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https://issues.apache.org/jira/browse/FLINK-6091?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15980126#comment-15980126
 ] 

ASF GitHub Bot commented on FLINK-6091:
---------------------------------------

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

    https://github.com/apache/flink/pull/3733#discussion_r112817984
  
    --- Diff: 
flink-libraries/flink-table/src/main/scala/org/apache/flink/table/runtime/aggregate/GroupAggProcessFunction.scala
 ---
    @@ -68,12 +70,35 @@ class GroupAggProcessFunction(
         var accumulators = state.value()
     
         if (null == accumulators) {
    +      previous = null
           accumulators = new Row(aggregates.length)
           i = 0
           while (i < aggregates.length) {
             accumulators.setField(i, aggregates(i).createAccumulator())
             i += 1
           }
    +    } else {
    +      // get previous row
    +      if (generateRetraction) {
    +        if (null == previous) {
    +          previous = new Row(groupings.length + aggregates.length)
    --- End diff --
    
    done


> Implement and turn on the retraction for aggregates
> ---------------------------------------------------
>
>                 Key: FLINK-6091
>                 URL: https://issues.apache.org/jira/browse/FLINK-6091
>             Project: Flink
>          Issue Type: Sub-task
>          Components: Table API & SQL
>            Reporter: Shaoxuan Wang
>            Assignee: Hequn Cheng
>
> Implement functions for generating and consuming retract messages for 
> different aggregates. 
> 1. add delete/add property to Row
> 2. implement functions for generating retract messages for unbounded groupBy
> 3. implement functions for handling retract messages for different aggregates.
> 4. handle retraction messages in CommonCorrelate and CommonCalc (retain 
> Delete property).
> Note: Currently, only unbounded groupby generates retraction and it is 
> working under unbounded and processing time mode. Hence, retraction is only 
> supported for unbounded and processing time aggregations so far. We can add 
> more retraction support later.
> supported now: unbounded groupby, unbounded and processing time over window
> unsupported now: group window, event time or bounded over window.



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