初始化空聚合时sumMerge与maxMerge返回0而非NULL的疑问

我执行了以下ClickHouse查询：

SELECT 
  uniqMerge (uniq_state) as uniq_merge  -- 0: ok
, sumMerge  (sum_state)  as sum_merge   -- 0: why not NULL?
, maxMerge  (max_state)  as max_merge   -- 0: why not NULL?
FROM 
(
  SELECT 
    initializeAggregation ('uniqStateIf', '', 0) as uniq_state
  , initializeAggregation ('sumStateIf',   0, 0) as sum_state
  , initializeAggregation ('maxStateIf',   0, 0) as max_state
)

查询结果：

我不清楚为什么sumMerge和maxMerge返回0而非NULL，这属于预期行为吗？

更新：与基础列可空性相关的测试结果 #

我做了进一步测试，发现结果似乎和基础列的可空性有关：

SELECT 
  sumMerge (sum_state_null)  as sum_merge_null
, sumMerge (sum_state_0)     as sum_merge_0
FROM
(
  SELECT 
    sumState (cast (null as Nullable(UInt8)))   as sum_state_null
  , sumState (0)                                as sum_state_0
/*  
    UNION ALL
  SELECT 
    sumState (cast (1 as Nullable(UInt8)))
  , sumState (0)
*/  
)

查询结果：

内容的提问来源于stack exchange，提问作者Mark Barinstein