Github user hvanhovell commented on a diff in the pull request:
https://github.com/apache/spark/pull/12705#discussion_r61134754
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala
---
@@ -1405,6 +1405,27 @@ object ReplaceIntersectWithSemiJoin extends
Rule[LogicalPlan] {
}
/**
+ * Replaces logical [[Except]] operator with a left-anti [[Join]]
operator.
+ * {{{
+ * SELECT a1, a2 FROM Tab1 EXCEPT SELECT b1, b2 FROM Tab2
+ * ==> SELECT DISTINCT a1, a2 FROM Tab1 LEFT ANTI JOIN Tab2 ON a1<=>b1
AND a2<=>b2
+ * }}}
+ *
+ * Note:
+ * 1. This rule is only applicable to EXCEPT DISTINCT. Do not use it for
INTERSECT ALL.
+ * 2. This rule has to be done after de-duplicating the attributes;
otherwise, the generated
+ * join conditions will be incorrect.
+ */
+object ReplaceIntersectWithAntiJoin extends Rule[LogicalPlan] {
--- End diff --
We could also integrate this with `ReplaceIntersectWithSemiJoin`. But I'll
leave that up to you.
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