13.索引优化(五)

mysql在使用不等于(!=或<>)的时候无法使用索引,会导致全表扫描

先看等于的情况

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_staffs_nameAgePos | idx_staffs_nameAgePos | 74      | const |    1 |   100.00 | NULL  |
+----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.01 sec)

再看不等于的情况

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       | ALL  | idx_staffs_nameAgePos | NULL | NULL    | NULL |    3 |    66.67 | Using where |
+----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)


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       | ALL  | idx_staffs_nameAgePos | NULL | NULL    | NULL |    3 |    66.67 | Using where |
+----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec) 
 

两个不等于的情况,都没有用到索引