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From Gabriel Reid <>
Subject Re: PGroupedTable.combineValues question
Date Fri, 05 Apr 2013 15:48:55 GMT
Hi Dave,

On Fri, Apr 5, 2013 at 5:05 PM, Dave Beech <> wrote:
> I have a PGroupedTable<A,B> and I want to aggregate / combine the values
> to produce a PCollection<C> - in other words, I need the type of the
> aggregate to be different to the original value type.
> What's the best approach? The combineValues method takes either an
> Aggregator or a CombineFn but as far as I can see, both of these assume the
> end result will be of the same type as the values.
The approach that I always use for this is just creating a custom DoFn to
operate on the PGroupedTable and construct the instance of type C based in
the incoming Iterable fromt he PGroupedTable. This basically works out to
the same as a Aggregator.

I don't think that this scenario would be technically applicable to a
CombineFn, because the CombineFn can be called any number of times on an
incoming set of values, on both the map and reduce sides of a job. In order
to map values to another type, the intermediate value of type C would
somehow need to be given to the CombineFn each time it was used.

- Gabriel

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