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 │   │
└────────────┴───┴───┘
```

for (const auto & key : key_columns)
        key_names[key.second] = key.first;
    WriteBufferFromOwnString buf;
    std::function<void(const Node *)> describe;
    describe = [&describe, &key_names, &is_key_used, &buf](const Node * node)
    {
        switch (node->type)
        {
            case Node::Type::Leaf:
            {
                is_key_used[node->element->key_column] = true;
                
                
                
                
                if (node->negate)
                    buf << "not(";
                buf << node->element->toString(key_names[node->element->key_column], true);
                if (node->negate)
                    buf << ")";
                break;
            }
            case Node::Type::True:
                buf << "true";
                break;
            case Node::Type::False:
                buf << "false";
                break;
            case Node::Type::And:
                buf << "and(";