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.type.RelDataType;
  
  import java.util.List;
  
  /**
   * Node in a planner.
   */
  public interface RelOptNode {
    /**
     * Returns the ID of this relational expression, unique among all relational
     * expressions created since the server was started.
     *
     * @return Unique ID
     */
    int getId();
  
    /**
     * Returns a string which concisely describes the definition of this
     * relational expression. Two relational expressions are equivalent if
     * their digests and {@link #getRowType()} (except the field names) are the same.
     *
     * <p>The digest does not contain the relational expression's identity --
     * that would prevent similar relational expressions from ever comparing
     * equal -- but does include the identity of children (on the assumption
     * that children have already been normalized).
     *
     * <p>If you want a descriptive string which contains the identity, call
     * {@link Object#toString()}, which always returns "rel#{id}:{digest}".
     *
     * @return Digest string of this {@code RelNode}
     */
    String getDigest();
  
    /**
     * Retrieves this RelNode's traits. Note that although the RelTraitSet
     * returned is modifiable, it <b>must not</b> be modified during
     * optimization. It is legal to modify the traits of a RelNode before or
     * after optimization, although doing so could render a tree of RelNodes
     * unimplementable. If a RelNode's traits need to be modified during
     * optimization, clone the RelNode and change the clone's traits.
     *
     * @return this RelNode's trait set
     */
    RelTraitSet getTraitSet();
  
    // TODO: We don't want to require that nodes have very detailed row type. It
    // may not even be known at planning time.
    RelDataType getRowType();
  
    /**
     * Returns a string which describes the relational expression and, unlike
     * {@link #getDigest()}, also includes the identity. Typically returns
     * "rel#{id}:{digest}".
     *
     * @return String which describes the relational expression and, unlike
     *   {@link #getDigest()}, also includes the identity
     */
    @Deprecated // to be removed before 2.0
    String getDescription();
  
    /**
     * Returns an array of this relational expression's inputs. If there are no
     * inputs, returns an empty list, not {@code null}.
     *
     * @return Array of this relational expression's inputs
     */
    List<? extends RelOptNode> getInputs();
  
    /**
     * Returns the cluster this relational expression belongs to.
     *
     * @return cluster
     */
    RelOptCluster getCluster();
  }
  