Unexpected result of sumMerge & maxMerge on empty initialization

Question

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
)

uniq_merge	sum_merge	max_merge
0	0	0

It's not clear why I get 0 instead of NULL on sumMerge & maxMerge.
Is this expected?

Update:
Seems, it depends on the base column's nullability:

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)
*/  
)

sum_merge_null	sum_merge_0
	0

Answer 1

By default Clickhouse server avoids Nulls (Nullable types), because processing of Nullable types can be up to twice slower.

There is a settings for more SQL ANSI compatibility

select sum(x) sx from (select 1 x where 0);
┌─sx─┐
│  0 │
└────┘

set aggregate_functions_null_for_empty=1;

select sum(x) sx from (select 1 x where 0);
┌───sx─┐
│ ᴺᵁᴸᴸ │
└──────┘

Unfortunately this functionality is not implemented for State/Merge.

And actually you don't need AggregateFunction for SUM/MAX because these two functions does not need state. So you can use SimpleAggregateFunction.

see for example Clickhouse Materialized View on ReplicatedAggregatingMergeTree

So in real-life examples with AggregatingMergeTree+SimpleAggregateFunction+Nullable types it's possible to get Nulls here.

---

Upd:

also it's possible like this

CREATE TABLE test
ENGINE = AggregatingMergeTree
ORDER BY i AS
SELECT
    1 AS i,
    uniqState('a') AS uniq_state,
    sumState(CAST('10', 'Nullable(Int64)')) AS sum_state,
    maxState(CAST('10', 'Nullable(Int64)')) AS max_state
WHERE 0

SELECT
    uniqMerge(uniq_state) AS uniq_merge,
    sumMerge(sum_state) AS sum_merge,
    maxMerge(max_state) AS max_merge
FROM test
┌─uniq_merge─┬─sum_merge─┬─max_merge─┐
│          0 │      ᴺᵁᴸᴸ │      ᴺᵁᴸᴸ │
└────────────┴───────────┴───────────┘