c21 commented on pull request #30395:
URL: https://github.com/apache/spark/pull/30395#issuecomment-729427520


   Just to add my thought, and would like to get opinions from previous authors 
and reviewers in this area for sure.
   
   * In terms of scalability, reliability, etc, full outer join stores same 
amount of data in state store, compared to left outer/right outer/inner join. 
The only difference is full outer join would output more rows (all rows from 
both stream sides). So I think from system perspective, we probably can support 
it, similar to other joins.
   
   * The motivation from my side is that, I am investigating the performance 
difference between spark structured streaming (micro-batch) vs other internal 
streaming systems, and plan to add some more stuff on top of it to support 
internal use cases. So it would be good to have a basic full outer join 
implementation here (merge to upstream instead of forking for each spark 
version upgrade for me).


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