presto-main/src/main/java/io/prestosql/operator/aggregation/DoubleHistogramAggregation.java/DoubleHistogramAggregation/merge
class DoubleHistogramAggregation:    @CombineFunction
    public static void merge(@AggregationState State state, State other)
    {
        NumericHistogram input = other.get();
        NumericHistogram previous = state.get();

        if (previous == null) {
            state.set(input);
        }
        else {
            previous.mergeWith(input);
        }
    }