Thank you.  I see it on the wikipedia page for map reduce.   

On Monday, January 14, 2019 at 5:12:37 PM UTC-6, Oscar Boykin wrote:
>
> This is a consequence of how map-reduce works. It does hash partitioning, 
> but then on each reducer the keys are sorted before it begins the reducing. 
> It does not do hash partitioning *within* reducers.
>
> On Mon, Jan 14, 2019 at 12:49 PM Russell Carden <[email protected] 
> <javascript:>> wrote:
>
>> I do a sort by grouping on a key and then doing a sortby on the values.  
>> I then write out the results.   Since I have multiple reducers, I will have 
>> multiple files.   I have observed that in the output for each reducer, the 
>> keys are also sorted.  That is not only are the values within each group 
>> sorted, the groups themselves are sorted in each file.   Is this a 
>> consequence of how the results for each group are merged within a reducer?
>>
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