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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