Github user hequn8128 commented on a diff in the pull request:

    https://github.com/apache/flink/pull/3733#discussion_r112817995
  
    --- Diff: 
flink-libraries/flink-table/src/main/scala/org/apache/flink/table/runtime/aggregate/GroupAggProcessFunction.scala
 ---
    @@ -84,16 +109,38 @@ class GroupAggProcessFunction(
         }
     
         // Set aggregate result to the final output
    -    i = 0
    -    while (i < aggregates.length) {
    -      val index = groupings.length + i
    -      val accumulator = accumulators.getField(i).asInstanceOf[Accumulator]
    -      aggregates(i).accumulate(accumulator, 
input.getField(aggFields(i)(0)))
    -      output.setField(index, aggregates(i).getValue(accumulator))
    -      i += 1
    +    if (input.command == Command.Delete) {
    +      i = 0
    +      while (i < aggregates.length) {
    +        val index = groupings.length + i
    +        val accumulator = 
accumulators.getField(i).asInstanceOf[Accumulator]
    +        aggregates(i).retract(accumulator, input.getField(aggFields(i)(0)))
    +        output.setField(index, aggregates(i).getValue(accumulator))
    +        i += 1
    +      }
    +    } else {
    +      i = 0
    +      while (i < aggregates.length) {
    +        val index = groupings.length + i
    +        val accumulator = 
accumulators.getField(i).asInstanceOf[Accumulator]
    +        aggregates(i).accumulate(accumulator, 
input.getField(aggFields(i)(0)))
    +        output.setField(index, aggregates(i).getValue(accumulator))
    +        i += 1
    +      }
         }
    -    state.update(accumulators)
     
    +    // if previous is not null, do retraction process
    +    if (null != previous) {
    +      if (previous.equals(output)) {
    +        // ignore same output
    +        return
    --- End diff --
    
    hi, we need not to output a same row, think about batch, only one result 
will be output, is it right?


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