How can I improve this Clojure function?
===

I just wrote my first Clojure function based on my very limited knowledge of the language. I would love some feedback in regards to performance and use of types. For example, I'm not sure 
if I should be using lists or vectors.

    (defn actor-ids-for-subject-id [subject-id]
      (sql/with-connection (System/getenv "DATABASE_URL")
        (sql/with-query-results results
          ["SELECT actor_id FROM entries WHERE subject_id = ?" subject-id]
          (let [res (into [] results)]
            (map (fn [row] (get row :actor_id)) res)))))

It passes the following test (given proper seed data):

    (deftest test-actor-ids-for-subject-id
      (is (= ["123" "321"] (actor-ids-for-subject-id "123"))))

If it makes a difference (and I imagine it does) my usage characteristics of the returned data will almost exclusively involve generating the union and intersection of another set returned by the same function.