Why a GroupBy Operator need Two MapReduce jobs to solved?

[Alexis De La Cruz Toledo]

Hi! I have a doubt, Why a GroupBy Operator is solved
in two MapReduce Job.
1. First the aggregation functions(sum(), count(), avg(), max(), etc) are
solved partial
2. After in another MapReduce Job the aggregation function is final.
Why?

Thanks.
Regards

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Ing. Alexis de la Cruz Toledo.
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