presto-main/src/main/java/io/prestosql/operator/aggregation/VarianceAggregation.java/VarianceAggregation/combine
class VarianceAggregation:    @CombineFunction
    public static void combine(@AggregationState VarianceState state, @AggregationState VarianceState otherState)
    {
        mergeVarianceState(state, otherState);
    }