sym_difference( geometry a, geometry b)
geom_to_wkt( sym_difference( geom_from_wkt( 'LINESTRING(3 3 , 4 4 , 5 5)' ) , geom_from_wkt( 'LINESTRING(3 3 , 8 8)' )) ) → returns LINESTRING(5 5, 8 8)