MySQL查詢優化的MAX（）

Question

我有3個表有以下模式：

CREATE TABLE `devices` (
    `device_id` int(11) NOT NULL auto_increment, 
    `name` varchar(20) default NULL, 
    `appliance_id` int(11) default '0', 
    `sensor_type` int(11) default '0', 
    `display_name` VARCHAR(100), 
    PRIMARY KEY USING BTREE (`device_id`) 
) 

CREATE TABLE `channels` (
    `channel_id` int(11) NOT NULL AUTO_INCREMENT, 
    `device_id` int(11) NOT NULL, 
    `channel` varchar(10) NOT NULL, 
    PRIMARY KEY (`channel_id`), 
    KEY `device_id_idx` (`device_id`) 
) 

CREATE TABLE `historical_data` (
    `date_time` datetime NOT NULL, 
    `channel_id` int(11) NOT NULL, 
    `data` float DEFAULT NULL, 
    `unit` varchar(10) DEFAULT NULL, 
    KEY `devices_datetime_idx` (`date_time`) USING BTREE, 
    KEY `channel_id_idx` (`channel_id`) 
) 

的設置是一個設備可以具有一個或多個通道，每個通道具有許多（歷史的）的數據。

我用下面的查詢來獲取一個設備的最後歷史數據以及所有它的相關渠道：

SELECT c.channel_id, c.channel, max(h.date_time), h.data 
FROM devices d 
INNER JOIN channels c ON c.device_id = d.device_id 
INNER JOIN historical_data h ON h.channel_id = c.channel_id 
WHERE d.name = 'livingroom' AND d.appliance_id = '0' 
AND d.sensor_type = 1 AND (c.channel = 'ch1') 
GROUP BY c.channel 
ORDER BY h.date_time, channel 

查詢計劃如下所示：

+----+-------------+-------+--------+-----------------------+----------------+---------+---------------------------+--------+-------------+ 
| id | select_type | table | type | possible_keys   | key   | key_len | ref      | rows | Extra  | 
+----+-------------+-------+--------+-----------------------+----------------+---------+---------------------------+--------+-------------+ 
| 1 | SIMPLE  | c  | ALL | PRIMARY,device_id_idx | NULL   | NULL | NULL      |  34 | Using where | 
| 1 | SIMPLE  | d  | eq_ref | PRIMARY    | PRIMARY  | 4  | c.device_id    |  1 | Using where | 
| 1 | SIMPLE  | h  | ref | channel_id_idx  | channel_id_idx | 4  | c.channel_id    | 322019 |    | 
+----+-------------+-------+--------+-----------------------+----------------+---------+---------------------------+--------+-------------+ 
3 rows in set (0.00 sec) 

以上查詢是目前大約需要15秒，我想知道是否有任何提示或方法來改善查詢？

編輯：從historical_data 實施例數據

+---------------------+------------+------+------+ 
| date_time   | channel_id | data | unit | 
+---------------------+------------+------+------+ 
| 2011-11-20 21:30:57 |   34 | 23.5 | C | 
| 2011-11-20 21:30:57 |   9 | 68 | W | 
| 2011-11-20 21:30:54 |   34 | 23.5 | C | 
| 2011-11-20 21:30:54 |   5 | 316 | W | 
| 2011-11-20 21:30:53 |   34 | 23.5 | C | 
| 2011-11-20 21:30:53 |   2 | 34 | W | 
| 2011-11-20 21:30:51 |   34 | 23.4 | C | 
| 2011-11-20 21:30:51 |   9 | 68 | W | 
| 2011-11-20 21:30:49 |   34 | 23.4 | C | 
| 2011-11-20 21:30:49 |   4 | 193 | W | 
+---------------------+------------+------+------+ 
10 rows in set (0.00 sec) 

編輯2： 複式信道選擇例如：

SELECT c.channel_id, c.channel, max(h.date_time), h.data 
FROM devices d 
INNER JOIN channels c ON c.device_id = d.device_id 
INNER JOIN historical_data h ON h.channel_id = c.channel_id 
WHERE d.name = 'livingroom' AND d.appliance_id = '0' 
AND d.sensor_type = 1 AND (c.channel = 'ch1' OR c.channel = 'ch2' OR c.channel = 'ch2') 
GROUP BY c.channel 
ORDER BY h.date_time, channel 

我使用或c.channel where子句，因爲它更容易以語法形式生成，但如果需要可以更改爲使用IN。

編輯3：我想要達到 結果舉例：

+-----------+------------+---------+---------------------+-------+ 
| device_id | channel_id | channel | max(h.date_time) | data | 
+-----------+------------+---------+---------------------+-------+ 
|  28 |   9 | ch1  | 2011-11-21 20:39:36 |  0 | 
|  28 |   35 | ch2  | 2011-11-21 20:30:55 | 32767 | 
+-----------+------------+---------+---------------------+-------+ 

