关于在hadoop中，如何让reduce阶段同一个key下的values有序，一篇很好的文章，写的比《Hadoop权威指南》清楚！

转载自：

http://www.bigdataspeak.com/2013/02/hadoop-how-to-do-secondary-sort-on_25.html

The problem at hand here is that you need to work upon a sorted values set in your reducer.[......]

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