[GitHub] flink issue #4350: [FLINK-7204] [core] CombineHint.NONE

[fhueske <gi...@git.apache.org>]

Github user fhueske commented on the issue:

    https://github.com/apache/flink/pull/4350
  
    Hi @greghogan, I think this is definitely a valuable improvement.
    However, I looked at the plan of a query with a `ReduceFunction` with `CombineHint.None` and found that it still has a combine node:
    
    ```
    [IN] -FWD-> [COMBINE(SORTED_REDUCE)] -HASH-> [REDUCE(SORTED_REDUCE)] -> [OUT]
    ```
    
    I think we should
    1. set `combinerStrategy` to `DriverStrategy.NONE` in `ReduceNode` for `CombineHint.NONE`.
    2. change `ReduceProperties.instantiate()` with a check for `combinerStrategy == NONE` and not add a node for the combiner in that case.
    3. add a test in `ReduceCompilationTest` similar to `testGroupedReduceWithHint()` to check that the resulting plan has no combine node.
    
    Best, Fabian


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