HeartSaVioR commented on issue #23634: [SPARK-26154][SS] Streaming left/right 
outer join should not return outer nulls for already matched rows
URL: https://github.com/apache/spark/pull/23634#issuecomment-457789591
 
 
   > In update mode it may be ok to emit null value for one side and later when 
the matching events arrive on the other side the new rows be re-emitted.
   
   In this case it produces incorrect result, because matched row will be 
emitted first, and null-matched row will be emitted later, which may 
"overwrite" the result and treat the final result as null-matched.
   
   IMHO regardless of modes, the final result should be same per key, same 
result between batch and streaming. Suppose the query is running as a batch 
query, then null-matched row will never be produced. So I'm not sure this is 
related to output mode.

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