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.plan.RelOptPlanner;
  import org.apache.calcite.plan.RelTraitDef;
  import org.apache.calcite.plan.RelTraitSet;
  import org.apache.calcite.rel.core.Sort;
  import org.apache.calcite.rel.logical.LogicalSort;
  
  import org.checkerframework.checker.nullness.qual.Nullable;
  
  /**
   * Definition of the ordering trait.
   *
   * <p>Ordering is a physical property (i.e. a trait) because it can be changed
   * without loss of information. The converter to do this is the
   * {@link org.apache.calcite.rel.core.Sort} operator.
   *
   * <p>Unlike other current traits, a {@link RelNode} can have more than one
   * value of this trait simultaneously. For example,
   * <code>LogicalTableScan(table=TIME_BY_DAY)</code> might be sorted by
   * <code>{the_year, the_month, the_date}</code> and also by
   * <code>{time_id}</code>. We have to allow a RelNode to belong to more than
   * one RelSubset (these RelSubsets are always in the same set).
   */
  public class RelCollationTraitDef extends RelTraitDef<RelCollation> {
    public static final RelCollationTraitDef INSTANCE =
        new RelCollationTraitDef();
  
    private RelCollationTraitDef() {
    }
  
    @Override public Class<RelCollation> getTraitClass() {
      return RelCollation.class;
    }
  
    @Override public String getSimpleName() {
      return "sort";
    }
  
    @Override public boolean multiple() {
      return true;
    }
  
    @Override public RelCollation getDefault() {
      return RelCollations.EMPTY;
    }
  
    @Override public @Nullable RelNode convert(
        RelOptPlanner planner,
        RelNode rel,
        RelCollation toCollation,
        boolean allowInfiniteCostConverters) {
      if (toCollation.getFieldCollations().isEmpty()) {
        // An empty sort doesn't make sense.
        return null;
      }
  
      // Create a logical sort, then ask the planner to convert its remaining
      // traits (e.g. convert it to an EnumerableSortRel if rel is enumerable
      // convention)
      final Sort sort = LogicalSort.create(rel, toCollation, null, null);
      RelNode newRel = planner.register(sort, rel);
      final RelTraitSet newTraitSet = rel.getTraitSet().replace(toCollation);
      if (!newRel.getTraitSet().equals(newTraitSet)) {
        newRel = planner.changeTraits(newRel, newTraitSet);
      }
      return newRel;
    }
  
    @Override public boolean canConvert(
        RelOptPlanner planner, RelCollation fromTrait, RelCollation toTrait) {
      return true;
    }
  }
  