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 * 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,
 * See the License for the specific language governing permissions and
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package org.apache.calcite.rel.core;

import org.apache.calcite.plan.RelOptCluster;
import org.apache.calcite.plan.RelOptCost;
import org.apache.calcite.plan.RelOptPlanner;
import org.apache.calcite.plan.RelTraitSet;
import org.apache.calcite.rel.RelDistribution;
import org.apache.calcite.rel.RelDistributionTraitDef;
import org.apache.calcite.rel.RelDistributions;
import org.apache.calcite.rel.RelInput;
import org.apache.calcite.rel.RelNode;
import org.apache.calcite.rel.RelWriter;
import org.apache.calcite.rel.SingleRel;
import org.apache.calcite.rel.metadata.RelMetadataQuery;
import org.apache.calcite.util.Util;

import java.util.List;
import java.util.Objects;

 * Relational expression that imposes a particular distribution on its input
 * without otherwise changing its content.
 * @see org.apache.calcite.rel.core.SortExchange
public abstract class Exchange extends SingleRel {
  //~ Instance fields --------------------------------------------------------

  public final RelDistribution distribution;

  //~ Constructors -----------------------------------------------------------

   * Creates an Exchange.
   * @param cluster   Cluster this relational expression belongs to
   * @param traitSet  Trait set
   * @param input     Input relational expression
   * @param distribution Distribution specification
  protected Exchange(RelOptCluster cluster, RelTraitSet traitSet, RelNode input,
      RelDistribution distribution) {
    super(cluster, traitSet, input);
    this.distribution = Objects.requireNonNull(distribution);

    assert traitSet.containsIfApplicable(distribution)
        : "traits=" + traitSet + ", distribution" + distribution;
    assert distribution != RelDistributions.ANY;

   * Creates an Exchange by parsing serialized output.
  public Exchange(RelInput input) {
    this(input.getCluster(), input.getTraitSet().plus(input.getCollation()),

  //~ Methods ----------------------------------------------------------------

  @Override public final Exchange copy(RelTraitSet traitSet,
      List<RelNode> inputs) {
    return copy(traitSet, sole(inputs), distribution);

  public abstract Exchange copy(RelTraitSet traitSet, RelNode newInput,
      RelDistribution newDistribution);

  /** Returns the distribution of the rows returned by this Exchange. */
  public RelDistribution getDistribution() {
    return distribution;

  @Override public RelOptCost computeSelfCost(RelOptPlanner planner,
      RelMetadataQuery mq) {
    // Higher cost if rows are wider discourages pushing a project through an
    // exchange.
    double rowCount = mq.getRowCount(this);
    double bytesPerRow = getRowType().getFieldCount() * 4;
    return planner.getCostFactory().makeCost(
        Util.nLogN(rowCount) * bytesPerRow, rowCount, 0);

  public RelWriter explainTerms(RelWriter pw) {
    return super.explainTerms(pw)
        .item("distribution", distribution);