public OptionalDouble getValueSum() {
        if (StatisticType.COUNT.equals(type)) {
            return OptionalDouble.empty();
        } else {
            return OptionalDouble.of(valueSum);
        }
    }
@return The sum of all stat event values for this time bucket.
Only applicable to VALUE stats
stroom-statistics/stroom-statistics-impl-sql/src/main/java/stroom/statistics/impl/sql/SQLStatValSourceDO.java