Nulls in order by

Nulls in order by
CREATE TABLE x
(
    `a` Nullable(UInt32),
    `b` Nullable(UInt32),
    `cnt` UInt32
)
ENGINE = SummingMergeTree
ORDER BY (a, b)
SETTINGS allow_nullable_key = 1;
INSERT INTO x VALUES (Null,2,1), (Null,Null,1), (3, Null, 1), (4,4,1);
INSERT INTO x VALUES (Null,2,1), (Null,Null,1), (3, Null, 1), (4,4,1);
SELECT * FROM x;
┌────a─┬────b─┬─cnt─┐
    3   null    2 
    4     4    2 
  null     2    2 
  null   null    2 
└──────┴──────┴─────┘