**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

{
    LOG_TRACE(log, "dropIfEmpty");
    auto lock = lockParts();
    if (!data_parts_by_info.empty())
        return;
    for (const auto & [path, disk] : getRelativeDataPathsWithDisks())
    {
        
        disk->removeFile(path + MergeTreeData::FORMAT_VERSION_FILE_NAME);
        disk->removeDirectory(path + MergeTreeData::DETACHED_DIR_NAME);
        disk->removeDirectory(path);
    }
}
namespace
{
bool isSafeForKeyConversion(const IDataType * from, const IDataType * to)