Github user gatorsmile commented on the pull request:
https://github.com/apache/spark/pull/11854#issuecomment-200676690
The motivation of this function `distinctSet` is to obtain the uniqueness
constraint from the child operators. The output of `Distinct`, `Intersect`,
`Except`, and `Aggregate` (iff its aggregate expressions is identical to its
grouping expressions) can always guarantee the uniqueness. Thus, the parent
operators can use it for query optimization.
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