**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

{
    return total_active_size_rows.load(std::memory_order_acquire);
}
size_t MergeTreeData::getPartsCount() const
{
    return total_active_size_parts.load(std::memory_order_acquire);
}
size_t MergeTreeData::getMaxPartsCountForPartitionWithState(DataPartState state) const
{
    auto lock = lockParts();
    size_t res = 0;
    size_t cur_count = 0;
    const String * cur_partition_id = nullptr;
    for (const auto & part : getDataPartsStateRange(state))
    {
        if (cur_partition_id && part->info.partition_id == *cur_partition_id)
        {
            ++cur_count;
        }
        else
        {
            cur_partition_id = &part->info.partition_id;
            cur_count = 1;
        }
        res = std::max(res, cur_count);
    }
    return res;
}
size_t MergeTreeData::getMaxPartsCountForPartition() const