mysql优化一则（20170703）

1.表结构

CREATE TABLE `room_break_history_tmp_test ` (
`id` INT(11) NOT NULL AUTO_INCREMENT,
`break_type` INT(11) DEFAULT NULL,
`app_id` INT(11) DEFAULT NULL,
`room_id` INT(11) DEFAULT NULL,
`from_user_id` INT(11) DEFAULT NULL,
`to_user_id` INT(11) DEFAULT NULL,
`content_type` INT(11) DEFAULT NULL,
`content_name` VARCHAR(300) DEFAULT NULL,
`source_message` VARCHAR(1536) DEFAULT NULL,
`send_message` VARCHAR(1536) DEFAULT NULL,
`request_type` INT(4) DEFAULT NULL,
`report_relation` VARCHAR(1536) DEFAULT NULL,
`handle_type` INT(11) DEFAULT NULL,
`handle_uid` INT(11) DEFAULT NULL,
`create_time` DATETIME DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `idx_from_user_id` (`room_id`,`from_user_id`,`handle_type`,`create_time`)
) ENGINE=INNODB AUTO_INCREMENT=3416971 DEFAULT CHARSET=utf8mb4

2.执行语句

DESC SELECT
COUNT(1)
FROM
(SELECT
COUNT(1)
FROM
room_break_history_tmp_test
WHERE `create_time` BETWEEN '2017-07-01 22:25:33'
AND '2017-07-01 22:27:00'
AND handle_type = 5
GROUP BY room_id,
from_user_id) AS keywordtemp

3.执行计划

    id  select_type  table               type    possible_keys     key               key_len  ref        rows  Extra                     
------  -----------  ------------------  ------  ----------------  ----------------  -------  ------  -------  --------------------------
     1  PRIMARY      <derived2>          ALL     (NULL)            (NULL)            (NULL)   (NULL)  3438331  (NULL)                    
     2  DERIVED      room_break_history  index   idx_from_user_id  idx_from_user_id  21       (NULL)  3438331  Using where; Using index

4.执行时长：

Execution Time : 17.182 sec
Transfer Time : 0.001 sec
Total Time : 17.184 sec

5.描述，就执行计划看，type为index，key及key_len正常，看似是走了索引，但是rows几乎是全表记录（不准确，就是全表扫描），300多万的数据执行时长居然17秒。

思考：将字段的nullable改为not null后，key_len变短了，是不是将是否为空的判断逻辑添加到了数据上？

有关null的文章：

https://dev.mysql.com/doc/refman/5.6/en/problems-with-null.html

改进：

1.添加索引

ALTER TABLE `test`.`room_break_history_tmp_test`
-> ADD INDEX `idx_handle_time` (`handle_type`, `create_time`);

2.执行计划

    id  select_type  table                        type    possible_keys                     key              key_len  ref       rows  Extra                                                   
------  -----------  ---------------------------  ------  --------------------------------  ---------------  -------  ------  ------  --------------------------------------------------------
     1  PRIMARY      <derived2>                   ALL     (NULL)                            (NULL)           (NULL)   (NULL)       2  (NULL)                                                  
     2  DERIVED      room_break_history_tmp_test  range   idx_from_user_id,idx_handle_time  idx_handle_time  7        (NULL)       1  Using index condition; Using temporary; Using filesort

3.执行时长

Execution Time : 0.178 sec
Transfer Time : 0 sec
Total Time : 0.179 sec