**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

column_info.default_expression = queryToString(column.default_desc.expression);
        }
        columns_info[column.name] = column_info;
    }
    
    MergeTreeData::DataPartStateVector all_parts_state;
    MergeTreeData::DataPartsVector all_parts;
    all_parts = info.getParts(all_parts_state, has_state_column);
    for (size_t part_number = 0; part_number < all_parts.size(); ++part_number)
    {
        const auto & part = all_parts[part_number];
        auto part_state = all_parts_state[part_number];
        auto columns_size = part->getTotalColumnsSize();
        
        auto use_count = part.use_count() - 1;
        auto min_date = part->getMinDate();
        auto max_date = part->getMaxDate();
        auto index_size_in_bytes = part->getIndexSizeInBytes();
        auto index_size_in_allocated_bytes = part->getIndexSizeInAllocatedBytes();
        using State = IMergeTreeDataPart::State;
        size_t column_position = 0;
        for (const auto & column : part->getColumns())
        {
            ++column_position;
            size_t src_index = 0, res_index = 0;
            if (columns_mask[src_index++])
            {
                WriteBufferFromOwnString out;
                part->partition.serializeText(*info.data, out, format_settings);
                columns[res_index++]->insert(out.str());