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.convert;
  
  import org.apache.calcite.plan.RelTraitDef;
  import org.apache.calcite.plan.RelTraitSet;
  import org.apache.calcite.rel.RelNode;
  
  import org.checkerframework.checker.nullness.qual.Nullable;
  
  /**
   * A relational expression implements the interface <code>Converter</code> to
   * indicate that it converts a physical attribute, or
   * {@link org.apache.calcite.plan.RelTrait trait}, of a relational expression
   * from one value to another.
   *
   * <p>Sometimes this conversion is expensive; for example, to convert a
   * non-distinct to a distinct object stream, we have to clone every object in
   * the input.
   *
   * <p>A converter does not change the logical expression being evaluated; after
   * conversion, the number of rows and the values of those rows will still be the
   * same. By declaring itself to be a converter, a relational expression is
   * telling the planner about this equivalence, and the planner groups
   * expressions which are logically equivalent but have different physical traits
   * into groups called <code>RelSet</code>s.
   *
   * <p>In principle one could devise converters which change multiple traits
   * simultaneously (say change the sort-order and the physical location of a
   * relational expression). In which case, the method {@link #getInputTraits()}
   * would return a {@link org.apache.calcite.plan.RelTraitSet}. But for
   * simplicity, this class only allows one trait to be converted at a
   * time; all other traits are assumed to be preserved.
   */
  public interface Converter extends RelNode {
    //~ Methods ----------------------------------------------------------------
  
    /**
     * Returns the trait of the input relational expression.
     *
     * @return input trait
     */
    RelTraitSet getInputTraits();
  
    /**
     * Returns the definition of trait which this converter works on.
     *
     * <p>The input relational expression (matched by the rule) must possess
     * this trait and have the value given by {@link #getInputTraits()}, and the
     * traits of the output of this converter given by {@link #getTraitSet()} will
     * have one trait altered and the other orthogonal traits will be the same.
     *
     * @return trait which this converter modifies
     */
    @Nullable RelTraitDef getTraitDef();
  
    /**
     * Returns the sole input relational expression.
     *
     * @return child relational expression
     */
    RelNode getInput();
  }
  