  year =	{2019},
  volume =	{127},
  editor =	{Barcelo, Pablo and Calautti, Marco},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops-dev.dagstuhl.de/entities/document/10.4230/LIPIcs.ICDT.2019.14},
  URN =		{urn:nbn:de:0030-drops-103165},
  doi =		{10.4230/LIPIcs.ICDT.2019.14},
  annote =	{Keywords: Datalog, distributed databases, distributed evaluation, decision problems, complexity}
}
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