Will already sorted Mapper output improve speed of Sort in reducer?

[Le Zhao]

Hi,

Does anybody know whether sorted Mapper output will decrease the Sort in 
the reduce phase?

I'm teaching a class, and am curious to know how much of a difference 
will sorted vs. unsorted mapper output be.  If the merge sort is 
implemented to deal with already sorted input, then I guess it will be 
fast.  Am I right?

Thanks,
Le