presto-main/src/main/java/io/prestosql/operator/aggregation/QuantileDigestAggregationFunction.java/QuantileDigestAggregationFunction/specialize
class QuantileDigestAggregationFunction:    @Override
    public InternalAggregationFunction specialize(BoundVariables boundVariables, int arity, Metadata metadata)
    {
        Type valueType = boundVariables.getTypeVariable("V");
        QuantileDigestType outputType = (QuantileDigestType) metadata.getParameterizedType(
                StandardTypes.QDIGEST,
                ImmutableList.of(TypeSignatureParameter.typeParameter(valueType.getTypeSignature())));
        return generateAggregation(valueType, outputType, arity);
    }