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