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.plan;
  
  import org.apache.calcite.rel.RelNode;
  
  import org.apiguardian.api.API;
  
  /**
   * The digest is the exact representation of the corresponding {@code RelNode},
   * at anytime, anywhere. The only difference is that digest is compared using
   * {@code #equals} and {@code #hashCode}, which are prohibited to override
   * for RelNode, for legacy reasons.
   *
   * <p>INTERNAL USE ONLY.
   */
  @API(since = "1.24", status = API.Status.INTERNAL)
  public interface RelDigest {
    /**
     * Reset state, possibly cache of hash code.
     */
    void clear();
  
    /**
     * Returns the relnode that this digest is associated with.
     */
    RelNode getRel();
  }
  