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(k2, [v3])
Merge: ((k2, [v3]), (k3, [v4])) → (k4, [v5])
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Communications of the ACM
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http://www.tpc.org/tpch/results/tpch_result_detail.asp?id=107061802
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http://cs.baylor.edu/~speegle/5335/2007slides/MapReduceMerge.pdf