Question

I have 3 tables with the following schema:

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`)
)

The setup is that a device can have one or more channels and each channel has many (historical) data.

I use the following query to get the last historical data for one device and all it’s related channels:

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

The query plan looks as follows:

+----+-------------+-------+--------+-----------------------+----------------+---------+---------------------------+--------+-------------+
| 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)

The above query is currently taking approximately 15 secs and I wanted to know if there are any tips or way to improve the query?

Edit:
Example data from historical_data

+---------------------+------------+------+------+
| 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)

Edit 2:
Mutliple channel SELECT example:

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

I’ve used OR in the c.channel where clause because it was easier to generated pro grammatically but it can be changed to use IN if necessary.

Edit 3:
Example result of what I’m trying to achieve:

+-----------+------------+---------+---------------------+-------+
| 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 |
+-----------+------------+---------+---------------------+-------+

I have added the device_id to the example but my select will only need to return channel_id, channel, last date_time i.e max and the data. The results should be the last record from the historical_data table for each channel for one device.

Answer 1

It seems that removing an re-creating the index on date_time by deleting and creating it again sped up my original SQL up to around 2secs