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https://issues.apache.org/jira/browse/FLINK-925?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14045758#comment-14045758
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Fabian Hueske commented on FLINK-925:
-------------------------------------
Using a CustomComparable is not supported right now, however the second
example, i.e., grouping a Tuple2<MyComparable, Integer> on the Integer field,
should work. Not sure why it fails in this situation, but in principle it
should be possible.
For this issue, I would focus on enabling KeySelectors that return Tuples of
types which are supported for grouping such as
{code}
KeySelector<Tuple3<Integer, String, Double>, Tuple2<Integer, String>>
{code}
> Support KeySelector function returning Tuples
> ---------------------------------------------
>
> Key: FLINK-925
> URL: https://issues.apache.org/jira/browse/FLINK-925
> Project: Flink
> Issue Type: Improvement
> Affects Versions: 0.6-incubating
> Reporter: Fabian Hueske
> Assignee: Tobias
> Priority: Minor
> Labels: starter
>
> KeySelector functions are used to extract keys on which DataSets can be
> grouped or joined.
> Currently, the keys types returned by KeySelector function are restricted to
> be comparable. However, Flinks Tuple data types are not comparable (because
> this depends on the types of its fields) which makes grouping and joining on
> composite keys difficult.
> We should change the signature of the groupBy(), join(), and coGroup()
> methods to allow also non-comparable keys as return types of a KeySelector
> function.
> Instead we will check at optimization time whether the returned type is
> comparable (which is true for tuples if all elements are comparable).
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