perf: avoid checking success tasks when save limit is disabled (#255)

- avoid extra queries when checking the count of success in db

django_q/monitor.py

@@ -107,13 +107,16 @@ def save_task(task, broker: Broker):
                 value = get_func_repr(value)
             filters[Conf.SAVE_LIMIT_PER] = value
 
-        with db.transaction.atomic(using=db.router.db_for_write(Success)):
-            list(Success.objects.filter(**filters).select_for_update())
-            if (
-                task["success"]
-                and 0 < Conf.SAVE_LIMIT <= Success.objects.filter(**filters).count()
-            ):
-                Success.objects.filter(**filters).last().delete()
+        # check if we should clean the success tasks
+        if Conf.SAVE_LIMIT > 0:
+            with db.transaction.atomic(using=db.router.db_for_write(Success)):
+                success_tasks_qs = Success.objects.filter(**filters)
+                success_tasks_pks = [
+                    success_task.pk
+                    for success_task in success_tasks_qs.select_for_update()
+                ]
+                if task["success"] and len(success_tasks_pks) >= Conf.SAVE_LIMIT:
+                    success_tasks_qs.last().delete()
 
         # check if this task has previous results
         try: