Github user dawidwys commented on the issue:
https://github.com/apache/flink/pull/4153
@dianfu sorry I have not reviewed it yet, but I do think this feature would
benefit from reworking of the Pattern API I propose in
[FLINK-3414](https://issues.apache.org/jira/browse/FLINK-3414).
Instead of checking for head/tail of group pattern and caching them, we
could more or less use current code for plain sequence creation that could
return begin/end states. Then the code for joining groups would be much easier,
cause it would operate on already translated sequences.
Also the new API would make
[FLINK-4641]](https://issues.apache.org/jira/browse/FLINK-4641)
much easier I think.
As it would require API rework I would really like to hear @kl0u opinion.
If we agree though not to change the API I will go straight to reviewing this
PR.
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