I am wondering if my time dimension is in millisecond granularity, how it will be used in star-tree? Should I truncate it to day or week first? P.S. I see that star tree will automatically include dictionary-encoded Time/DateTime columns to the dimensionsSplitOrder property.

If you have different millisecond value for each row, then you are preventing pre-aggregation. At this point, you need to ask if the application really needs to slice/dice at millis level? If not, then you can either use a different time unit (say days), or you can snap the millis for the entire day to the beginning of the day or some fixed value.

I understand. Because I see this in the doc, I am wondering will the enableDefaultStarTree append millisecond cols to the dimension?

It will append it to the split order. As in the after splitting data on specified dimension columns, it will be further split by time.