改进 MySQL 联合声明

Question

我有以下 mySql 语句，我确信它可以变得更有效率，我只是不确定什么是最好的方法......唯一的区别是 equip_id 在 WHERE 子句中发生变化...

==============

 (SELECT 
        `receipt_ts`, 
        `driver_id`,
        `equip_id`, 
        `pos_lon`, 
        `pos_lat`, 
        `pos_timestamp`  
   FROM `log_messaging` 
  WHERE `equip_id`='207' AND `tran_type`='T.2.12.0' 
  ORDER BY receipt_ts DESC LIMIT 1 ) 

 UNION  

(SELECT 
       `receipt_ts`, 
       `driver_id`, 
       `equip_id`, 
       `pos_lon`, 
       `pos_lat`, 
       `pos_timestamp`  
  FROM `log_messaging` 
 WHERE `equip_id`='212' AND `tran_type`='T.2.12.0' 
 ORDER BY receipt_ts DESC LIMIT 1 ) 

 UNION

  (SELECT 
         `receipt_ts`, 
         `driver_id`, 
         `equip_id`, 
         `pos_lon`, 
         `pos_lat`, 
         `pos_timestamp`  
    FROM `log_messaging`
   WHERE `equip_id`='213' AND `tran_type`='T.2.12.0' 
   ORDER BY receipt_ts DESC LIMIT 1 );

================

我正在联合每个返回的行来为多个 equip_id 构建一个 max(receipt_ts) 的数据集。（我需要获取设备的最新定位。

有时查询最终会针对 100 多个唯一 equip_id。

我正在尝试使查询执行得比现在更快（如上约 100 个 UNIONS 大约 7 秒...

给我指明正确的方向??

谢谢！

Answer 1

我会使用 IN 子句：

SELECT receipt_ts, driver_id, equip_id, pos_lon, pos_lat, pos_timestamp 
FROM log_messaging a
JOIN (SELECT c.equip_id, max(c.receipt_ts) as receipt
      FROM log_messaging c
      WHERE equip_id in ('207', '212', '213')
        AND tran_type='T.2.12.0'
      GROUP by c.equip_id) b USING(equip_id)
WHERE b.receipt = a.receipt_ts
ORDER BY a.receipt_ts

请注意，如果您真的想使用 UNION（我不明白您为什么要这样做）但又想对其进行优化，您可以使用 UNION ALL，它的性能会更好UNION检查数据是否重复删除，消耗更多进程。

Answer 2

首先，您应该使用 union all 而不是 union。所以从这个查询开始：

(SELECT receipt_ts, driver_id, equip_id, pos_lon, pos_lat, pos_timestamp
 FROM log_messaging
 WHERE equip_id = '207' AND tran_type = 'T.2.12.0'
 ORDER BY receipt_ts DESC
 LIMIT 1
) UNION ALL
(SELECT receipt_ts, driver_id, equip_id, pos_lon, pos_lat, pos_timestamp
 FROM log_messaging
 WHERE equip_id = '212' AND tran_type = 'T.2.12.0'
 ORDER BY receipt_ts DESC
 LIMIT 1
) UNION ALL
(SELECT receipt_ts, driver_id, equip_id, pos_lon, pos_lat, pos_timestamp
 FROM log_messaging
 WHERE equip_id = '213' AND tran_type = 'T.2.12.0'
 ORDER BY receipt_ts DESC
 LIMIT 1
) ;

这是一个合理的查询。我建议您在 log_messaging(tran_type, equip_id, recipt_ts).

上创建一个索引

改进 MySQL 联合声明

Improve MySQL Union Statement

mysql

union

join