cloud-fan commented on issue #25295: [SPARK-28560][SQL] Optimize shuffle reader 
to local shuffle reader when smj converted to bhj in adaptive execution
URL: https://github.com/apache/spark/pull/25295#issuecomment-530372266
 
 
   I think this is a good idea, but the implementation needs more polishing. 
What I expect to see is:
   1. a rule to convert `ShuffleQueryStageExec` to `LocalShuffleReaderExec`. 
This rule must be run before `ReduceNumShufflePartitions`.
   2. A special `ShuffledRDD` that reads shuffle blocks grouped by mapId 
instead of reduceId.
   
   I'm a little worried to see the invasive changes in the underlying shuffle 
component. Can you briefly explain how your special  `ShuffledRDD` is 
implemented?

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