Yeah, you and us are on the same page (heh). We don’t want the format to require row grouping. The file format has a layout schema written in a footer. A row group style layout is supported but not required. Specification of the layout will probably evolve, but currently the in-memory structure becomes the on-disk structure. So, if you have a ChunkedArray of StructArray of ChunkedArray you’ll get row groups and pages within them. If you had a StructArray of ChunkedArray you’ll just get per-column pages.

I’m working on the Python API now. I think we probably want the user to specify, on write, whether they want row groups or not and then we can enforce that as we write.