Clarify support for zero geometry columns

[himikof]

Clarify support for zero geometry columns

[jorisvandenbossche]

What is the advantage of having the metadata without columns (in which case it is basically just a version, i.e. "geo": {"columns": [], "version": "1.0.0-dev"}), compared to just leaving out the "geo" metadata?

[himikof]

I think that would depend on whether the "geo" metadata would be made optional in the specification (that is, if any other parquet file would be a valid GeoParquet file, just without geo-columns). If that is true, then leaving out "geo" metadata could be indeed another solution. But if the spec would require a valid GeoParquet file to always have this metadata (as it currently does), then it would be very strange for a GeoDataFrame (or something like that) GeoParquet serialization function to write a file that:

1. is not valid according to the GeoParquet specification;
2. would error out due to the missing required metadata in many (if not all) readers.

It seems like a trade-off between supporting missing "geo" metadata (and possibly interpreting a totally unrelated parquet file as GeoParquet) and empty "columns" (which likely requires a bit of careful handling) in GeoParquet readers.