Github user gatorsmile commented on a diff in the pull request: https://github.com/apache/spark/pull/19451#discussion_r143284824 --- Diff: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala --- @@ -1242,6 +1243,53 @@ object ReplaceIntersectWithSemiJoin extends Rule[LogicalPlan] { } /** + * If one or both of the datasets in the logical [[Except]] operator are purely transformed using + * [[Filter]], this rule will replace logical [[Except]] operator with a [[Filter]] operator by + * flipping the filter condition of the right child. + * {{{ + * SELECT a1, a2 FROM Tab1 WHERE a2 = 12 EXCEPT SELECT a1, a2 FROM Tab1 WHERE a1 = 5 + * ==> SELECT a1, a2 FROM Tab1 WHERE a2 = 12 AND a1 <> 5 --- End diff -- This is wrong. See the correct description of `EXCEPT` > EXCEPT returns distinct rows from the left input query that arenât output by the right input query.
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