Here is an example that works in a single query. Since you have two different orders you want the data back in, you need to use subqueries to get the proper data back, but it works, and is very fast.
CREATE TEMPORARY TABLE foo AS
SELECT generate_series as bar
FROM generate_series(1, 1000000);
CREATE INDEX idx_foo_bar ON foo (bar);
SELECT *
FROM (
SELECT bar
FROM foo
ORDER BY bar asc
LIMIT 1
) x
UNION ALL
SELECT *
FROM (
SELECT bar
FROM foo
ORDER BY bar desc
LIMIT 1
) y;
DROP TABLE foo;
