Question

Will it leave one remained and delete the others, or pass them all into mapper and reducer?

Answer 1

Same KeyValue Pair

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Map Phase

Since Key-Value pairs are independent of each other , so Mapper will never look/know for a identical Key-Value Pair

ex:

key  value
1       2
1       2
2       5
3       19


map(k,v)
{
 emit(k,v)
}

emit: 1,2 1,2 2,5 3,19

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Reduce Phase

Identical Key-Value pair are handled by sorting the value on Key , so the value associated with the Key is ignored ,so each value is treated as unique.

ex:

key    value 
1      {2,2}
2      {5}
3      {19}

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