run sql in parallel using future: but the sql is not executed

Question

I have the following function

(defn run [x]          
  (doseq [i  (range 1 x)]
     (println i)
     (future (j/execute! vertica-db ["insert /*+ direct */ into a select * from a limit 1"]))
   ))

when call it using

(run 100)

it will print 1..99, however if check the row number of table a, the row number is not increased which mean the sql is not executed. How to run the sql in parallel?

Answer 1

The only suspicious thing I see in your code is the fact that you never wait for the futures to finish (so maybe they don't ?).

You need to collect the values returned by future calls and then block until they finish by using (deref f)/@f (i.e. dereferencing the future) where f is one of those values.

Something like this should work:

(defn run [x]
  (let [db-insert (fn [i] ((println i) (future (j/execute! vertica-db ["insert /*+ direct */ into a select * from a limit 1"]))))
        inserts (doall (map db-insert (range 1 x)))] ;force execution of all the db-insert calls
        (doseq [insert inserts] @insert))) ;wait for all the futures to finish