处理数据分布不均，orace数据库使用额外的统计数据直方图来完成，而MYSQL
中统计数据只有索引的不同值这样一个统计数据，那么我们制出如下数据：
mysql> select * from test.testf;
+------+----------+
| id   | name     |
+------+----------+
|    1 | gaopeng  |
|    2 | gaopeng1 |
|    3 | gaopeng1 |
|    4 | gaopeng1 |
|    5 | gaopeng1 |
|    6 | gaopeng1 |
|    7 | gaopeng1 |
|    8 | gaopeng1 |
|    9 | gaopeng1 |
|   10 | gaopeng1 |
+------+----------+
10 rows in set (0.00 sec)
name 上有一个普通二级索引
mysql> analyze table test.testf;
+------------+---------+----------+----------+
| Table      | Op      | Msg_type | Msg_text |
+------------+---------+----------+----------+
| test.testf | analyze | status   | OK       |
+------------+---------+----------+----------+
1 row in set (0.21 sec)

分别作出如下执行计划：
mysql> explain select * from test.testf where name='gaopeng';
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref   | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | testf | NULL       | ref  | name          | name | 63      | const |    1 |   100.00 | NULL  |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

mysql> explain select * from test.testf where name='gaopeng1';
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | testf | NULL       | ALL  | name          | NULL | NULL    | NULL |   10 |    90.00 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

可以看到执行计划是正确的，name='gaopeng'的只有一行选择了索引，name='gaopeng1'的有9行走了全表。
按理说如果只是记录不同的那么这两个语句的选择均为1/2,应该会造成执行计划错误，而MYSQL 5.6 5.7中
都做了正确的选择，那是为什么呢？
其实原因就在于 eq_range_index_dive_limit这个参数，我们来看一下trace
T@2: | | | | | | | | | | | opt: (null): "gaopeng1 <= name <=  | T@3: | | | | | | | | | | | opt: (null): "gaopeng <= name <= g
T@2: | | | | | | | | | | | opt: ranges: ending struct         | T@3: | | | | | | | | | | | opt: ranges: ending struct
T@2: | | | | | | | | | | | opt: index_dives_for_eq_ranges: 1  | T@3: | | | | | | | | | | | opt: index_dives_for_eq_ranges: 1
T@2: | | | | | | | | | | | opt: rowid_ordered: 1              | T@3: | | | | | | | | | | | opt: rowid_ordered: 1
T@2: | | | | | | | | | | | opt: using_mrr: 0                  | T@3: | | | | | | | | | | | opt: using_mrr: 0
T@2: | | | | | | | | | | | opt: index_only: 0                 | T@3: | | | | | | | | | | | opt: index_only: 0
T@2: | | | | | | | | | | | opt: rows: 9                       | T@3: | | | | | | | | | | | opt: rows: 1
T@2: | | | | | | | | | | | opt: cost: 11.81                   | T@3: | | | | | | | | | | | opt: cost: 2.21


我们可以看到 index_dives_for_eq_ranges均为1，rows: 9 rows: 1都是正确的，那么可以确定是index_dives_for_eq_ranges的作用，实际上
这是一个参数eq_range_index_dive_limit来决定的(equality range optimization of many-valued comparisions)，默认为
mysql> show variables like '%eq%';
+--------------------------------------+-------+
| Variable_name                        | Value |
+--------------------------------------+-------+
| eq_range_index_dive_limit            | 200   |

在官方文档说这个取值是等值范围比较的时候有多少个需要比较的值
如：
id=1 or id=2 or id=3 那么他取值就是3+1=4
而这种方法会得到精确的数据，但是增加的是时间成本，如果将
eq_range_index_dive_limit 设置为1：则禁用此功能
eq_range_index_dive_limit 设置为0：则始终开启
eq_range_index_dive_limit 设置为N：则满足N-1个这样的域。
那么我们设置为eq_range_index_dive_limit=1 后看看
mysql> explain select * from test.testf where name='gaopeng1';
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref   | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | testf | NULL       | ref  | name          | name | 63      | const |    5 |   100.00 | NULL  |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)


mysql> explain select * from test.testf where name='gaopeng';
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref   | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | testf | NULL       | ref  | name          | name | 63      | const |    5 |   100.00 | NULL  |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

可以看到执行计划已经错误 name='gaopeng1' 明显不应该使用索引，我们再来看看trace
T@3: | | | | | | | | | | | opt: ranges: ending struct
T@3: | | | | | | | | | | | opt: index_dives_for_eq_ranges: 0
T@3: | | | | | | | | | | | opt: rowid_ordered: 1
T@3: | | | | | | | | | | | opt: using_mrr: 0
T@3: | | | | | | | | | | | opt: index_only: 0
T@3: | | | | | | | | | | | opt: rows: 5
T@3: | | | | | | | | | | | opt: cost: 7.01
index_dives_for_eq_ranges: 0 rows: 5这个5就是10*1/2导致的,而index_dives_for_eq_ranges=0就是禁用了