**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

MinMaxIndex(DayNum min_date, DayNum max_date)
            : hyperrectangle(1, Range(min_date, true, max_date, true))
            , initialized(true)
        {
        }
        void load(const MergeTreeData & data, const DiskPtr & disk_, const String & part_path);
        void store(const MergeTreeData & data, const DiskPtr & disk_, const String & part_path, Checksums & checksums) const;
        void store(const Names & column_names, const DataTypes & data_types, const DiskPtr & disk_, const String & part_path, Checksums & checksums) const;
        void update(const Block & block, const Names & column_names);
        void merge(const MinMaxIndex & other);
    };
    MinMaxIndex minmax_idx;
    Checksums checksums;
    
    NameSet expired_columns;
    CompressionCodecPtr default_codec;
    
    UInt64 getIndexSizeInBytes() const;
    UInt64 getIndexSizeInAllocatedBytes() const;
    UInt64 getMarksCount() const;
    UInt64 getBytesOnDisk() const { return bytes_on_disk; }