Subselect with limit - Mailing list pgsql-general

I guess I can't do like this
SELECT   id,a,b  FROM  T  WHERE id NOT IN (SELECT id FROM T2 WHERE
c=5) LIMIT 20;

I only want 20 rows.
How do I do it?

I have table
T (
id ,a,b integer, primary key(id) );
T2 (id