**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

{
    assertOnDisk();
    
    
    
    MemoryTracker::BlockerInThread temporarily_disable_memory_tracker(VariableContext::Global);
    loadUUID();
    loadColumns(require_columns_checksums);
    loadChecksums(require_columns_checksums);
    loadIndexGranularity();
    calculateColumnsSizesOnDisk();
    loadIndex();     
    loadRowsCount(); 
    loadPartitionAndMinMaxIndex();
    loadTTLInfos();
    if (check_consistency)
        checkConsistency(require_columns_checksums);
    loadDefaultCompressionCodec();
}
void IMergeTreeDataPart::loadIndexGranularity()
{
    throw Exception("Method 'loadIndexGranularity' is not implemented for part with type " + getType().toString(), ErrorCodes::NOT_IMPLEMENTED);
}
void IMergeTreeDataPart::loadIndex()
{
    
    if (!index_granularity.isInitialized())
        throw Exception("Index granularity is not loaded before index loading", ErrorCodes::LOGICAL_ERROR);
    auto metadata_snapshot = storage.getInMemoryMetadataPtr();
    const auto & primary_key = metadata_snapshot->getPrimaryKey();