**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

{
    if (!initialized)
        hyperrectangle.reserve(column_names.size());
    for (size_t i = 0; i < column_names.size(); ++i)
    {
        FieldRef min_value;
        FieldRef max_value;
        const ColumnWithTypeAndName & column = block.getByName(column_names[i]);
        column.column->getExtremes(min_value, max_value);
        if (!initialized)
            hyperrectangle.emplace_back(min_value, true, max_value, true);
        else
        {
            hyperrectangle[i].left = std::min(hyperrectangle[i].left, min_value);
            hyperrectangle[i].right = std::max(hyperrectangle[i].right, max_value);
        }
    }
    initialized = true;
}
void IMergeTreeDataPart::MinMaxIndex::merge(const MinMaxIndex & other)