[Question][Kafka SQL Connector] When will decoding format emits multiple keys for a Kafka key; why do a cartesian product.

[Yufei Zhang]

I was reading the source code of Kafka SQL Source Connector, I noticed that
in  DynamicKafkaDeserializationSchema[1], when the schema emits multiple
keys, the code is doing a cartesian product of the key rows and value rows.
I know that in CDC, a format can emit multiple rows (UPDATE_BEFORE and
UPDATE_AFTER rows) for a single message, but I'm wondering
1. In what case will a key emits multiple rows?
2. what does the cartesian product of key and value rows represent? (if
there is only 1 keyRow, then it make sense, but when both keyRows and
valueRows have more than 1, I failed to infer the possible use case)

```

// otherwise emit a value for each key
for (RowData physicalKeyRow : physicalKeyRows) {
    emitRow((GenericRowData) physicalKeyRow, (GenericRowData) physicalValueRow);
}


I searched in the docs but failed to understand the design here.  Any hints
would be appreciated~ Thanks~







[1]
https://github.com/apache/flink/blob/1fc26192cf794961af4f6933c155daf86eb8a0fe/flink-connectors/flink-connector-kafka/src/main/java/org/apache/flink/streaming/connectors/kafka/table/DynamicKafkaDeserializationSchema.java#L224


Cheers,
Affe