EmmyMiao87 commented on issue #1725: The null result of union stmt is incorrect
URL: 
https://github.com/apache/incubator-doris/issues/1725#issuecomment-529177921
 
 
   ISSUES-1725: The result of union stmt whose child is outer join stmt is 
incorrect.
   
   Example:
   sql: (select k1 from empty) union all (select b.k1 k1 from left_table a left 
join empty  b on a.k2 = b.k2);
   context: the empty table has no data.
   error result: 0
   expect result: null
   
   Reason:
   The judgment (columns k1 who belongs to union tuple is nullalbe ) is 
incorrect.
   It could not be determinated by slot attribute of children when the slot is 
producted by the outer join.
   The slot A is not nullalbe while the result of outer join is nullable which 
is same as slot A.
   So, the judgment needs to consider if the slot comes from the outer join.

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