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For these two options, there is a common limitation, which is the number of splits should not be larger than the number of key groups. Because finally, we want to build a mapping from a split to a key group to ensure correctness during rescaling.

Compatibility, Deprecation, and Migration Plan

This improvement

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will not impact

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the current users.

Test Plan

Additional unit tests will be added to test if the API works well when the input streaming is "Pre-KeyedStream". 

Rejected Alternatives

No option is rejected yey.