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

    https://github.com/apache/spark/pull/16985#discussion_r132620278
  
    --- Diff: 
sql/core/src/main/scala/org/apache/spark/sql/execution/joins/ReorderJoinPredicates.scala
 ---
    @@ -0,0 +1,93 @@
    +/*
    + * 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.execution.joins
    +
    +import scala.collection.mutable.ArrayBuffer
    +
    +import org.apache.spark.sql.catalyst.expressions.Expression
    +import org.apache.spark.sql.catalyst.plans.physical.{HashPartitioning, 
Partitioning}
    +import org.apache.spark.sql.catalyst.rules.Rule
    +import org.apache.spark.sql.execution.SparkPlan
    +
    +/**
    + * When the physical operators are created for JOIN, the ordering of join 
keys is based on order
    + * in which the join keys appear in the user query. That might not match 
with the output
    + * partitioning of the join node's children (thus leading to extra sort / 
shuffle being
    + * introduced). This rule will change the ordering of the join keys to 
match with the
    + * partitioning of the join nodes' children.
    + */
    +class ReorderJoinPredicates extends Rule[SparkPlan] {
    +  private def reorderJoinKeys(
    +      leftKeys: Seq[Expression],
    +      rightKeys: Seq[Expression],
    +      leftPartitioning: Partitioning,
    +      rightPartitioning: Partitioning): (Seq[Expression], Seq[Expression]) 
= {
    +
    +    def reorder(expectedOrderOfKeys: Seq[Expression],
    +                currentOrderOfKeys: Seq[Expression]): (Seq[Expression], 
Seq[Expression]) = {
    +      val leftKeysBuffer = ArrayBuffer[Expression]()
    +      val rightKeysBuffer = ArrayBuffer[Expression]()
    +
    +      expectedOrderOfKeys.foreach(expression => {
    +        val index = currentOrderOfKeys.indexWhere(e => 
e.semanticEquals(expression))
    +        leftKeysBuffer.append(leftKeys(index))
    +        rightKeysBuffer.append(rightKeys(index))
    +      })
    +      (leftKeysBuffer, rightKeysBuffer)
    +    }
    +
    +    if (leftKeys.forall(_.deterministic) && 
rightKeys.forall(_.deterministic)) {
    +      leftPartitioning match {
    +        case HashPartitioning(leftExpressions, _)
    +          if leftExpressions.length == leftKeys.length &&
    --- End diff --
    
    The contract for reordering is that the set of join keys must be equal to 
the set of child's partitioning columns (implemented at L58-L59 in this file). 
Thus there won't be reordering for the case you pointed out. I have added a 
test case of the same.


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