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From David Ortiz <>
Subject Re: PGroupedTable.combineValues should allow composition with PGroupedTable.mapValues
Date Tue, 17 May 2016 13:15:22 GMT
You can still do parallelDo on a PGroupedTable to allow it to map to a
different type.  Just would be new DoFn<Pair<Key, Set<String>>, Pair<Key,

On Tue, May 17, 2016 at 2:01 AM Stan Rosenberg <>

> Hi,
> I couldn't seem to find sufficient documentation or examples of using
> combiners in non-trivial ways. Say my map emits values of type Set<String>;
> after grouping by key I want to emit the _size_ of the union of the sets of
> strings, i.e., size(union(Iterable<Set<String>>))  Thus, the combiner's
> type is Iterable<Set<String>> -> Set<String> but the reduce's type
> Iterable<Set<String>> -> Int
> To my knowledge, both MapReduce and Spark allow a combiner to have a
> result type different from reducer's.  However, unless I missed something,
> this is not expressible in Crunch.  Shouldn't PGroupedTable.combineValues
> return PGroupedTable to allow composition with mapValues?
> Thanks,
> stan

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