Geometry fields have a result type string and a 
  special subclass to cater for the differences
  between them and the base class (just like 
  DATE/TIME).
  When creating temporary tables for results of 
  functions that return results of type GEOMETRY
  we must construct fields of the derived class 
  instead of the base class.
  Fixed by creating a GEOMETRY field (Field_geom) 
  instead of a generic BLOB (Field_blob) in temp 
  tables for the results of GIS functions that 
  have GEOMETRY return type (Item_geometry_func).