leanken edited a comment on pull request #29304:
URL: https://github.com/apache/spark/pull/29304#issuecomment-667783096


   
![image](https://user-images.githubusercontent.com/17242071/89143099-fb2dd500-d57b-11ea-881e-9d248403db9d.png)
   
   this is quite the same with expansion, first it go through all the data in 
buildSide, to gather information about which column might have exists with null 
values, and let's say there are c1 c2 c3 in buildSide, after scan, found that 
only c1 c2 with null values
   
   then left record (1, 2, 3) will try to found match (1, null, 3) (null, 2, 3) 
(null, null, 3)
   
   And what i am trying to do is that I will not scan the buildSide to gather 
null information, I just assume that every column might have the null value, 
and with all combination of null padding.
   


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