Improve the performance of the query to get the last template usage.

By adding the service id, the query performance has improved greatly. It went from 6200ms to 0.04ms.
This should stop the 500s when a template is deleted.

app/dao/notifications_dao.py

@@ -84,11 +84,15 @@ def dao_get_template_usage(service_id, day):
 
 
 @statsd(namespace="dao")
-def dao_get_last_template_usage(template_id, template_type):
+def dao_get_last_template_usage(template_id, template_type, service_id):
+    # By adding the service_id to the filter the performance of the query is greatly improved.
+    # Using a max(Notification.created_at) is better than order by and limit one.
+    # But the effort to change the endpoint to return a datetime only is more than the gain.
     return Notification.query.filter(
         Notification.template_id == template_id,
         Notification.key_type != KEY_TYPE_TEST,
-        Notification.notification_type == template_type
+        Notification.notification_type == template_type,
+        Notification.service_id == service_id
     ).order_by(
         desc(Notification.created_at)
     ).first()

app/template_statistics/rest.py

@@ -47,7 +47,7 @@ def get_template_statistics_for_template_id(service_id, template_id):
     template = dao_get_template_by_id_and_service_id(template_id, service_id)
 
     data = None
-    notification = dao_get_last_template_usage(template_id, template.template_type)
+    notification = dao_get_last_template_usage(template_id, template.template_type, template.service_id)
     if notification:
         data = notification_with_template_schema.dump(notification).data