An empirical evaluation of cost-based federated SPARQL query Processing Engines
Finding a good query plan is key to the optimization of query runtime. This holds in particular for cost-based federation engines, which make use of cardinality estimations to achieve this goal. A number of studies compare SPARQL federation engines across different performance metrics, including query runtime, result set completeness and correctness, number of sources selected and number of requests sent. Albeit informative, these metrics are generic and unable to quantify and evaluate the accuracy of the cardinality estimators of cost-based federation engines. To thoroughly evaluate cost-based federation engines, the effect of estimated cardinality errors on the overall query runtime performance must be measured. In this paper, we address this challenge by presenting novel evaluation metrics targeted at a fine-grained benchmarking of cost-based federated SPARQL query engines. We evaluate five cost-based federated SPARQL query engines using existing as well as novel evaluation metrics by using LargeRDFBench queries. Our results provide a detailed analysis of the experimental outcomes that reveal novel insights, useful for the development of future cost-based federated SPARQL query processing engines.
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An empirical evaluation of cost-based federated SPARQL query Processing Engines
- 1. An Empirical Evaluation of Cost-based Federated SPARQL Query Processing Engines Umair Qudus Muhammad Saleem Axel-Cyrille Ngonga Ngomo Young-koo Lee
- 2. INTRODUCTION • Finding a good query plan is of key step of the optimization of query runtime. • Different metrics proposed to measure the quality of query plan, including query runtime, result set completeness and correctness, number of sources selected and number of requests sent. • Although informative, these metrics are generic and unable to quantify and evaluate the accuracy of the cardinality estimators of the cost-based federation engines. • We present a novel evaluation metrics targeted at a fine-grained benchmarking of cost-based federated SPARQL query engines
- 4. We need methods to measure the quality of cost estimations for better query planning. Motivation (2)
- 5. RELATED WORK
- 7. METRICS:
- 8. Definitions (1) • q-error: • Example – Cr(TP1):100 Ce(TP1) = 90 – q-error = max(90/100,100/90) = 1.11 – q-error of all TPs = max(1.11,1.25,1) = 1.25 – q-error of whole plan(TPs+Joins) = max(1.11,1.25,1,1.3,3) = 3.
- 9. Definitions (2) • Proposed Similarity Error: • real = (100,200,300,50,50) estimated = (90,250,300,65,150) Ep(engine 1) = 2*0.1391 = 0.2784 Ep(engine2) = 2*0.3838 = 0.7676
- 11. Experimental Settings • Federated Query Engines – CostFed – SPLENDID – SemaGrow – LHD – Odyssey • Queries and datasets – FedBench and LargeRDFBench benchmarks. 13 Virtuoso endpoints. • Technical Specifications: Each Virtuoso was deployed on a physical machine (32 GB RAM, Core i7 processor and 500 GB hard disc). We ran the selected federation engines on a local client machine with same specifications
- 12. Overall Plan Error (Similarity Error vs. q-error)
- 13. Join Error (Similarity Error vs. q-error)
- 14. Triple pattern error (Similarity Error vs. q-error)
- 15. Correlating metrics with runtime
- 20. Conclusion • Positive correlation with the runtimes. • The higher coefficients (R values) with cosine-based errors as compared to q-error. • The smaller p-values of the cosine-based errors as compared to q- error. • Joins has higher correlation to runtimes as compared to the error in the cardinality estimation of triple patterns. • On average, the CostFed engine produce the smallest estimation errors and has the smallest execution time for majority of the LargeRDFBench queries.
- 21. Twitter: @UQudus Paper Link: http://www.semantic-web-journal.net/system/files/swj2604.pdf https://dice-research.org/UmairQudus