presto-hive/src/main/java/io/prestosql/plugin/hive/util/Statistics.java/Statistics/merge
class Statistics:    public static PartitionStatistics merge(PartitionStatistics first, PartitionStatistics second)
    {
        return new PartitionStatistics(
                reduce(first.getBasicStatistics(), second.getBasicStatistics(), ADD),
                merge(first.getColumnStatistics(), second.getColumnStatistics()));
    }