cal's questions

@符号在此sql表达式中表示什么？

update some_table set some_number=(@some_number+1) where some_id=1;

它似乎返回与以下相同的结果：

update some_table set some_number=some_number+1 where some_id=1;

据我了解，主键在定义上是唯一的。如果这是真的，有人会想要在主键上选择 DISTINCT 吗？这是否实用/有用？

这个查询

SELECT *
FROM
  ( SELECT event_id
   FROM events_201806
   WHERE registry_name='$VCZN://externalcenter/ninaExtCenter_ZN'
   UNION ALL SELECT event_id
   FROM events_201805
   WHERE registry_name='$VCZN://externalcenter/ninaExtCenter_ZN'
   UNION ALL SELECT event_id
   FROM events_201804
   WHERE registry_name='$VCZN://externalcenter/ninaExtCenter_ZN'
   UNION ALL SELECT event_id
   FROM events_201803
   WHERE registry_name='$VCZN://externalcenter/ninaExtCenter_ZN') AS ids 
NATURAL INNER JOIN events;

返回 456 个结果，耗时约 20 毫秒。

添加 ORDER BY

SELECT *
FROM
  (SELECT event_id
   FROM events_201806
   WHERE registry_name='$VCZN://externalcenter/ninaExtCenter_ZN'
   UNION ALL SELECT event_id
   FROM events_201805
   WHERE registry_name='$VCZN://externalcenter/ninaExtCenter_ZN'
   UNION ALL SELECT event_id
   FROM events_201804
   WHERE registry_name='$VCZN://externalcenter/ninaExtCenter_ZN'
   UNION ALL SELECT event_id
   FROM events_201803
   WHERE registry_name='$VCZN://externalcenter/ninaExtCenter_ZN') AS ids 
NATURAL INNER JOIN events
ORDER BY event_ts DESC;

导致查询需要很长时间才能完成，大约 10 分钟。

关于如何解决这个问题的任何指示？