presto-main/src/main/java/io/prestosql/operator/aggregation/RealAverageAggregation.java/RealAverageAggregation/combine
class RealAverageAggregation:    public static void combine(LongState count, DoubleState sum, LongState otherCount, DoubleState otherSum)
    {
        count.setLong(count.getLong() + otherCount.getLong());
        sum.setDouble(sum.getDouble() + otherSum.getDouble());
    }