这是我的Flask-sqlalchemy代码，它不断轮询某些数据：
start_time = datetime.now()
query = Controller.query
sid = request.args.get('sid', "", type=str)
if sid:
safe_sid = sanitize_user_input(sid, escape="/")
query = query.filter(Controller.sid.startswith(safe_sid, escape="/"))
else:
query = query.filter(Controller.sid != None)
if 'name' in request.args:
query = query.name_like(request.args['name'])
head = request.args.get('head', 0, type=int)
if head == 1:
query = query.head_only()
query = query.options(joinedload('monitors'), joinedload('maintenance'))
query = query.order_by(Controller.id)
controllers = query.all()
logger.info('controllers/ delta after query: {}'.format(str(datetime.now()-start_time)))
它在日志中显示以下时间：
2015-07-12T01:16:34 controllers/ delta after query: 0:00:00.850631
2015-07-12T01:16:37 controllers/ delta after query: 0:00:00.859993
所以基本上它大概是900毫秒。
但是，当我查看postgres日志时，我会看到更小的值：
2015-07-12 01:16:33.400 EDT >LOG: duration: 136.405 ms statement:
SELECT controller.sid AS controller_sid, controller.id AS controller_id,
controller.type AS controller_type, controller.name AS controller_name,
controller.description AS controller_description, controller.address AS controller_address,
controller.last_response AS controller_last_response, controller.gis_id AS controller_gis_id,
controller.tag AS controller_tag, controller.parent_id AS controller_parent_id,
controller.rdp_id AS controller_rdp_id, controller.root_node_id AS controller_root_node_id,
asuno_roles_1.id AS asuno_roles_1_id, asuno_roles_1.slug AS asuno_roles_1_slug,
asuno_roles_1.name AS asuno_roles_1_name, asuno_roles_1.description AS asuno_roles_1_description,
asuno_users_1.id AS asuno_users_1_id, asuno_users_1.name AS asuno_users_1_name,
asuno_users_1.username AS asuno_users_1_username, asuno_users_1.password_hash AS asuno_users_1_password_hash,
asuno_users_1.active AS asuno_users_1_active, rdp_dispatcher_1.user_id AS rdp_dispatcher_1_user_id,
rdp_dispatcher_1.rdp_id AS rdp_dispatcher_1_rdp_id, maintenance_1.id AS maintenance_1_id,
maintenance_1.date_from AS maintenance_1_date_from, maintenance_1.date_to AS maintenance_1_date_to,
maintenance_1.reason AS maintenance_1_reason, maintenance_1.controller_id AS maintenance_1_controller_id,
maintenance_1.user_id AS maintenance_1_user_id, controller_monitor_1.id AS controller_monitor_1_id,
controller_monitor_1.sid AS controller_monitor_1_sid,
controller_monitor_1.node_type AS controller_monitor_1_node_type,
controller_monitor_1.name AS controller_monitor_1_name,
controller_monitor_1.tag AS controller_monitor_1_tag,
controller_monitor_1.denotation AS controller_monitor_1_denotation,
controller_monitor_1.alias AS controller_monitor_1_alias,
controller_monitor_1.controller_id AS controller_monitor_1_controller_id,
controller_monitor_1.value AS controller_monitor_1_value,
controller_monitor_1.silent AS controller_monitor_1_silent,
controller_monitor_1.last_reading_timestamp AS controller_monitor_1_last_reading_timestamp,
controller_monitor_1.payload AS controller_monitor_1_payload
FROM controller LEFT OUTER JOIN maintenance AS maintenance_1 ON controller.id = maintenance_1.controller_id
AND maintenance_1.date_from <= TIMEZONE('utc', CURRENT_TIMESTAMP)
AND maintenance_1.date_to >= TIMEZONE('utc', CURRENT_TIMESTAMP)
LEFT OUTER JOIN asuno_users AS asuno_users_1 ON asuno_users_1.id = maintenance_1.user_id
LEFT OUTER JOIN (roles_users AS roles_users_1
JOIN asuno_roles AS asuno_roles_1 ON asuno_roles_1.id = roles_users_1.role_id)
ON asuno_users_1.id = roles_users_1.user_id
LEFT OUTER JOIN rdp_dispatcher AS rdp_dispatcher_1 ON asuno_users_1.id = rdp_dispatcher_1.user_id
LEFT OUTER JOIN controller_monitor AS controller_monitor_1 ON controller.id = controller_monitor_1.controller_id
WHERE (controller.sid LIKE 'obekty-no:artiukhinoi' || '%' ESCAPE '/')
AND controller.parent_id IS NULL ORDER BY controller.id, maintenance_1.date_from
额外600毫秒来自哪里？