文章目录
- 1、覆盖索引
- 1.1、查看索引
- 1.2、删除单列索引 idx_user_pro
- 1.3、查询 profession='软件工程' and age=31 and status='0'
- 1.4、执行计划 profession='软件工程' and age=31 and status='0'
- 1.5、执行计划 select id,profession,age,status
- 1.6、执行计划 select id,profession,age,status,name
- 2、思考题
1、覆盖索引
尽量使用覆盖索引（查询使用了索引，并且需要返回的列，在该索引中已经全部能够找到），减少select * 。
1.1、查看索引
mysql> show index from tb_user;
+---------+------------+----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | Expression |
+---------+------------+----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| tb_user | 0 | PRIMARY | 1 | id | A | 24 | NULL | NULL | | BTREE | | | YES | NULL |
| tb_user | 1 | idx_user_pro_age_sta | 1 | profession | A | 16 | NULL | NULL | YES | BTREE | | | YES | NULL |
| tb_user | 1 | idx_user_pro_age_sta | 2 | age | A | 22 | NULL | NULL | YES | BTREE | | | YES | NULL |
| tb_user | 1 | idx_user_pro_age_sta | 3 | status | A | 24 | NULL | NULL | YES | BTREE | | | YES | NULL |
| tb_user | 1 | idx_user_pro | 1 | profession | A | 16 | NULL | NULL | YES | BTREE | | | YES | NULL |
+---------+------------+----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
5 rows in set (0.00 sec)
mysql>
1.2、删除单列索引 idx_user_pro
mysql> drop index idx_user_pro on tb_user;
Query OK, 0 rows affected (0.02 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> show index from tb_user;
+---------+------------+----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | Expression |
+---------+------------+----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| tb_user | 0 | PRIMARY | 1 | id | A | 24 | NULL | NULL | | BTREE | | | YES | NULL |
| tb_user | 1 | idx_user_pro_age_sta | 1 | profession | A | 16 | NULL | NULL | YES | BTREE | | | YES | NULL |
| tb_user | 1 | idx_user_pro_age_sta | 2 | age | A | 22 | NULL | NULL | YES | BTREE | | | YES | NULL |
| tb_user | 1 | idx_user_pro_age_sta | 3 | status | A | 24 | NULL | NULL | YES | BTREE | | | YES | NULL |
+---------+------------+----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
4 rows in set (0.01 sec)
mysql>
1.3、查询 profession=‘软件工程’ and age=31 and status=‘0’
mysql> select * from tb_user where profession='软件工程' and age=31 and status='0';
+----+------+-------------+-------------------+------------+------+--------+--------+---------------------+
| id | name | phone | email | profession | age | gender | status | createtime |
+----+------+-------------+-------------------+------------+------+--------+--------+---------------------+
| 16 | 妲己 | 17799990015 | 2783238293@qq.com | 软件工程 | 31 | 2 | 0 | 2001-01-30 00:00:00 |
+----+------+-------------+-------------------+------------+------+--------+--------+---------------------+
1 row in set (0.00 sec)
mysql>
1.4、执行计划 profession=‘软件工程’ and age=31 and status=‘0’
mysql> explain select * from tb_user where profession='软件工程' and age=31 and status='0';
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------------------+------+----------+-----------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------------------+------+----------+-----------------------+
| 1 | SIMPLE | tb_user | NULL | ref | idx_user_pro_age_sta | idx_user_pro_age_sta | 54 | const,const,const | 1 | 100.00 | Using index condition |
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------------------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)
mysql>
1.5、执行计划 select id,profession,age,status
mysql> explain select id,profession,age,status from tb_user where profession='软件工程' and age=31 and status='0';
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------------------+------+----------+--------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------------------+------+----------+--------------------------+
| 1 | SIMPLE | tb_user | NULL | ref | idx_user_pro_age_sta | idx_user_pro_age_sta | 54 | const,const,const | 1 | 100.00 | Using where; Using index |
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------------------+------+----------+--------------------------+
1 row in set, 1 warning (0.00 sec)
mysql>
using where;using index：查找使用了索引，但是需要的数据都在索引列中能找到，所以不需要回表查询数据
1.6、执行计划 select id,profession,age,status,name
mysql> explain select id,profession,age,status,name from tb_user where profession='软件工程' and age=31 and status='0';
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------------------+------+----------+-----------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------------------+------+----------+-----------------------+
| 1 | SIMPLE | tb_user | NULL | ref | idx_user_pro_age_sta | idx_user_pro_age_sta | 54 | const,const,const | 1 | 100.00 | Using index condition |
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------------------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)
mysql>
using index condition：查找使用了索引，但是需要回表查询数据
2、思考题
一张表，有四个字段（id，username，password，status），由于数据量大，需要对一下SQL语句进行优化，该如何进行才是最优方案：
select id,username,password from tb_user where username=‘csdn’
select id,username,password from tb_user where username='csdn';
答案：针对于 username，password 建立联合索引，sql 为：create index idx_user_name_pass on tb_user(username,password);这样就可以避免上述的SQL语句，在查询的过程中，出现回表查询。
create index idx_user_name_pass on tb_user(username,password);
