public RelNode convert(RelNode rel) {

      final Sort sort = (Sort) rel;

      final RelTraitSet traitSet =

          sort.getTraitSet().replace(out)

              .replace(sort.getCollation());

      return new MongoSort(rel.getCluster(), traitSet,

          convert(sort.getInput(), traitSet.replace(RelCollations.EMPTY)),

          sort.getCollation(), sort.offset, sort.fetch);

    }

    public void registerConverterRule(

        RelOptPlanner planner,

        ConverterRule converterRule) {

      if (!converterRule.isGuaranteed()) {

        return;

      }



      RelTrait fromTrait = converterRule.getInTrait();

      RelTrait toTrait = converterRule.getOutTrait();



      conversionMap.put(fromTrait, Pair.of(toTrait, converterRule));

    }