Github user wuchong commented on a diff in the pull request:

    https://github.com/apache/flink/pull/3266#discussion_r99987780
  
    --- Diff: 
flink-libraries/flink-table/src/main/scala/org/apache/flink/table/plan/nodes/dataset/DataSetWindowAggregate.scala
 ---
    @@ -271,19 +269,46 @@ class DataSetWindowAggregate(
               isInputCombined = true)
     
             mappedInput
    -          .groupBy(groupingKeys: _*)
    -          .sortGroup(rowTimeFieldPos, Order.ASCENDING)
    -          .combineGroup(combineGroupFunction)
    -          .groupBy(groupingKeys: _*)
    -          .sortGroup(windowStartPos, Order.ASCENDING)
    -          .sortGroup(windowEndPos, Order.ASCENDING)
    -          .reduceGroup(groupReduceFunction)
    -          .returns(rowTypeInfo)
    -          .name(aggregateOperatorName)
    -          .asInstanceOf[DataSet[Any]]
    +        .groupBy(groupingKeys: _*)
    +        .sortGroup(rowTimeFieldPos, Order.ASCENDING)
    +        .combineGroup(combineGroupFunction)
    +        .groupBy(groupingKeys: _*)
    +        .sortGroup(windowStartPos, Order.ASCENDING)
    +        .sortGroup(windowEndPos, Order.ASCENDING)
    +        .reduceGroup(groupReduceFunction)
    +        .returns(rowTypeInfo)
    +        .name(aggregateOperatorName)
    +        .asInstanceOf[DataSet[Any]]
    +
    +      } else {
    +        // non-grouping window
    +        val preMapPartitionFunction = 
createDataSetWindowAggregationMapPartitionFunction(
    +          window,
    +          namedAggregates,
    +          inputType)
    +
    +        val mapPartitionFunction = 
createDataSetWindowAggregationMapPartitionFunction(
    +          window,
    +          namedAggregates,
    +          inputType,
    +          rowRelDataType,
    +          namedProperties,
    +          isPreMapPartition = false,
    +          isInputCombined = true)
    +
    +        mappedInput.sortPartition(rowTimeFieldPos, 
Order.ASCENDING).setParallelism(1)
    +        .mapPartition(preMapPartitionFunction).setParallelism(1)
    --- End diff --
    
    Please indent every operator method in a single line.
    
    I think the first `sortPartition` and the first `mapPartition` can be run 
in parallel, do not need to set `setParallelism(1)`. Otherwise, the performance 
will be poorer than the approach of non-incremental agg & non-grouping. 
    
    What do you think ?   


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