Re:

How come when I pass in a list in it doesn't use it as a list of integers? Do I need to somehow make $1 be interpreted as a list of ints? Rather than just passing a text value that contains the list?

CREATE TABLE my_tbl (u_id int);

INSERT INTO my_tbl (u_id) values (1);
INSERT INTO my_tbl (u_id) values (2);
INSERT INTO my_tbl (u_id) values (3);

CREATE OR REPLACE FUNCTION getlist(text) RETURNS SETOF my_tbl as $$
SELECT * FROM my_tbl
WHERE u_id IN (0, $1);

$$ LANGUAGE SQL;

SELECT * from getlist('1,2,3');
(0 rows)

I'm sure it's probably trival but I'm still learning how psql  :) Thanks