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Plan space analysis: an early warning system to detect plan regressions in cost-based optimizers

Published: 13 June 2011 Publication History

Abstract

Plan regressions pose a significant problem in commercial database systems: Seemingly innocuous changes to a query optimizer component such as the cost model or the search strategy in order to enhance optimization results may result in unexpected and detrimental changes to previously satisfactory query plans.
Database vendors spend substantial resources on quality assurance to guard against this very issue, yet, testing for plan regressions in optimizers has proven hard and inconclusive. This is due to the nature of the problem: the optimizer chooses a single plan---Best Plan Found (bpf)---from a search space of literally up to hundreds of millions of different plan alternatives. It is standard practice to use a known good bpf and test for changes to this plan, i. e., ensure that no changes have occurred. However, in the vast majority of cases the bpf is not be affected by a code-level change, even though the change is known to affect many plans in the search space.
In this paper, we propose a holistic approach to address this issue. Instead of focusing on test suites consisting of BPFS we take the entire search space into account. We introduce a metric to assess the optimizer's accuracy across the entire search space.
We present preliminary results using a commercial database system, demonstrate the usefulness of our methodology with a standard benchmark, and illustrate how to build such an early warning system.

References

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M. Elhemali and L. Giakoumakis. Unit Testing Query Transformation Rules. In Proc. Int'l. Workshop on Database Testing (DBTest), 2008.
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H. G. Elmongui, V. Narasayya, and R. Ramamurthy. A Framework for Testing Query Transformation Rules. In Proc. Int'l. Conf. ACM SIGMOD, 2009.
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L. Giakoumakis and C. Galindo-Legaria. Testing SQL Servers Query Optimizer: Challenges, Techniques and Experiences. IEEE Data Engineering Bulletin, 31(1), 2008.
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G. Graefe, A.-C. König, H. Kuno, V. Markl, and K.-U. Sattler. Robust Query Processing. Technical Report Workshop 10381, Dagstuhl, 2010.
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J. S. Maritz. Distribution-Free Statistical Methods. Chapman & Hall, 1981.
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N. Reddy and J. Haritsa. Analyzing Plan Diagrams of Database Query Optimizers. In Proc. Int'l. Conf. on Very Large Databases, 2005.
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F. Waas and C. A. Galindo-Legaria. Counting, Enumerating, and Sampling of Execution Plans in a Cost-Based Query Optimizer. In Proc. Int'l. Conf. ACM SIGMOD, 2000.

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cover image ACM Conferences
DBTest '11: Proceedings of the Fourth International Workshop on Testing Database Systems
June 2011
51 pages
ISBN:9781450306553
DOI:10.1145/1988842
Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. Copyrights for components of this work owned by others than ACM must be honored. Abstracting with credit is permitted. To copy otherwise, or republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee. Request permissions from [email protected]

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Published: 13 June 2011

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