The recursive part of the query builds a table r that has all objects at all levels. This is done by iteratively joining the table itself with the geo_objects_attributes and geo_objects tables. The top level region is added in the first part of the union (select 1) and then the geo_objects_attributes and geo_objects tables are joined with the table r itself. This is done for all levels of the hierarchy. In the end, the result is simply joined with the geo_polygons table.

WITH RECURSIVE r AS (
  SELECT 1 AS level, id, name, parent_id
  FROM geo_objects_attributes
  WHERE parent_id IS NULL
  UNION ALL
  SELECT r.level + 1, geo_objects_attributes.id, geo_objects_attributes.name, geo_objects_attributes.parent_id
  FROM geo_objects_attributes, r
  WHERE geo_objects_attributes.parent_id = r.id
)
SELECT r.id, r.name, geo_polygons.polygon
FROM r
LEFT JOIN geo_polygons ON r.id = geo_polygons.object_id;