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 │   │
└────────────┴───┴───┘
```

{
    return unknownOrAlwaysTrue(false);
}
bool KeyCondition::anyUnknownOrAlwaysTrue() const
{
    return unknownOrAlwaysTrue(true);
}
bool KeyCondition::unknownOrAlwaysTrue(bool unknown_any) const
{
    std::vector<UInt8> rpn_stack;
    for (const auto & element : rpn)
    {
        if (element.function == RPNElement::FUNCTION_UNKNOWN)
        {
            
            
            if (unknown_any)
                return true;
            
            rpn_stack.push_back(true);
        }
        else if (element.function == RPNElement::ALWAYS_TRUE)
        {
            rpn_stack.push_back(true);
        }
        else if (element.function == RPNElement::FUNCTION_NOT_IN_RANGE
            || element.function == RPNElement::FUNCTION_IN_RANGE
            || element.function == RPNElement::FUNCTION_IN_SET
            || element.function == RPNElement::FUNCTION_NOT_IN_SET
            || element.function == RPNElement::ALWAYS_FALSE)