Question

我有一张约5kk记录的表

CREATE TABLE IF NOT EXISTS `termo` (
  `id` int(11) NOT NULL auto_increment,
  `date` timestamp NOT NULL default '0000-00-00 00:00:00' on update     CURRENT_TIMESTAMP,
  `sensor` varchar(16) NOT NULL,
  `temp` float NOT NULL default '-255'
) ENGINE=MyISAM DEFAULT CHARSET=cp1251 AUTO_INCREMENT=5466795 ;

索引

ALTER TABLE `termo` ADD PRIMARY KEY  (`id`), ADD KEY `date_3` (`date`,`sensor`);

查询

select pressure, humidity, temp_a, voltage, aqua_temp, home_temp, home_hum, current from
(select temp as pressure from termo force index(date_3) where sensor='000461965E3901' order by date desc limit 0,1) as pressure,
(select temp as humidity from termo force index(date_3) where sensor='000461965E3903' order by date desc limit 0,1) as humidity,
(select temp as temp_a from termo force index(date_3) where sensor='000461965E3902' order by date desc limit 0,1) as temp_a,
(select temp as voltage from termo force index(date_3) where sensor='000461965E3904' order by date desc limit 0,1) as voltage,
(select temp as aqua_temp from termo force index(date_3) where sensor='000461965E3907' order by date desc limit 0,1) as aqua_temp,
(select temp as home_temp from termo force index(date_3) where sensor='000461965E3905' order by date desc limit 0,1) as home_temp,
(select temp as current from termo force index(date_3) where sensor='000461965E3911' order by date desc limit 0,1) as current,
(select temp as home_hum from termo force index(date_3) where sensor='000461965E3906' order by date desc limit 0,1) as home_hum

工作很慢......大约30秒。请帮助优化。

解释看起来像

id  select_type     table   type    possible_keys   key     key_len     ref     rows    Extra   
1   PRIMARY     <derived2>  system  NULL    NULL    NULL    NULL    1   
1   PRIMARY     <derived3>  system  NULL    NULL    NULL    NULL    1   
1   PRIMARY     <derived4>  system  NULL    NULL    NULL    NULL    1   
1   PRIMARY     <derived5>  system  NULL    NULL    NULL    NULL    1   
1   PRIMARY     <derived6>  system  NULL    NULL    NULL    NULL    1   
1   PRIMARY     <derived7>  system  NULL    NULL    NULL    NULL    1   
1   PRIMARY     <derived8>  system  NULL    NULL    NULL    NULL    1   
1   PRIMARY     <derived9>  system  NULL    NULL    NULL    NULL    1   
9   DERIVED     termo   index   NULL    date_3  3232    NULL    5193950     Using where

8   DERIVED     termo   index   NULL    date_3  3232    NULL    5193950     Using where

7   DERIVED     termo   index   NULL    date_3  3232    NULL    5193950     Using where

6   DERIVED     termo   index   NULL    date_3  3232    NULL    5193950     Using where

5   DERIVED     termo   index   NULL    date_3  3232    NULL    5193950     Using where

4   DERIVED     termo   index   NULL    date_3  3232    NULL    5193950     Using where

3   DERIVED     termo   index   NULL    date_3  3232    NULL    5193950     Using where

2   DERIVED     termo   index   NULL    date_3  3232    NULL    5193950     Using where

Answer 1

我认为这可能是更好的方式，但只需INDEX添加sensor，因此IN语句的工作速度更快。

SELECT termo.`sensor`, `temp`
FROM termo
JOIN (
    SELECT `sensor`, MAX(`date`) as `lastTemp`
    FROM termo 
    WHERE sensor IN ('000461965E3901', '000461965E3903', '000461965E3902',
                     '000461965E3904', '000461965E3907', '000461965E3905',
                     '000461965E3911', '000461965E3906')
    GROUP BY sensor
    ) T
 ON termo.`date` = T.`lastTemp`
AND termo.`sensor` = T.`sensor`

还要考虑使用复合INDEX (temp, date, sensor)

进行测试

要使用正确的标签

SELECT 
   MAX(CASE WHEN termo.sensor = '000461965E3901' THEN temp END) as pressure,
   MAX(CASE WHEN termo.sensor = '000461965E3903' THEN temp END) as humidity ,
   MAX(CASE WHEN termo.sensor = '000461965E3902' THEN temp END) as temp_a,
   MAX(CASE WHEN termo.sensor = '000461965E3904' THEN temp END) as voltage ,
   MAX(CASE WHEN termo.sensor = '000461965E3907' THEN temp END) as aqua_temp ,
   MAX(CASE WHEN termo.sensor = '000461965E3905' THEN temp END) as home_temp ,
   MAX(CASE WHEN termo.sensor = '000461965E3911' THEN temp END) as current ,
   MAX(CASE WHEN termo.sensor = '000461965E3906' THEN temp END) as home_hum
FROM termo
JOIN (
    SELECT `sensor`, MAX(`date`) as `lastTemp`
    FROM termo 
    WHERE sensor IN ('000461965E3901', '000461965E3903', '000461965E3902',
                     '000461965E3904', '000461965E3907', '000461965E3905',
                     '000461965E3911', '000461965E3906')
    GROUP BY sensor
    ) T
 ON termo.`date` = T.`lastTemp`
AND termo.`sensor` = T.`sensor`

Answer 2

尝试修改这样的每个子选择。我知道这不是最好的方式。我稍后会发送一个较短的查询。

(SELECT temp AS pressure FROM termo WHERE id = (SELECT max(id) FROM termo WHERE sensor='000461965E3901'))

并添加密钥

ALTER TABLE `termo`  ADD KEY `sensor` (`sensor`);

MySQL慢查询：按日期时间排序

2 个答案: