presto-main/src/main/java/io/prestosql/operator/aggregation/MapUnionAggregation.java/MapUnionAggregation/createInputParameterMetadata
class MapUnionAggregation:    private static List<ParameterMetadata> createInputParameterMetadata(Type inputType)
    {
        return ImmutableList.of(
                new ParameterMetadata(STATE),
                new ParameterMetadata(INPUT_CHANNEL, inputType));
    }