MySQL 优化实例—添加索引
http://chlotte.blog.51cto.com/318402/361132
近来webgame在维护后起动的速度越来越慢，竞然超过了1个小时30分钟，以前一直以为是数据量大的缘故，清理了无效的数据之后，速度没有任何改变，执行show full processlist发现异常：
mysql> show full processlist\G;
*************************** 14. row ***************************
Id: 16
User: programs
Host: localhost:53912
db: sword
Command: Query
Time: 1843
State: Sending data
Info: SELECT userId,leaveDttm, costTime, raid as layer FROM TongTianRecord as a WHERE times != 0 and raid!=0 and raid = (select max(raid) from TongTianRecord as b where a.userId=b.userId ) group by userId order by layer Desc , leaveDttm asc limit 50
*************************** 15. row ***************************
查了一下，表将近10W条记录
mysql> select count(*) from TongTianRecord;
+———-+
| count(*) |
+———-+
| 99090 |
+———-+
1 row in set (0.03 sec)
手工执行了这条异常语句，速度惊人，执行这条语句花费了近43分钟，这是什么概念呀
mysql> SELECT userId,leaveDttm, costTime, raid as layer FROM TongTianRecord as a WHERE times != 0 and raid!=0 and raid = (select max(raid) from TongTianRecord as b where a.userId=b.userId ) group by userId order by layer Desc , leaveDttm asc limit 50
-> ;
+——–+———————+———-+——-+
| userId | leaveDttm | costTime | layer |
+——–+———————+———-+——-+
| 432676 | 2010-03-03 20:44:25 | 47271 | 142 |
| 437123 | 2010-03-07 19:40:43 | 38798 | 142 |
| 385063 | 2010-03-02 19:05:52 | 14772 | 140 |
| 370529 | 2010-03-11 10:00:40 | 68756 | 140 |
| 416174 | 2010-05-22 10:03:24 | 72971 | 133 |
| 385938 | 2010-06-07 13:47:52 | 28274 | 129 |
| 442027 | 2010-06-09 11:08:35 | 48048 | 128 |
| 107397 | 2010-01-09 23:58:35 | 9954 | 123 |
| 129437 | 2010-01-17 08:13:40 | 41907 | 123 |
| 227342 | 2010-02-17 12:19:09 | 6170 | 123 |
| 441531 | 2010-03-20 12:52:47 | 5901 | 123 |
| 180382 | 2010-04-07 21:53:42 | 19133 | 123 |
| 212991 | 2010-06-20 08:48:32 | 57859 | 123 |
| 252337 | 2010-07-23 00:41:53 | 4867 | 123 |
| 216937 | 2010-01-07 06:27:14 | 24580 | 122 |
| 73227 | 2010-02-05 18:09:50 | 8336 | 122 |
| 187937 | 2010-03-18 23:55:45 | 7375 | 122 |
| 220040 | 2010-03-20 13:48:14 | 45294 | 122 |
| 185100 | 2010-05-06 02:34:09 | 13080 | 122 |
| 187953 | 2010-05-11 20:54:49 | 3571 | 122 |
| 118332 | 2010-05-20 19:19:37 | 9057 | 122 |
| 303014 | 2010-06-06 15:35:23 | 9638 | 122 |
| 218924 | 2010-07-14 19:53:41 | 93286 | 122 |
| 218689 | 2010-01-03 00:00:09 | 2999 | 121 |
| 245938 | 2010-01-17 19:39:56 | 13599 | 121 |
| 425601 | 2010-05-18 17:11:56 | 5007 | 121 |
| 217315 | 2010-05-22 09:33:52 | 45245 | 121 |
| 368088 | 2010-01-12 20:10:09 |
