As mentioned in geoarrow specification, a mixed type geometry column can be implemented in the form of a union column.

ST_distance, as well as many other operations to be implemented in the future computes between mixed geometry columns and it should be implemented in libcuspatial. After discussion, libcuspatial will implement Mixed-type geometry column with a specialization of union-column in arrow. This union column contains 6 children, standing for 6 single-typed geometry column respectively: point column, linestring column, polygon column, multipoint column, multilinestring column and multipolygon column. This will expand the expressiveness of libcuspatial library as well as reducing maintenance cost of arbitrary type union column.