我已經加入了DEVICE_ID的例子，但我的選擇將只需要返回CHANNEL_ID，通道，最後DATE_TIME即最多和數據。結果應該是history_data表中每個設備的每個通道的最後一條記錄。

Answer 1

它似乎刪除重新創建date_time索引通過刪除並重新創建它加快了我的原始SQL高達2secs左右

Answer 2

我還沒有能夠測試這個，所以我想問你運行它，讓我們知道會發生什麼..如果它給你所需的結果，如果它比你當前的運行速度更快：

CREATE DEFINER=`root`@`localhost` PROCEDURE `GetLatestHistoricalData_EXAMPLE` 
    (
     IN param_device_name VARCHAR(20) 
    , IN param_appliance_id INT 
    , IN param_sensor_type INT 
    , IN param_channel VARCHAR(10) 
) 
BEGIN 

    SELECT 
     h.date_time, h.data 
    FROM 
     historical_data h 
     INNER JOIN 
     (
      SELECT c.channel_id 
      FROM devices d 
      INNER JOIN channels c ON c.device_id = d.device_id 
      WHERE 
       d.name = param_device_name 
      AND d.appliance_id = param_appliance_id 
      AND d.sensor_type = param_sensor_type 
      AND c.channel = param_channel 
     ) 
     c ON h.channel_id = c.channel_id 
    ORDER BY h.date_time DESC 
    LIMIT 1; 

END 

然後運行一個測試：

CALL GetLatestHistoricalData_EXAMPLE ('livingroom', 0, 1, 'ch1'); 

我努力的在它變成一個存儲過程，這樣即使你使用此一臺設備所期望的結果，你可以與其它設備嘗試，看到結果...謝謝！

[編輯]：在回答丹尼的評論這裏是一個更新的測試版本：

CREATE DEFINER=`root`@`localhost` PROCEDURE `GetLatestHistoricalData_EXAMPLE_3Channel` 
    (
     IN param_device_name VARCHAR(20) 
    , IN param_appliance_id INT 
    , IN param_sensor_type INT 
    , IN param_channel_1 VARCHAR(10) 
    , IN param_channel_2 VARCHAR(10) 
    , IN param_channel_3 VARCHAR(10) 
) 
BEGIN 

    SELECT 
     h.date_time, h.data 
    FROM 
     historical_data h 
     INNER JOIN 
     (
      SELECT c.channel_id 
      FROM devices d 
      INNER JOIN channels c ON c.device_id = d.device_id 
      WHERE 
       d.name = param_device_name 
      AND d.appliance_id = param_appliance_id 
      AND d.sensor_type = param_sensor_type 
      AND (
       c.channel IN (param_channel_1 
          ,param_channel_2 
          ,param_channel_3 
       ) 
     c ON h.channel_id = c.channel_id 
    ORDER BY h.date_time DESC 
    LIMIT 1; 

END 

然後運行一個測試：

CALL GetLatestHistoricalData_EXAMPLE_3Channel ('livingroom', 0, 1, 'ch1', 'ch2' , 'ch3'); 

同樣，這只是爲了測試，所以你將能夠看到它是否滿足您的需求..

Answer 3

我會首先在設備表（appliance_id，sensor_type，名稱）上添加一個索引以匹配您的查詢。我不知道這個表中有多少個條目，但是如果每個設備都有很大的元素，就可以正確使用它。

其次，在你的頻道表，索引上（DEVICE_ID，通道）

三，關於你的歷史數據，指數（CHANNEL_ID，DATE_TIME）

然後，

SELECT STRAIGHT_JOIN 
     PreQuery.MostRecent, 
     PreQuery.Channel_ID, 
     PreQuery.Channel, 
     H2.Data, 
     H2.Unit 
    from 
     (select 
       c.channel_id, 
       c.channel, 
       max(h.date_time) as MostRecent 
      from 
       devices d 

       join channels c 
        on d.device_id = c.device_id 
        and c.channel in ('ch1', 'ch2', 'ch3') 

        join historical_data h 
         on c.channel_id = c.Channel_id 
      where 
        d.appliance_id = 0 
       and d.sensor_type = 1 
       and d.name = 'livingroom' 

      group by 
       c.channel_id) PreQuery 

     JOIN Historical_Data H2 
     on PreQuery.Channel_ID = H2.Channel_ID 
     AND PreQuery.MostRecent = H2.Date_Time 
    order by 
     PreQuery.MostRecent, 
     PreQuery.Channel