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); }