presto-main/src/main/java/io/prestosql/operator/aggregation/AverageAggregations.java/AverageAggregations/combine
class AverageAggregations:    @CombineFunction
    public static void combine(@AggregationState LongAndDoubleState state, @AggregationState LongAndDoubleState otherState)
    {
        state.setLong(state.getLong() + otherState.getLong());
        state.setDouble(state.getDouble() + otherState.getDouble());
    }