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From davies <...@git.apache.org>
Subject [GitHub] spark pull request: [SPARK-13523] [SQL] Reuse exchanges in a query
Date Tue, 08 Mar 2016 22:22:58 GMT
Github user davies commented on a diff in the pull request:

    https://github.com/apache/spark/pull/11403#discussion_r55439108
  
    --- Diff: sql/core/src/main/scala/org/apache/spark/sql/execution/exchange/Exchange.scala
---
    @@ -0,0 +1,91 @@
    +/*
    + * 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 scala.collection.mutable.ArrayBuffer
    +
    +import org.apache.spark.broadcast
    +import org.apache.spark.rdd.RDD
    +import org.apache.spark.sql.SQLContext
    +import org.apache.spark.sql.catalyst.InternalRow
    +import org.apache.spark.sql.catalyst.expressions.Attribute
    +import org.apache.spark.sql.catalyst.rules.Rule
    +import org.apache.spark.sql.execution.{LeafNode, SparkPlan, UnaryNode}
    +import org.apache.spark.sql.types.StructType
    +
    +/**
    + * An interface for exchanges.
    + */
    +abstract class Exchange extends UnaryNode {
    +  override def output: Seq[Attribute] = child.output
    +}
    +
    +/**
    + * A wrapper for reused exchange to have different output, which is required to resolve
the
    + * attributes in following plans.
    + */
    +case class ReusedExchange(override val output: Seq[Attribute], child: Exchange) extends
LeafNode {
    +
    +  override def sameResult(plan: SparkPlan): Boolean = {
    +    // Ignore this wrapper. `plan` could also be a ReusedExchange, so we reverse the
order here.
    +    plan.sameResult(child)
    +  }
    +
    +  def doExecute(): RDD[InternalRow] = {
    +    child.execute()
    +  }
    +
    +  override protected[sql] def doExecuteBroadcast[T](): broadcast.Broadcast[T] = {
    +    child.executeBroadcast()
    +  }
    +
    +  // Do not repeat the same tree in explain.
    +  override def treeChildren: Seq[SparkPlan] = Nil
    +}
    +
    +/**
    + * Find out duplicated exchanges in the spark plan, then use the same exchange for all
the
    + * references.
    + */
    +private[sql] case class ReuseExchange(sqlContext: SQLContext) extends Rule[SparkPlan]
{
    +
    +  def apply(plan: SparkPlan): SparkPlan = {
    +    if (!sqlContext.conf.exchangeReuseEnabled) {
    +      return plan
    +    }
    +    // Build a hash map using schema of exchanges to avoid O(N*N) sameResult calls.
    +    val exchanges = mutable.HashMap[StructType, ArrayBuffer[Exchange]]()
    --- End diff --
    
    Good question, it could be false negative.
    
    But usually if two plan have the same result, they should have the same inputs also the
same plan and expressions, they should generate the same name (does not include the random
ExprId).


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