presto-main/src/main/java/io/prestosql/operator/aggregation/VarianceAggregation.java/VarianceAggregation/stddevPop
class VarianceAggregation:    @AggregationFunction("stddev_pop")
    @Description("Returns the population standard deviation of the argument")
    @OutputFunction(StandardTypes.DOUBLE)
    public static void stddevPop(@AggregationState VarianceState state, BlockBuilder out)
    {
        long count = state.getCount();
        if (count == 0) {
            out.appendNull();
        }
        else {
            double m2 = state.getM2();
            double result = m2 / count;
            result = Math.sqrt(result);
            DOUBLE.writeDouble(out, result);
        }
    }