15.索引优化(七)

两边都有百分号的,有全表扫描

mysql> EXPLAIN SELECT * FROM staffs where name LIKE '%july%';
+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table  | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | staffs | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |    33.33 | Using where |
+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

百分号在左边,有全表扫描

mysql> EXPLAIN SELECT * FROM staffs where name LIKE '%july';
+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table  | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | staffs | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |    33.33 | Using where |
+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

百分号在右边的,使用到了索引

mysql> EXPLAIN SELECT * FROM staffs where name LIKE 'july%';
+----+-------------+--------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+
| id | select_type | table  | partitions | type  | possible_keys         | key                   | key_len | ref  | rows | filtered | Extra                 |
+----+-------------+--------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+
|  1 | SIMPLE      | staffs | NULL       | range | idx_staffs_nameAgePos | idx_staffs_nameAgePos | 74      | NULL |    1 |   100.00 | Using index condition |
+----+-------------+--------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)

口诀: 百分Like加右边

解决 like '%字符串%'时索引不被使用的方法

准备数据

CREATE TABLE `tbl_user`(
  `id` INT(11) NOT NULL AUTO_INCREMENT,
  `name` VARCHAR(20) DEFAULT NULL ,
  `age` INT(11) DEFAULT NULL ,
  `email` VARCHAR(20) DEFAULT NULL ,
  PRIMARY KEY (`id`)
) ENGINE=INNODB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;

INSERT INTO tbl_user(`name`,`age`, `email`)VALUES ('1aa1', 21, 'b@163.com');
INSERT INTO tbl_user(`name`,`age`, `email`)VALUES ('2aa2', 222, 'a@163.com');
INSERT INTO tbl_user(`name`,`age`, `email`)VALUES ('3aa3', 265, 'c@163.com');
INSERT INTO tbl_user(`name`,`age`, `email`)VALUES ('4aa4', 21, 'd@163.com');

CREATE INDEX idx_user_nameAge ON tbl_user(`name`,`age`);

以下查询的字段只要全部是索引中的字段,就能使用索引

mysql> EXPLAIN SELECT name, age FROM tbl_user WHERE name LIKE '%aa%';
+----+-------------+----------+------------+-------+---------------+------------------+---------+------+------+----------+--------------------------+
| id | select_type | table    | partitions | type  | possible_keys | key              | key_len | ref  | rows | filtered | Extra                    |
+----+-------------+----------+------------+-------+---------------+------------------+---------+------+------+----------+--------------------------+
|  1 | SIMPLE      | tbl_user | NULL       | index | NULL          | idx_user_nameAge | 68      | NULL |    4 |    25.00 | Using where; Using index |
+----+-------------+----------+------------+-------+---------------+------------------+---------+------+------+----------+--------------------------+
1 row in set, 1 warning (0.00 sec)
mysql> EXPLAIN SELECT id FROM tbl_user WHERE name LIKE '%aa%';
+----+-------------+----------+------------+-------+---------------+------------------+---------+------+------+----------+--------------------------+
| id | select_type | table    | partitions | type  | possible_keys | key              | key_len | ref  | rows | filtered | Extra                    |
+----+-------------+----------+------------+-------+---------------+------------------+---------+------+------+----------+--------------------------+
|  1 | SIMPLE      | tbl_user | NULL       | index | NULL          | idx_user_nameAge | 68      | NULL |    4 |    25.00 | Using where; Using index |
+----+-------------+----------+------------+-------+---------------+------------------+---------+------+------+----------+--------------------------+
1 row in set, 1 warning (0.00 sec)
mysql> EXPLAIN SELECT name FROM tbl_user WHERE name LIKE '%aa%';
+----+-------------+----------+------------+-------+---------------+------------------+---------+------+------+----------+--------------------------+
| id | select_type | table    | partitions | type  | possible_keys | key              | key_len | ref  | rows | filtered | Extra                    |
+----+-------------+----------+------------+-------+---------------+------------------+---------+------+------+----------+--------------------------+
|  1 | SIMPLE      | tbl_user | NULL       | index | NULL          | idx_user_nameAge | 68      | NULL |    4 |    25.00 | Using where; Using index |
+----+-------------+----------+------------+-------+---------------+------------------+---------+------+------+----------+--------------------------+
1 row in set, 1 warning (0.01 sec)
mysql> EXPLAIN SELECT age FROM tbl_user WHERE name LIKE '%aa%';
+----+-------------+----------+------------+-------+---------------+------------------+---------+------+------+----------+--------------------------+
| id | select_type | table    | partitions | type  | possible_keys | key              | key_len | ref  | rows | filtered | Extra                    |
+----+-------------+----------+------------+-------+---------------+------------------+---------+------+------+----------+--------------------------+
|  1 | SIMPLE      | tbl_user | NULL       | index | NULL          | idx_user_nameAge | 68      | NULL |    4 |    25.00 | Using where; Using index |
+----+-------------+----------+------------+-------+---------------+------------------+---------+------+------+----------+--------------------------+
1 row in set, 1 warning (0.00 sec)
mysql> EXPLAIN SELECT id, name FROM tbl_user WHERE name LIKE '%aa%';
+----+-------------+----------+------------+-------+---------------+------------------+---------+------+------+----------+--------------------------+
| id | select_type | table    | partitions | type  | possible_keys | key              | key_len | ref  | rows | filtered | Extra                    |
+----+-------------+----------+------------+-------+---------------+------------------+---------+------+------+----------+--------------------------+
|  1 | SIMPLE      | tbl_user | NULL       | index | NULL          | idx_user_nameAge | 68      | NULL |    4 |    25.00 | Using where; Using index |
+----+-------------+----------+------------+-------+---------------+------------------+---------+------+------+----------+--------------------------+
1 row in set, 1 warning (0.00 sec)
mysql> EXPLAIN SELECT id, name, age FROM tbl_user WHERE name LIKE '%aa%';
+----+-------------+----------+------------+-------+---------------+------------------+---------+------+------+----------+--------------------------+
| id | select_type | table    | partitions | type  | possible_keys | key              | key_len | ref  | rows | filtered | Extra                    |
+----+-------------+----------+------------+-------+---------------+------------------+---------+------+------+----------+--------------------------+
|  1 | SIMPLE      | tbl_user | NULL       | index | NULL          | idx_user_nameAge | 68      | NULL |    4 |    25.00 | Using where; Using index |
+----+-------------+----------+------------+-------+---------------+------------------+---------+------+------+----------+--------------------------+
1 row in set, 1 warning (0.00 sec)
mysql> EXPLAIN SELECT * FROM tbl_user WHERE name LIKE '%aa%';
+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table    | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | tbl_user | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    4 |    25.00 | Using where |
+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
mysql> EXPLAIN SELECT id, name, age, email FROM tbl_user WHERE name LIKE '%aa%';
+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table    | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | tbl_user | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    4 |    25.00 | Using where |
+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

如果两边非要用%模糊查询,就要用覆盖索引