Github user gatorsmile commented on a diff in the pull request:
https://github.com/apache/spark/pull/19451#discussion_r147508867
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/ReplaceExceptWithFilter.scala
---
@@ -0,0 +1,104 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.sql.catalyst.optimizer
+
+import scala.annotation.tailrec
+
+import org.apache.spark.sql.catalyst.expressions._
+import org.apache.spark.sql.catalyst.plans.logical._
+import org.apache.spark.sql.catalyst.rules.Rule
+
+
+/**
+ * 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 DISTINCT a1, a2 FROM Tab1 WHERE a2 = 12 AND (a1 is null
OR a1 <> 5)
+ * }}}
+ *
+ * Note:
+ * Before flipping the filter condition of the right node, we should:
+ * 1. Combine all it's [[Filter]].
+ * 2. Apply InferFiltersFromConstraints rule (to support NULL values of
the condition).
+ */
+object ReplaceExceptWithFilter extends Rule[LogicalPlan] {
+
+ def apply(plan: LogicalPlan): LogicalPlan = {
+ if (!plan.conf.replaceExceptWithFilter) {
+ return plan
+ }
+
+ plan.transform {
+ case Except(left, right) if isEligible(left, right) =>
+ Distinct(Filter(Not(transformCondition(left, skipProject(right))),
left))
+ }
+ }
+
+ private def transformCondition(left: LogicalPlan, right: LogicalPlan):
Expression = {
+ val filterCondition = InferFiltersFromConstraints(combineFilters(right)
+ ).asInstanceOf[Filter].condition
--- End diff --
Also need to leave a comment to explain why we need to call the rule
`InferFiltersFromConstraints `
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