presto-mongodb/src/main/java/io/prestosql/plugin/mongodb/MongoIndex.java/MongoIndex/MongodbIndexKey
class MongoIndex:        public MongodbIndexKey(String name, Optional<SortOrder> sortOrder, Optional<String> type)
        {
            this.name = requireNonNull(name, "name is null");
            this.sortOrder = sortOrder;
            this.type = type;
        }