content: /*
   * 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.calcite.rel.rules;
  
  import org.apache.calcite.plan.hep.HepPlanner;
  import org.apache.calcite.plan.volcano.VolcanoPlanner;
  import org.apache.calcite.rel.PhysicalNode;
  
  /**
   * Logical transformation rule, only logical operator can be rule operand,
   * and only generate logical alternatives. It is only visible to
   * {@link VolcanoPlanner}, {@link HepPlanner} will ignore this interface.
   * That means, in {@link HepPlanner}, the rule that implements
   * {@link TransformationRule} can still match with physical operator of
   * {@link PhysicalNode} and generate physical alternatives.
   *
   * <p>But in {@link VolcanoPlanner}, {@link TransformationRule} doesn't match
   * with physical operator that implements {@link PhysicalNode}. It is not
   * allowed to generate physical operators in {@link TransformationRule},
   * unless you are using it in {@link HepPlanner}.
   *
   * @see VolcanoPlanner
   * @see SubstitutionRule
   */
  public interface TransformationRule {
  }
  