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