Hintable.java

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 * 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
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 * distributed under the License is distributed on an "AS IS" BASIS,
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package org.apache.calcite.rel.hint;

import org.apache.calcite.linq4j.function.Experimental;
import org.apache.calcite.rel.RelNode;

import com.google.common.collect.ImmutableList;

import java.util.ArrayList;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Set;

import static java.util.Objects.requireNonNull;

/**
 * {@link Hintable} is a kind of {@link RelNode} that can attach {@link RelHint}s.
 *
 * <p>This interface is experimental, {@link RelNode}s that implement it
 * have a constructor parameter named "hints" used to construct relational expression
 * with given hints.
 *
 * <p>Current design is not that elegant and mature, because we have to
 * copy the hints whenever these relational expressions are copied or used to
 * derive new relational expressions.
 * Even though we have implemented the mechanism to propagate the hints, for large queries,
 * there would be many cases where the hints are not copied to the right RelNode,
 * and the effort/memory is wasted if we are copying the hint to a RelNode
 * but the hint is not used.
 */
@Experimental
public interface Hintable {

  /**
   * Attaches list of hints to this relational expression.
   *
   * <p>This method is only for internal use during sql-to-rel conversion.
   *
   * <p>Sub-class should return a new copy of the relational expression.
   *
   * <p>The default implementation merges the given hints with existing ones,
   * put them in one list and eliminate the duplicates; then
   * returns a new copy of this relational expression with the merged hints.
   *
   * @param hintList The hints to attach to this relational expression
   * @return Relational expression with the hints {@code hintList} attached
   */
  default RelNode attachHints(List<RelHint> hintList) {
    final Set<RelHint> hints = new LinkedHashSet<>(getHints());
    hints.addAll(requireNonNull(hintList, "hintList"));
    return withHints(new ArrayList<>(hints));
  }

  /**
   * Returns a new relational expression with the specified hints {@code hintList}.
   *
   * <p>This method should be overridden by every logical node that supports hint.
   * It is only for internal use during decorrelation.
   *
   * <p>Sub-class should return a new copy of the relational expression.
   *
   * <p>The default implementation returns the relational expression directly
   * only because not every kind of relational expression supports hints.
   *
   * @return Relational expression with set up hints
   */
  default RelNode withHints(List<RelHint> hintList) {
    return (RelNode) this;
  }

  /**
   * Returns the hints of this relational expressions as an immutable list.
   */
  ImmutableList<RelHint> getHints();
}