If you use intarray type it is convenient to call buil-in intarray
functions for your purpose.
http://www.postgresql.org/docs/8.3/static/intarray.html
SELECT t.a[1] AS "min", t.a[array_upper(t.a, 1)] AS "max"
FROM (SELECT sort(string_to_array('2,3,4,15,6,7',',')::int[]) AS a)t;
--
Best regards, Yuri.