**Describe the bug**
When using ORDER BY with multiple expressions on a MergeTree table, the ordering is not correct.


**How to reproduce**
` yandex/clickhouse-server:20.5.2.7`

Create the table:
```sql
create table order_test
(
    timestamp DateTime64(3),
    color     LowCardinality(Nullable(String))
) engine = MergeTree()
      PARTITION BY toYYYYMM(timestamp)
      ORDER BY timestamp
      SETTINGS index_granularity = 8192;
```

Fill the table (run a few time times to get a range of timestamps):
```sql
insert into order_test
select now64(),
       arrayElement(['red', 'green', 'blue', null], modulo(number, 4) + 1) as color
from (select number from system.numbers limit 1000000);
```

Query:
```sql
SELECT count(),
       color,
       toStartOfSecond(timestamp) AS `second`
FROM order_test AS i
GROUP BY color, `second`
ORDER BY color, `second` desc
LIMIT 500;
```


See out of order results:
```
500000,blue,2020-08-21 18:30:06.000
500000,blue,2020-08-21 18:30:09.000
250000,blue,2020-08-21 18:30:03.000
250000,blue,2020-08-21 18:30:08.000
250000,blue,2020-08-21 18:30:07.000
```

**Expected behavior**
See results in order by descending second:
```
500000,blue,2020-08-21 18:30:09.000
250000,blue,2020-08-21 18:30:08.000
250000,blue,2020-08-21 18:30:07.000
500000,blue,2020-08-21 18:30:06.000
250000,blue,2020-08-21 18:30:03.000
```

Notes:

It appears when I take out `LowCardinality` from the color column, the order works as expected. 



{
    auto check_for = [&](auto type) { return typeid_cast<const ColumnVector<decltype(type)> *>(&column) != nullptr; };
    auto try_get_size_for = [&](auto type) -> size_t { return check_for(type) ? sizeof(decltype(type)) : 0; };
    if (hint)
    {
        size_t size = 0;
        callForType([&](auto type) { size = try_get_size_for(type); }, hint);
        if (size)
            return size;
    }
    if (auto size = try_get_size_for(UInt8()))
        return size;
    if (auto size = try_get_size_for(UInt16()))
        return size;
    if (auto size = try_get_size_for(UInt32()))
        return size;
    if (auto size = try_get_size_for(UInt64()))
        return size;
    throw Exception("Unexpected indexes type for ColumnLowCardinality. Expected UInt, got " + column.getName(),
                    ErrorCodes::ILLEGAL_COLUMN);
}
void ColumnLowCardinality::Index::attachPositions(ColumnPtr positions_)