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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