maropu commented on issue #27077: [SPARK-30408][SQL] Should not remove orderBy 
in sortBy clause in Optimizer
URL: https://github.com/apache/spark/pull/27077#issuecomment-571891836
 
 
   > The content of sortData should be same as groupData.
   
   I might miss your point, but why do you think so?
   It seems the plans below looks ok to me;
   ```
   scala> groupData.explain(true)
   == Parsed Logical Plan ==
   'Sort ['b ASC NULLS FIRST], true
   +- Project [_1#3 AS a#10, _2#4 AS b#11, _3#5 AS c#12]
      +- LocalRelation [_1#3, _2#4, _3#5]
   
   == Analyzed Logical Plan ==
   a: string, b: int, c: int
   Sort [b#11 ASC NULLS FIRST], true
   +- Project [_1#3 AS a#10, _2#4 AS b#11, _3#5 AS c#12]
      +- LocalRelation [_1#3, _2#4, _3#5]
   
   == Optimized Logical Plan ==
   Sort [b#11 ASC NULLS FIRST], true
   +- LocalRelation [a#10, b#11, c#12]
   
   == Physical Plan ==
   *(1) Sort [b#11 ASC NULLS FIRST], true, 0  /// <==== global sort
   +- Exchange rangepartitioning(b#11 ASC NULLS FIRST, 200), true, [id=#7]
      +- LocalTableScan [a#10, b#11, c#12]
   
   
   scala> sortData.explain(true)
   == Parsed Logical Plan ==
   'Sort ['c ASC NULLS FIRST], false
   +- Sort [b#11 ASC NULLS FIRST], true
      +- Project [_1#3 AS a#10, _2#4 AS b#11, _3#5 AS c#12]
         +- LocalRelation [_1#3, _2#4, _3#5]
   
   == Analyzed Logical Plan ==
   a: string, b: int, c: int
   Sort [c#12 ASC NULLS FIRST], false
   +- Sort [b#11 ASC NULLS FIRST], true
      +- Project [_1#3 AS a#10, _2#4 AS b#11, _3#5 AS c#12]
         +- LocalRelation [_1#3, _2#4, _3#5]
   
   == Optimized Logical Plan ==
   Sort [c#12 ASC NULLS FIRST], false
   +- LocalRelation [a#10, b#11, c#12]
   
   == Physical Plan ==
   *(1) Sort [c#12 ASC NULLS FIRST], false, 0 /// <==== local sort
   +- *(1) LocalTableScan [a#10, b#11, c#12]
   ```

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