mysql数据检索过多导致索引没有生效的问题
查询的时候，在数据量过多时出现索引失效的情况，请问是什么原因？
```
具体情况，mysql版本5.7.7,ad_stat_day表总数据在250W左右，day_time字段上有建立索引
```
```
-- 25号至30号总数据
mysql> select count(*) from ad_stat_day where dayTime BETWEEN  '2017-10-25' and '2017-10-30';
+----------+
| count(*) |
+----------+
|   107063 |
+----------+
1 row in set
```
```
-- 只查id的情况下会走索引
mysql> EXPLAIN select id from ad_stat_day where dayTime BETWEEN  '2017-10-25' and '2017-10-30';
+----+-------------+-------------+------------+-------+---------------+-------------+---------+------+--------+----------+--------------------------+
| id | select_type | table       | partitions | type  | possible_keys | key         | key_len | ref  | rows   | filtered | Extra                    |
+----+-------------+-------------+------------+-------+---------------+-------------+---------+------+--------+----------+--------------------------+
|  1 | SIMPLE      | ad_stat_day | NULL       | range | nk_day_time   | nk_day_time | 4       | NULL | 189566 |      100 | Using where; Using index |
+----+-------------+-------------+------------+-------+---------------+-------------+---------+------+--------+----------+--------------------------+
1 row in set
```
```
-- 查id,fee就不走索引了
mysql> EXPLAIN select id,fee from ad_stat_day where dayTime BETWEEN  '2017-10-25' and '2017-10-30';
+----+-------------+-------------+------------+------+---------------+------+---------+------+--------+----------+-------------+
| id | select_type | table       | partitions | type | possible_keys | key  | key_len | ref  | rows   | filtered | Extra       |
+----+-------------+-------------+------------+------+---------------+------+---------+------+--------+----------+-------------+
|  1 | SIMPLE      | ad_stat_day | NULL       | ALL  | nk_day_time   | NULL | NULL    | NULL | 646016 |    27.18 | Using where |
+----+-------------+-------------+------------+------+---------------+------+---------+------+--------+----------+-------------+
1 row in set
```
```
--缩小条件范围，查询id,fee也会走索引
mysql> EXPLAIN select id,fee from ad_stat_day where dayTime BETWEEN  '2017-10-27' and '2017-10-30';
+----+-------------+-------------+------------+-------+---------------+-------------+---------+------+--------+----------+----------------------------------+
| id | select_type | table       | partitions | type  | possible_keys | key         | key_len | ref  | rows   | filtered | Extra                            |
+----+-------------+-------------+------------+-------+---------------+-------------+---------+------+--------+----------+----------------------------------+
|  1 | SIMPLE      | ad_stat_day | NULL       | range | nk_day_time   | nk_day_time | 4       | NULL | 124092 |      100 | Using index condition; Using MRR |
+----+-------------+-------------+------------+-------+---------------+-------------+---------+------+--------+----------+----------------------------------+
1 row in set
```
求大佬告知下mysql是根据什么情况来选择使用索引和不使用索引的？
```
这次范围扫描数据(189566行)不到表总数的10%，而加上一个字段就成了(646016行)占到表总数的25%来了，这个646016行数据mysql是怎么算出来的？还有mysql有明确的配置来说明超过某个阀值(百分比)就不使用索引了吗？
```
索引:hi，MySQL，这次我给你带来了一个很大范围的索引片哦，并且我的索引结构上面还有一个只在表结构里面存在的数据，你看看怎么查询起来快吧...
MySQL:我擦，你的数据量太大了，我还不如直接去表里面查快呢...
索引:hi，MySQL，我为上次的事情道歉，这次虽然给你带来的范围还是比较大，但是呢，我需要的字段都在我的索引结构上面，你帮我处理一下吧...
MySQL:嗯，这次你不需要通过表结构，我就直接在你自己身上给你处理吧...
上文中的`MySQL`就是我们知道的`MySQL`的优化器在处理，《数据库索引结构和设计》这本书中有讲一个三星索引的概念，像你遇到的这种情况，你可以考虑一下是否需要建立一个`<day_time , fee>`的联合索引，不知道我是否有阐述清楚。