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From Keith Turner <ke...@deenlo.com>
Subject Re: org.apache.accumulo.core.iterators.Combiner: key scope?
Date Mon, 19 Mar 2012 20:35:24 GMT
On Mon, Mar 19, 2012 at 4:09 PM, Aaron Cordova <aaron@cordovas.org> wrote:
> The original semantics of reduce() from lisp is to compact everything down into one object
.. but the original MapReduce semantics allow reduce and map functions to emit() as many new
KV pairs as one desires. To bring Accumulo's reduce() function closer to the usage of MapReduce's
reduce() might not introduce a huge amount of cognitive load on users, especially if they
are coming from the MapReduce world.

Another thing that map reduce allows is for a reducer to emit zero KV.
 Users have asked if this was possible in a combiner/aggregator
before, the ability to filter.  Allowing a combiner to do this can be
more efficient than a Combiner+Filter, because the Filter may need to
redo computation that the Combiner just did inorder to make a
decision.

Keith

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