Github user chiwanpark commented on the pull request:
https://github.com/apache/flink/pull/572#issuecomment-94188129
Hi. I updated this PR. The changes are following.
* Re-implement this feature with generalizing `ExpressionKeys`.
* Modify `CoGroupOperatorBase`, `GroupCombineOperatorBase` and
`GroupReduceOperatorBase` to allow use of AtomicType as Key.
* Add unit tests to test invalid usage.
Because a mention for wildcard expression with atomic type exists already
in documentation. I didn't modify documentation.
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