presto-main/src/main/java/io/prestosql/operator/aggregation/state/QuantileDigestStateFactory.java/QuantileDigestStateFactory/getGroupedStateClass
class QuantileDigestStateFactory:    @Override
    public Class<? extends QuantileDigestState> getGroupedStateClass()
    {
        return GroupedQuantileDigestState.class;
    }