也可以使用summap函数：

select result.1 as name, result.2 as amt
from (
      select sumMap(items.productName, items.amount) sum_per_keys,
            arrayJoin(arrayZip(sum_per_keys.1, sum_per_keys.2)) result
      from nested_columns_test)
order by name;

对ch20.3.8.53的测试表明summap比array join慢得多。
准备测试环境：

create table nested_columns_test(
  id Int32,
  items Nested(productName String, amount Int32)
) Engine = MergeTree()
order by (id);

insert into nested_columns_test
select number as id,
      arrayMap(x -> concat('product_', toString(x)), range(number % 32)) as `items.productName`, 
      arrayMap(x -> number + x, range(number % 32)) as `items.amount` 
from numbers(100*1000*1000);

SELECT items.productName as name, sum(items.amount) as amt 
from nested_columns_test 
array join items
group by items.productName
order by name;