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;
  
  import org.apache.calcite.sql.SqlExplainLevel;
  import org.apache.calcite.util.Pair;
  
  import org.checkerframework.checker.nullness.qual.Nullable;
  
  import java.util.List;
  
  /**
   * Callback for an expression to dump itself to.
   *
   * <p>It is used for generating EXPLAIN PLAN output, and also for serializing
   * a tree of relational expressions to JSON.
   */
  public interface RelWriter {
    /**
     * Prints an explanation of a node, with a list of (term, value) pairs.
     *
     * <p>The term-value pairs are generally gathered by calling
     * {@link org.apache.calcite.rel.RelNode#explain(RelWriter)}.
     * Each sub-class of {@link org.apache.calcite.rel.RelNode}
     * calls {@link #input(String, org.apache.calcite.rel.RelNode)}
     * and {@link #item(String, Object)} to declare term-value pairs.
     *
     * @param rel       Relational expression
     * @param valueList List of term-value pairs
     */
    void explain(RelNode rel, List<Pair<String, @Nullable Object>> valueList);
  
    /** Returns detail level at which plan should be generated. */
    SqlExplainLevel getDetailLevel();
  
    /**
     * Adds an input to the explanation of the current node.
     *
     * @param term  Term for input, e.g. "left" or "input #1".
     * @param input Input relational expression
     */
    default RelWriter input(String term, RelNode input) {
      return item(term, input);
    }
  
    /**
     * Adds an attribute to the explanation of the current node.
     *
     * @param term  Term for attribute, e.g. "joinType"
     * @param value Attribute value
     */
    RelWriter item(String term, @Nullable Object value);
  
    /**
     * Adds an input to the explanation of the current node, if a condition
     * holds.
     */
    default RelWriter itemIf(String term, @Nullable Object value, boolean condition) {
      return condition ? item(term, value) : this;
    }
  
    /**
     * Writes the completed explanation.
     */
    RelWriter done(RelNode node);
  
    /**
     * Returns whether the writer prefers nested values. Traditional explain
     * writers prefer flattened values.
     */
    default boolean nest() {
      return false;
    }
  
    /**
     * Returns whether the writer needs to expand node's detail information when printing plan.
     * For example, LogicalSort(sort0=[$0], dir0=[ASC]) will be expanded to
     * LogicalSort(sort0=[$0], dir0=[ASC-nulls-last]).
     */
    default boolean expand() {
      return false;
    }
  }
  