SELECT FOR UPDATE NOWAIT

Question

Hi everyone!
I have procedure that looks like this:
procedure set_expired_users(
    p_QID   integer,
    p_Value integer default 1)
  is
    l_mdate timestamp(3) := null;
  begin
    l_mdate := get_mdate(p_Value);
    MERGE INTO cached_lists cl
    USING (SELECT distinct s.rootof as list_id, p_Value as is_expired, l_mdate as mdate
           FROM   zzz_userrelflat f, zzz_users u, zzz_temp_nn z, speclists s, lists_table lt
           WHERE  u.grp <> 1
                  AND f.userm = u.id
                  AND lt.id = z.id
                  AND f.userg = lt.ra
                  AND z.qid = p_QID
                  AND userm <> -2
                  AND s.ruser = userm) t
    ON (cl.list_id = t.list_id)
    when matched then
      UPDATE SET cl.is_expired = t.is_expired, cl.mdate = decode(t.mdate, null, cl.mdate, t.mdate)
    when not matched then
      INSERT VALUES (t.list_id, t.is_expired, l_mdate);
  end set_expired_users;As you can see there is no commit, commit executes in other stored procedures after that. in some cases I have bottleneck(enq TX), I rewrote procedure:
  procedure set_expired_users(
    p_QID   integer,
    p_Value integer default 1)
  is
    l_mdate timestamp(3) := null;
    busy_lock exception;
    PRAGMA exception_init(busy_lock,-54);
      cursor cur_upd  is
    SELECT 1 from cached_lists cl where cl.list_id in (SELECT distinct s.rootof as list_id
           FROM   zzz_userrelflat f, zzz_users u, zzz_temp_nn z, speclists s, lists_table lt
           WHERE  u.grp <> 1
                  AND f.userm = u.id
                  AND lt.id = z.id
                  AND f.userg = lt.ra
                  AND z.qid = p_QID
                  AND userm <> -2
                  AND s.ruser = userm) FOR UPDATE NOWAIT;
  begin
  open cur_upd;
    l_mdate := get_mdate(p_Value);
   MERGE INTO cached_lists cl
    USING (SELECT distinct s.rootof as list_id, p_Value as is_expired, l_mdate as mdate
           FROM   zzz_userrelflat f, zzz_users u, zzz_temp_nn z, speclists s, lists_table lt
           WHERE  u.grp <> 1
                  AND f.userm = u.id
                  AND lt.id = z.id
                  AND f.userg = lt.ra
                  AND z.qid = p_QID
                  AND userm <> -2
                  AND s.ruser = userm) t
    ON (cl.list_id = t.list_id)
    when matched then
      UPDATE SET cl.is_expired = t.is_expired, cl.mdate = decode(t.mdate, null, cl.mdate, t.mdate)
    when not matched then
      INSERT VALUES (t.list_id, t.is_expired, l_mdate);
        close cur_upd;  
      exception
  when busy_lock then
    null;
  end set_expired_users;Is this a good method to exclude enq TX?
Sincerely,
Pavel.

Answer 1

In my opinion, you shouldn't rewrite that procedure, but you should rethink (ie. streamline) the overall transaction and try to reduce the wait times between the procedure call and the commit/rollback.