[jira] [Created] (BEAM-1956) Flatten operation should respect input type hints.

["Vikas Kedigehalli (JIRA)" <ji...@apache.org>]

Vikas Kedigehalli created BEAM-1956:
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             Summary: Flatten operation should respect input type hints.
                 Key: BEAM-1956
                 URL: https://issues.apache.org/jira/browse/BEAM-1956
             Project: Beam
          Issue Type: Bug
          Components: sdk-py
            Reporter: Vikas Kedigehalli
            Assignee: Vikas Kedigehalli
             Fix For: First stable release


Input type hints are currently not respected by the Flatten operation and instead `Any` type is chosen as a fallback. This could lead to using a pickle coder even if there was a custom coder type hint provided for input PCollections. 

Also, this could lead to undesirable results, particularly, when a Flatten operation is followed by a GroupByKey operation which requires the key coder to be deterministic. Even if the user provides deterministic coder type hints to their PCollections, defaulting to Any would result in using the pickle coder (non-deterministic). As a result of this, CoGroupByKey is broken in such scenarios where input PCollection coder is deterministic for the type while pickle coder is not.   



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