hadoop - How to efficiently find top-k elements? -

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i have big sequence file storing tfidf values documents. each line represents line , columns value of tfidfs each term (the row sparse vector). i'd pick top-k words each document using hadoop. naive solution loop through columns each row in mapper , pick top-k file becomes bigger , bigger don't think solution. there better way in hadoop?

 1. in every map calculate topk (this local top k each map)  2. spawn signle reduce , top k mappers flow reducer , hence global top k evaluated.

think of problem 

 1. have been given results of x number of horse races.   2. need find top n fastest horse.

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