Dandandan edited a comment on pull request #1143:
URL: https://github.com/apache/arrow-datafusion/pull/1143#issuecomment-946377551


   In Spark, repartition is using `coalesce` by setting parameter 
`shuffle=true`.
   I think it might be cleaner to keep the `RepartitionExec` and 
`CoalescePartitionsExec` separated, otherwise you get two implementations in 
the same code without too much sharing?
   
   For implementing `CoalescePartitionsExec` we just have to have a scheme that 
combines partitions within `execute` (e.g. when reducing the number of 
partitions from 8 to 4 we can return partitions 0,1 for `execute(0)` 2,3 for 
`execute(1)` etc.
   For Ballista, we have to now (explicitly or implicitly) what partitions are 
living on what node to avoid shuffles.
   


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