Github user sathiyapk commented on a diff in the pull request:

    https://github.com/apache/spark/pull/19451#discussion_r146689987
  
    --- Diff: 
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/ReplaceExceptWithFilter.scala
 ---
    @@ -0,0 +1,114 @@
    +/*
    + * 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 = plan transform {
    +    case Except(left: Project, right) if isEligible(left, right) =>
    +      Project(left.projectList,
    +        Distinct(Filter(Not(transformCondition(left.child, 
skipProject(right))), left.child)))
    +
    +    case Except(left, right) if isEligible(left, right) =>
    +      Distinct(Filter(Not(transformCondition(left, skipProject(right))), 
left))
    +  }
    +
    +  private def transformCondition(left: LogicalPlan, right: LogicalPlan) = {
    +    val filterCondition = InferFiltersFromConstraints(combineFilters(right)
    +    ).asInstanceOf[Filter].condition
    +
    +    val attributeNameMap: Map[String, Attribute] = left.output.map(x => 
(x.name, x)).toMap
    +    val transformedCondition = filterCondition transform { case a : 
AttributeReference =>
    +      attributeNameMap(a.name)
    +    }
    +
    +    transformedCondition
    +  }
    +
    +  private def isEligible(left: LogicalPlan, right: LogicalPlan) = (left, 
right) match {
    +    case (_, right @ (Project(_, _: Filter) | Filter(_, _))) => 
verifyConditions(left, right)
    +    case _ => false
    +  }
    +
    +  private def verifyConditions(left: LogicalPlan, right: LogicalPlan) = {
    +    val leftProjectList = projectList(left)
    +    val rightProjectList = projectList(right)
    +
    +    verifyFilterCondition(skipProject(left)) && 
verifyFilterCondition(skipProject(right)) &&
    +      Project(leftProjectList, 
nonFilterChild(skipProject(left))).sameResult(
    +        Project(rightProjectList, nonFilterChild(skipProject(right))))
    +  }
    +
    +  private def verifyFilterCondition(plan: LogicalPlan) = {
    --- End diff --
    
    @dilipbiswal You are right! I first removed the first case of the rule and 
then tested for collision, that's why i didn't find any collision. Without 
removing the first case of the rule, yes there is a collision. Although it 
looks sufficient now, in order to be careful about any unanticipated cases, i 
will also add an additional check `left.output.size == 
left.output.distinct.size` in the verifyCondition function.


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