**Describe the bug**

I have a table like:

```
CREATE TABLE test (
    time DateTime64(3)
    ...
) ENGINE = MergeTree()
PARTITION BY toStartOfInterval(time, INTERVAL 1 HOUR)
...
```

But when I search in `system.parts` it does not appear to have the `min/max` columns appropriately set?

```
partition:                             2020-06-01 01:00:00
min_date:                              1970-01-01
max_date:                              1970-01-01
min_time:                              1970-01-01 00:00:00
max_time:                              1970-01-01 00:00:00
```

I assume that if these were correctly set, doing a `select toStartOfInterval(time, interval 1 day), count()` type query would be pretty instant rather than trying to scan everything? I am guessing this is due to the `DateTime64` type in use?

Clickhouse 20.11.3.3

{
    String path = getFullRelativePath() + "checksums.txt";
    if (volume->getDisk()->exists(path))
    {
        auto buf = openForReading(volume->getDisk(), path);
        if (checksums.read(*buf))
        {
            assertEOF(*buf);
            bytes_on_disk = checksums.getTotalSizeOnDisk();
        }
        else
            bytes_on_disk = calculateTotalSizeOnDisk(volume->getDisk(), getFullRelativePath());
    }
    else
    {
        if (require)
            throw Exception("No checksums.txt in part " + name, ErrorCodes::NO_FILE_IN_DATA_PART);
        
        
        LOG_WARNING(storage.log, "Checksums for part {} not found. Will calculate them from data on disk.", name);
        checksums = checkDataPart(shared_from_this(), false);
        {
            auto out = volume->getDisk()->writeFile(getFullRelativePath() + "checksums.txt.tmp", 4096);
            checksums.write(*out);
        }
        volume->getDisk()->moveFile(getFullRelativePath() + "checksums.txt.tmp", getFullRelativePath() + "checksums.txt");
        bytes_on_disk = checksums.getTotalSizeOnDisk();
    }
}
void IMergeTreeDataPart::loadRowsCount()