jackwener opened a new pull request, #7346:
URL: https://github.com/apache/arrow-datafusion/pull/7346

   ## Which issue does this PR close?
   
   Closes #.
   
   ## Rationale for this change
   
   original code will remove distinct when they are children of union.
   
   But we should't remove them, because
   
   1. distinct can decrease cardinaty/rowcount of union input
   2. sometime distinct can pushdown, such as distinct can pushdown through 
join.
   3. even if we want to remove distinct, we should create rule instead of 
eliminate distinct in plan builder
   
   ## What changes are included in this PR?
   
   union_distinct shouldn't remove child distinct
   
   ## Are these changes tested?
   
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   ## Are there any user-facing changes?
   
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