In support of the chosen answer I add:
Following on from this explanation
**Input**:
symbol time price
a 1 10
a 2 20
b 3 30
**Map output**: create composite key\values like so:
> symbol-time time-price
>
>**a-1** 1-10
>
>**a-2** 2-20
>
>**b-3** 3-30
The Partitioner: will route the a-1 and a-2 keys to the same reducer despite the keys being different. It will also route the b-3 to a separate reducer.
GroupComparator: once the composites key\value arrive at the reducer instead of the reducer getting
>(**a-1**,{1-10})
>
>(**a-2**,{2-20})
the above will happen due to the unique key values following composition.
the group comparator will ensure the reducer gets:
(a-1,{**1-10,2-20**})
The key of the grouped values will be the one which comes first in the group. This can be controlled by Key comparator.
**[[In a single reduce method call.]]**