presto-main/src/main/java/io/prestosql/operator/aggregation/AggregationUtils.java/AggregationUtils/mergeCovarianceState
class AggregationUtils:    public static void mergeCovarianceState(CovarianceState state, CovarianceState otherState)
    {
        if (otherState.getCount() == 0) {
            return;
        }

        updateCovarianceState(state, otherState);
    }