Hadoop combiners with aggregator functions

Question

When using one of the aggregator function in place of a reducer, will Hadoop be smart enough to use same function for combiner phase?

Answer 1

They fulfill partly the same purpose but the aggregator is more generic and can be used in cases where the combiner can not.

So it's a definite no, the aggregator will not be used as a combiner automatically. If you want it to be used as a combiner you will have to specify it as such.

Quote from cascade, "Combiners are limited to Associative and Commutative functions only, like 'sum' and 'max'. And in order to work, values emitted from the Map task must be serialized, sorted (deserialized and compared), deserialized again and operated on"