[GitHub] [spark] imback82 commented on a change in pull request #29655: [SPARK-32806][SQL] SortMergeJoin with partial hash distribution can be optimized to remove shuffle

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imback82 commented on a change in pull request #29655:
URL: https://github.com/apache/spark/pull/29655#discussion_r484636101



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File path: sql/core/src/main/scala/org/apache/spark/sql/execution/exchange/OptimizeSortMergeJoinWithPartialHashDistribution.scala
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@@ -0,0 +1,115 @@
+/*
+ * 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.exchange
+
+import scala.collection.mutable
+
+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.{SortExec, SparkPlan}
+import org.apache.spark.sql.execution.joins.SortMergeJoinExec
+import org.apache.spark.sql.internal.SQLConf
+
+/**
+ * This rule removes shuffle for the sort merge join if the following conditions are met:
+ * - The child of ShuffleExchangeExec has HashPartitioning with the same number of partitions
+ *   as the other side of join.
+ * - The child of ShuffleExchangeExec has output partitioning which has the subset of
+ *   join keys on the respective join side.
+ *
+ * If the above conditions are met, shuffle can be eliminated for the sort merge join
+ * because rows are sorted before join logic is applied.
+ */
+case class OptimizeSortMergeJoinWithPartialHashDistribution(conf: SQLConf) extends Rule[SparkPlan] {

Review comment:
       To do this inside `EnsureRequirements.ensureDistributionAndOrdering`, it would require a new `Partitioning` and `Distribution` that know both sides of join, so I didn't go that route. Doing this outside would be less intrusive, I thought. But please let me know if doing this inside `EnsureRequirements` makes more sense. Thanks.
   
   This is done after `EnsureRequirements` since reordering keys may eliminate shuffles in which case this rule is not applied.




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