Escludi gli elementi dell'array corrispondenti

Sembra XOR tra gli array:

WITH set1 AS
(
 SELECT * FROM unnest('{1, 2, 5, 15}'::int[])
), set2 AS
(
 SELECT * FROM unnest('{1, 2, 3, 6, 7, 9, 15}'::int[])
), xor AS
(
  (SELECT * FROM set1
   UNION 
   SELECT * FROM set2)
  EXCEPT
  (SELECT * FROM set1
   INTERSECT 
   SELECT * FROM set2)
)
SELECT array_agg(unnest ORDER BY unnest)
FROM xor

Uscita:

"{3,5,6,7,9}"

Come funziona:

1. Disnestare entrambi gli array
2. Calcola SOMMA
3. Calcola INTERSECT
4. Da SOMMA - INTERSEZIONE
5. Combina a array

In alternativa puoi usare la somma di entrambe le operazioni meno (tranne):

(A+B) - (A^B)
<=>
(A-B) + (B-A)

Utilizzando FULL JOIN :

WITH set1 AS
(
 SELECT *
FROM unnest('{1, 2, 5, 15}'::int[])
), set2 AS
(
 SELECT *
 FROM unnest('{1, 2, 3, 6, 7, 9, 15}'::int[])
)
SELECT array_agg(COALESCE(s1.unnest, s2.unnest) 
                 ORDER BY COALESCE(s1.unnest, s2.unnest))
FROM set1 s1
FULL JOIN set2 s2
  ON s1.unnest = s2.unnest
WHERE s1.unnest IS NULL
  OR s2.unnest IS NULL;

MODIFICA:

Se vuoi solo elementi del secondo array che non lo sono, usa prima semplice EXCEPT :

SELECT array_agg(unnest ORDER BY unnest)
FROM (SELECT * FROM unnest('{1, 2, 3, 6, 7, 9, 15}'::int[])
      EXCEPT
      SELECT * FROM unnest('{1, 2, 5, 15}'::int[])) AS sub

Uscita:

"{3,6,7,9}"