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https://issues.apache.org/jira/browse/FLINK-925?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14029109#comment-14029109 ]
Stephan Ewen commented on FLINK-925:
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I guess that is fine. I assume KeySelectors will become less important, once the expression language is in place, but it is a useful fallback.
+1
> 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
> Priority: Minor
>
> 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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