How to write a retract table to Kafka?

[Izual]

Hi community:

SQL contains aggregate functions, GROUP BY, etc，will generate a RetractStream, which type is DataStream[(Boolean, Row)].

It's not allowed to write to Kafka because kafka table is based on AppendStreamTableSink.

If I only need to write ADD message to Kafka, is it possible to achieve this only by SQL?

What is the classical usage for this? 

tableEnv.toRetractStream[Row](...).filter(_._1).map(_._2).addSink(new KafkaProducer)

or

How to convert a RetractStream to a DataStream, then use SQL to output to Kafka?