You've found an unported query. The following query (untested) should work:
"select c.category, c.category_id,
case when ui.category_id is NULL then NULL::char else 't'::char end as selected_p
from categories c, users_interests ui
where c.enabled_p = 't'
and ui.user_id = $user_id
and c.category_id = ui.category_id
union all
select c.category, c.category_id,
NULL::char as selected_p
from categories c
where c.enabled_p = 't'
and ui.user_id = $user_id
and not exists (select 1 from user_interests
where category_id = c.category_id
and user_id = $user_id)"
