clickhouse partition key in system.parts : tuple vs simple type

sample.sql

-- adding braces around partition name (even for simple types) allows to do cast safely
:) create table xxx(d Date, a UInt8) Engine = MergeTree() PARTITION by d ORDER BY a;

CREATE TABLE xxx
(
    d Date, 
    a UInt8
)
ENGINE = MergeTree()
PARTITION BY d
ORDER BY a

Ok.

0 rows in set. Elapsed: 0.006 sec. 

                                                    
:) SELECT CAST(concat('(',partition,')') as Tuple(Date)) val, toTypeName(val) val_type FROM system.parts  WHERE table = 'yyy' and active = 1;

SELECT 
    CAST(concat('(', partition, ')') AS Tuple(Date)) AS val, 
    toTypeName(val) AS val_type
FROM system.parts 
WHERE (table = 'xxx') AND (active = 1)

┌─val────────────┬─val_type────┐
│ ('2018-05-08') │ Tuple(Date) │
└────────────────┴─────────────┘

1 rows in set. Elapsed: 0.003 sec. 


-- basically clickhouse tolerates well when you give him a tuple with one val inside as partition id
:) insert into xxx values (today(),1),(today()-1,1),(today()-2,1);

INSERT INTO xxx VALUES

Ok.

3 rows in set. Elapsed: 0.003 sec. 

:) select count() from xxx;

SELECT count()
FROM xxx 

┌─count()─┐
│       3 │
└─────────┘

1 rows in set. Elapsed: 0.002 sec. 

--- alter table xxx drop partition today(); -- works exactly the same
:) alter table xxx drop partition tuple(today());

ALTER TABLE xxx
DROP PARTITION tuple(today())


Ok.

0 rows in set. Elapsed: 0.001 sec. 

:) select count() from xxx;

SELECT count()
FROM xxx 

┌─count()─┐
│       2 │
└─────────┘

1 rows in set. Elapsed: 0.002 sec. 
            
--- and it looks like clickhouse uses date when you use tuple(date) as expression.
-- so that behaves exactly the same as prv sample:
-- create table yyy(d Date, a UInt8) Engine = MergeTree() PARTITION by tuple(d) ORDER BY a;

-- but there is some inconsistency here:                                        

:) alter table xxx drop partition CAST(tuple(today()-1) AS Tuple(Date));

ALTER TABLE xxx
DROP PARTITION CAST(tuple(today()) AS Tuple(Date))


Received exception from server (version 1.1.54362):
Code: 53. DB::Exception: Received from localhost:9000, ::1. DB::Exception: Type mismatch in IN or VALUES section. Expected: Date. Got: Tuple. 

0 rows in set. Elapsed: 0.036 sec. 
                                            
-- with 'real' tuples clickhouse already adds braces around partition id in the system tables:
:) create table zzz (a UInt32, b UInt32, c String, d Array(String)) Engine=MergeTree PARTITION BY (a,b,c) ORDER BY (a,b,c)

CREATE TABLE zzz
(
    a UInt32, 
    b UInt32, 
    c String, 
    d Array(String)
)
ENGINE = MergeTree
PARTITION BY (a, b, c)
ORDER BY (a, b, c)

Ok.

0 rows in set. Elapsed: 0.007 sec. 

:) insert into zzz values (1,1,'1',['1']), (2,2,'22',['2','2']), (3,3,'333',['3','3','3'])

INSERT INTO zzz VALUES

Ok.

3 rows in set. Elapsed: 0.004 sec. 

-- it's easy to cast if you know the target type
SELECT CAST(partition AS Tuple(UInt32,UInt32,String)) AS val
FROM system.parts 
WHERE (table = 'zzz') AND (active = 1)

┌─val─────────┐
│ (3,3,'333') │
│ (2,2,'22')  │
│ (1,1,'1')   │
└─────────────┘

3 rows in set. Elapsed: 0.002 sec. 

-- but the same inconsistency appears
:) alter table zzz drop partition tuple(2,2,'22');

ALTER TABLE zzz
DROP PARTITION (2, 2, '22')


Ok.

0 rows in set. Elapsed: 0.001 sec. 

:) alter table zzz drop partition CAST( tuple(2,2,'22') AS Tuple(UInt32,UInt32,String));

ALTER TABLE zzz
DROP PARTITION CAST((2, 2, '22') AS Tuple(UInt32,UInt32,String))


Received exception from server (version 1.1.54362):
Code: 248. DB::Exception: Received from localhost:9000, ::1. DB::Exception: Wrong number of fields in the partition expression: 1, must be: 3. 

0 rows in set. Elapsed: 0.030 sec.