[GitHub] [spark] allisonwang-db commented on a change in pull request #29031: [SPARK-32216][SQL] Remove redundant ProjectExec

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allisonwang-db commented on a change in pull request #29031:
URL: https://github.com/apache/spark/pull/29031#discussion_r452583080



##########
File path: sql/core/src/main/scala/org/apache/spark/sql/execution/RemoveRedundantProjects.scala
##########
@@ -0,0 +1,88 @@
+/*
+ * 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
+
+import org.apache.spark.sql.catalyst.expressions.aggregate.{Final, PartialMerge}
+import org.apache.spark.sql.catalyst.rules.Rule
+import org.apache.spark.sql.execution.aggregate.BaseAggregateExec
+import org.apache.spark.sql.execution.datasources.v2.DataSourceV2ScanExecBase
+import org.apache.spark.sql.execution.window.WindowExec
+import org.apache.spark.sql.internal.SQLConf
+
+/**
+ * Remove redundant ProjectExec node from the spark plan. A ProjectExec node is redundant when
+ * - It has the same output attributes and orders as its child's output and the ordering of
+ *   the attributes is required.
+ * - It has the same output attributes as its child's output when attribute output ordering
+ *   is not required.
+ * This rule needs to be a physical rule because project nodes are useful during logical
+ * optimization to prune data. During physical planning, redundant project nodes can be removed
+ * to simplify the query plan.
+ */
+case class RemoveRedundantProjects(conf: SQLConf) extends Rule[SparkPlan] {
+  def apply(plan: SparkPlan): SparkPlan = {
+    if (!conf.getConf(SQLConf.REMOVE_REDUNDANT_PROJECTS_ENABLED)) {
+      plan
+    } else {
+      removeProject(plan, true)
+    }
+  }
+
+  private def removeProject(plan: SparkPlan, requireOrdering: Boolean): SparkPlan = {
+    plan match {
+      case p @ ProjectExec(_, child) =>
+        if (isRedundant(p, child, requireOrdering)) {
+          val newPlan = removeProject(child, requireOrdering)
+          newPlan.setLogicalLink(child.logicalLink.get)
+          newPlan
+        } else {
+          p.mapChildren(removeProject(_, false))
+        }
+      case op: TakeOrderedAndProjectExec =>
+        op.mapChildren(removeProject(_, false))
+      case a: BaseAggregateExec =>
+        // BaseAggregateExec require specific column ordering when mode is Final or PartialMerge.
+        // See comments in BaseAggregateExec inputAttributes method.
+        val keepOrdering = a.aggregateExpressions
+          .exists(ae => ae.mode.equals(Final) || ae.mode.equals(PartialMerge))
+        a.mapChildren(removeProject(_, keepOrdering))
+      case w: WindowExec => w.mapChildren(removeProject(_, false))

Review comment:
       Instead of setting require ordering to be true, I am wondering should `WindowExec` inherit this ordering requirement from its parent? For example in this case
   ```
   Project[a, avg, key]
     WindowExec[avg] [key] [a]
   ```
   `WindowExec` actually does't require column to be ordered. Is there any scenario where `WindowExec` must require child output column to be ordered? I am having trouble coming up with a test case for it.




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