SELECT row_number() over (order by a.id desc) as gid, a.nsg as nsg, b.ffh as ffh,
       ST_Union(a.geom,b.geom) as geom, st_area(St_union(a.geom,b.geom)) as area
FROM   p_gaiac.shape1 as a, p_gaiac.shape2 as b

SELECT geom FROM table1
UNION
SELECT geom FROM table2
UNION
SELECT geom FROM (
    (SELECT ST_Intersection(t3.geom, t4.geom) As geom
    FROM table1 as t3 JOIN table2 as t4 ON ST_Intersects(t3.geom, t4.geom))
    ) As subquery;