21.5.1
```sql

create table test(d Date, k Int64, s String) 
Engine=MergeTree partition by (toYYYYMM(d),k) order by (d, k)

insert into test values ('2020-01-01', 1, '');
insert into test values ('2020-01-02', 1, '');

SELECT * FROM test WHERE d != '2020-01-01'
0 rows in set. Elapsed: 0.003 sec.


SELECT * FROM test WHERE identity(d) !=  '2020-01-01';
┌──────────d─┬─k─┬─s─┐
│ 2020-01-02 │ 1 │   │
└────────────┴───┴───┘

SELECT * FROM test WHERE toDate(d) != '2020-01-01';
┌──────────d─┬─k─┬─s─┐
│ 2020-01-02 │ 1 │   │
└────────────┴───┴───┘
```

{
    /** Increment the last byte of the prefix by one. But if it is 255, then remove it and increase the previous one.
      * Example (for convenience, suppose that the maximum value of byte is `z`)
      * abcx -> abcy
      * abcz -> abd
      * zzz -> empty string
      * z -> empty string
      */
    String res = prefix;
    while (!res.empty() && static_cast<UInt8>(res.back()) == 255)
        res.pop_back();
    if (res.empty())
        return res;