WITH data1 AS(
SELECT f.x x1, f.y y1, ROW_NUMBER() OVER(ORDER BY f.x) RN1
FROM functions f
),

data2 AS(
SELECT f.x x2, f.y y2, ROW_NUMBER() OVER(ORDER BY f.x) RN2
FROM functions f
)

SELECT DISTINCT x1, y1
FROM data1
JOIN data2
    ON x1=y2 AND y1=x2 AND RN1 != RN2 AND x1 <= y1
ORDER BY x1

WITH NUM AS
    (SELECT ROW_NUMBER() OVER(ORDER BY X,Y) RN, X,Y
    FROM FUNCTIONS)

SELECT F1.X,F1.Y
FROM NUM F1
INNER JOIN NUM F2
ON F1.X =F2.Y AND F1.Y = F2.X AND F1.RN < F2.RN
ORDER BY F1.X

WITH RankedRows AS (SELECT ROW_NUMBER() OVER (ORDER BY X, Y) AS rn, X, Y FROM Functions),
Pairs AS (SELECT f1.X AS X1, f1.Y AS Y1, f2.X AS X2, f2.Y AS Y2  
          FROM RankedRows f1 JOIN RankedRows f2 
          ON f1.X = f2.Y AND f2.X = f1.Y AND f1.rn < f2.rn)
SELECT X1, Y1 FROM Pairs ORDER BY X1;

WITH _cte_functions_rules AS (
    SELECT * FROM (
        SELECT x, y, count(1) AS xy_count FROM Functions GROUP BY x, y
    ) a WHERE x <> y OR xy_count > 1
)
SELECT least(f1.x, f1.y) AS x, greatest(f1.y, f1.x) AS f
  FROM _cte_functions_rules f1
  JOIN _cte_functions_rules f2
    ON f2.y = f1.x
   AND f2.x = f1.y
 GROUP BY 1,2
 ORDER BY x