gistfile1.txt

table t1 contains 4 rows, with id = 1, 2, 3, 5

SELECT X, Y
FROM (
    SELECT A.id as X, (SELECT MIN(b.id) FROM t1 AS B WHERE B.id > A.id) as Y
    FROM t1 AS B
) C
WHERE Y IS NOT NULL

 id1 |  id2 
-----+------
   1 |    2
   2 |    3
   3 |    5
(4 rows)