mysql----索引優化（2）

索引失效：建了索引，但是查詢時沒有用到。
    staffs表：
    create table staffs(
        id int primary key auto_increment,
        name varchar(24) not null default '' comment '姓名',
        age int not null default 0 comment '年齡',
        pos varchar(20) not null default '' comment '職位',
        add_time timestamp not null default CURRENT_TIMESTAMP comment '入職時間'
    );
    insert into staffs(name,age,pos,add_time) values ('z3' ,22 ,'manager',now());
    insert into staffs(name,age,pos,add_time) values ('july' ,23 ,'manager',now());
    insert into staffs(name,age,pos,add_time) values ('2000' ,23 ,'manager',now());
    alter table staffs add index idx_name_age_pos(name,age,pos);
1.全值匹配最合適
2.最佳左前綴法則,索引的第一個字段不能丟，中間字段不能跳。
3.不在索引列上做任何操作（計算，比較，類型轉換等）
4.存儲引擎不能使用範圍查找中右邊的列
5.儘量使用覆蓋索引（只訪問索引的查詢，即減少使用select *）    
6.!=  >  <會導致索引失效（mysql8不會）
7.is null ,is not null無法使用索引（mysql8不會）
8.like以通配符開始（% _）會導致索引失效（mysql8不會）
   如何解決這個問題？
   用覆蓋索引解決。
9.字符串不加單引號導致索引失效（mysql8不會）
10.or導致索引失效

  mysql> explain select * from staffs where name = 'july';
+----+-------------+--------+------------+------+------------------+------------------+---------+-------+------+----------+-------+
| id | select_type | table  | partitions | type | possible_keys    | key              | key_len | ref   | rows | filtered | Extra |
+----+-------------+--------+------------+------+------------------+------------------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | staffs | NULL       | ref  | idx_name_age_pos | idx_name_age_pos | 74      | const |    1 |   100.00 | NULL  |
+----+-------------+--------+------------+------+------------------+------------------+---------+-------+------+----------+-------+   
   mysql> explain select * from staffs where name = 'july' and age = 23;
+----+-------------+--------+------------+------+------------------+------------------+---------+-------------+------+----------+-------+
| id | select_type | table  | partitions | type | possible_keys    | key              | key_len | ref         | rows | filtered | Extra |
+----+-------------+--------+------------+------+------------------+------------------+---------+-------------+------+----------+-------+
|  1 | SIMPLE      | staffs | NULL       | ref  | idx_name_age_pos | idx_name_age_pos | 78      | const,const |    1 |   100.00 | NULL  |
+----+-------------+--------+------------+------+------------------+------------------+---------+-------------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

mysql> explain select * from staffs where name = 'july' or age = 23;
+----+-------------+--------+------------+------+------------------+------+---------+------+------+----------+-------------+
| id | select_type | table  | partitions | type | possible_keys    | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+--------+------------+------+------------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | staffs | NULL       | ALL  | idx_name_age_pos | NULL | NULL    | NULL |    3 |    55.56 | Using where |
+----+-------------+--------+------------+------+------------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
  
     
     
(mysql8除外)
mysql> explain select name,age,pos from staffs where  name <> 'july' ;
+----+-------------+--------+------------+-------+------------------+------------------+---------+------+------+----------+--------------------------+
| id | select_type | table  | partitions | type  | possible_keys    | key              | key_len | ref  | rows | filtered | Extra                    |
+----+-------------+--------+------------+-------+------------------+------------------+---------+------+------+----------+--------------------------+
|  1 | SIMPLE      | staffs | NULL       | range | idx_name_age_pos | idx_name_age_pos | 74      | NULL |    2 |   100.00 | Using where; Using index |
+----+-------------+--------+------------+-------+------------------+------------------+---------+------+------+----------+--------------------------+
1 row in set, 1 warning (0.00 sec) 
mysql> explain select name,age from staffs where  pos <> 'man';
+----+-------------+--------+------------+-------+---------------+------------------+---------+------+------+----------+--------------------------+
| id | select_type | table  | partitions | type  | possible_keys | key              | key_len | ref  | rows | filtered | Extra                    |
+----+-------------+--------+------------+-------+---------------+------------------+---------+------+------+----------+--------------------------+
|  1 | SIMPLE      | staffs | NULL       | index | NULL          | idx_name_age_pos | 140     | NULL |    3 |    66.67 | Using where; Using index |
+----+-------------+--------+------------+-------+---------------+------------------+---------+------+------+----------+--------------------------+
1 row in set, 1 warning (0.00 sec)  
mysql 8> explain select age from staffs where age = '22';
+----+-------------+--------+------------+-------+------------------+------------------+---------+------+------+----------+--------------------------+
| id | select_type | table  | partitions | type  | possible_keys    | key              | key_len | ref  | rows | filtered | Extra                    |
+----+-------------+--------+------------+-------+------------------+------------------+---------+------+------+----------+--------------------------+
|  1 | SIMPLE      | staffs | NULL       | index | idx_name_age_pos | idx_name_age_pos | 140     | NULL |    3 |    33.33 | Using where; Using index |
+----+-------------+--------+------------+-------+------------------+------------------+---------+------+------+----------+--------------------------+
1 row in set, 1 warning (0.07 sec)