presto-tpch/src/main/java/io/prestosql/plugin/tpch/statistics/StatisticsEstimator.java/StatisticsEstimator/addDistinctValuesCount
class StatisticsEstimator:    private Optional<Long> addDistinctValuesCount(TpchColumn<?> partitionColumn, String columnName, ColumnStatisticsData leftStats, ColumnStatisticsData rightStats)
    {
        //unique values count can't be added between different partitions
        //for columns other than the partition column (because almost certainly there are duplicates)
        return combine(leftStats.getDistinctValuesCount(), rightStats.getDistinctValuesCount(), (a, b) -> a + b)
                .filter(v -> columnName.equals(partitionColumn.getColumnName()));
    }