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