記一次慢查詢的SQL優化

測試表結構

MariaDB [shoppings]> desc login_userinfo;
+------------+-------------+------+-----+---------+----------------+
| Field      | Type        | Null | Key | Default | Extra          |
+------------+-------------+------+-----+---------+----------------+
| num        | int(11)     | NO   | PRI | NULL    | auto_increment |
| name       | varchar(16) | NO   | UNI | NULL    |                |
| password   | varchar(40) | NO   |     | NULL    |                |
| nickname   | varchar(16) | NO   |     | NULL    |                |
| sex        | varchar(10) | NO   |     | NULL    |                |
| settime    | datetime(6) | NO   |     | NULL    |                |
| addr       | varchar(32) | NO   |     | NULL    |                |
| email      | varchar(32) | NO   |     | NULL    |                |
| phone      | varchar(11) | NO   |     | NULL    |                |
| receiver   | varchar(64) | NO   |     | NULL    |                |
| code       | varchar(6)  | NO   |     | NULL    |                |
| comment_id | int(11)     | YES  | MUL | NULL    |                |
+------------+-------------+------+-----+---------+----------------+
12 rows in set (0.003 sec)
 

表中資料量

MariaDB [shoppings]> select count(*) from login_userinfo;
+----------+
| count(*) |
+----------+
| 11809884 |
+----------+
1 row in set (2.503 sec)

分頁

MariaDB [shoppings]> select u.num, u.name, u.nickname, u.settime, u.addr,c.content from login_userinfo as u inner join login_comment as c on u.comment_id=c.id limit 10000000, 10;
+----------+--------------+--------------+----------------------------+------+---------+
| num      | name         | nickname     | settime                    | addr | content |
+----------+--------------+--------------+----------------------------+------+---------+
| 20010049 | test20014998 | test20014998 | 2021-02-20 00:00:00.000000 | 深圳 | 不太行  |
| 20010050 | test20014999 | test20014999 | 2021-02-20 00:00:00.000000 | 北京 | 不太行  |
| 20010051 | test20015000 | test20015000 | 2021-02-20 00:00:00.000000 | 廣州 | 不太行  |
| 20010052 | test20015001 | test20015001 | 2021-02-20 00:00:00.000000 | 北京 | 不太行  |
| 20010053 | test20015002 | test20015002 | 2021-02-20 00:00:00.000000 | 廣州 | 不太行  |
| 20010054 | test20015003 | test20015003 | 2021-02-20 00:00:00.000000 | 深圳 | 不太行  |
| 20010055 | test20015004 | test20015004 | 2021-02-20 00:00:00.000000 | 廣州 | 不太行  |
| 20010056 | test20015005 | test20015005 | 2021-02-20 00:00:00.000000 | 廣州 | 不太行  |
| 20010057 | test20015006 | test20015006 | 2021-02-20 00:00:00.000000 | 深圳 | 不太行  |
| 20010058 | test20015007 | test20015007 | 2021-02-20 00:00:00.000000 | 廣州 | 不太行  |
+----------+--------------+--------------+----------------------------+------+---------+
10 rows in set (1 min 6.340 sec)
 

這次查詢需要1分6秒，很明顯當資料量過大時 查詢效率會直線下降 ，使用者毫無體驗（#-_-）

MariaDB [shoppings]> explain select u.num, u.name, u.nickname, u.settime, u.addr,c.content from login_userinfo as u inner join login_comment as c on u.comment_id=c.id limit 10000000, 10;

檢視執行計劃會看到它可能進行了全表掃描

MariaDB [shoppings]> explain select u.num, u.name, u.nickname, u.settime, u.addr,c.content from login_userinfo as u inner join login_comment as c on u.comment_id=c.id limit 10000000, 10;
+------+-------------+-------+-------+--------------------------------------------------------+--------------------------------------------------------+---------+----------------+--------+-------------+
| id   | select_type | table | type  | possible_keys                                          | key                                                    | key_len | ref            | rows   | Extra       |
+------+-------------+-------+-------+--------------------------------------------------------+--------------------------------------------------------+---------+----------------+--------+-------------+
|    1 | SIMPLE      | c     | index | PRIMARY                                                | content                                                | 380     | NULL           | 17     | Using index |
|    1 | SIMPLE      | u     | ref   | Login_userinfo_comment_id_3d2eced3_fk_Login_comment_id | Login_userinfo_comment_id_3d2eced3_fk_Login_comment_id | 5       | shoppings.c.id | 681715 |             |
+------+-------------+-------+-------+--------------------------------------------------------+--------------------------------------------------------+---------+----------------+--------+-------------+
2 rows in set (0.001 sec)
 

優化

查詢主鍵num值

MariaDB [shoppings]> select num from login_userinfo limit 10000000,1;
+----------+
| num      |
+----------+
| 20010043 |
+----------+
1 row in set (3.259 sec)

MariaDB [shoppings]> explain select num from login_userinfo limit 10000000,1;
+------+-------------+----------------+-------+---------------+--------------------------------------------------------+---------+------+----------+-------------+
| id   | select_type | table          | type  | possible_keys | key  | key_len | ref  | rows     | Extra       |
+------+-------------+----------------+-------+---------------+--------------------------------------------------------+---------+------+----------+-------------+
|    1 | SIMPLE      | login_userinfo | index | NULL          | Login_userinfo_comment_id_3d2eced3_fk_Login_comment_id | 5       | NULL | 10907448 | Using index |
+------+-------------+----------------+-------+---------------+--------------------------------------------------------+---------+------+----------+-------------+
1 row in set (0.000 sec)

雖然我們也進行了全表掃描，但是我們用的是主鍵索引，所以效率會更高。

通過主鍵num值，查詢

MariaDB [shoppings]> select u.num, u.name, u.nickname, u.settime, u.addr,c.content from login_userinfo as u inner join login_comment as c on u.comment_id=c.id where u.num>20010043 limit 10;
+----------+--------------+--------------+----------------------------+------+---------+
| num      | name         | nickname     | settime                    | addr | content |
+----------+--------------+--------------+----------------------------+------+---------+
| 20010045 | test20014994 | test20014994 | 2021-02-20 00:00:00.000000 | 深圳 | 不太行  |
| 20010046 | test20014995 | test20014995 | 2021-02-20 00:00:00.000000 | 廣州 | 不太行  |
| 20010047 | test20014996 | test20014996 | 2021-02-20 00:00:00.000000 | 北京 | 不太行  |
| 20010048 | test20014997 | test20014997 | 2021-02-20 00:00:00.000000 | 上海 | 不太行  |
| 20010049 | test20014998 | test20014998 | 2021-02-20 00:00:00.000000 | 深圳 | 不太行  |
| 20010050 | test20014999 | test20014999 | 2021-02-20 00:00:00.000000 | 北京 | 不太行  |
| 20010051 | test20015000 | test20015000 | 2021-02-20 00:00:00.000000 | 廣州 | 不太行  |
| 20010052 | test20015001 | test20015001 | 2021-02-20 00:00:00.000000 | 北京 | 不太行  |
| 20010053 | test20015002 | test20015002 | 2021-02-20 00:00:00.000000 | 廣州 | 不太行  |
| 20010054 | test20015003 | test20015003 | 2021-02-20 00:00:00.000000 | 深圳 | 不太行  |
+----------+--------------+--------------+----------------------------+------+---------+
10 rows in set (0.020 sec)

MariaDB [shoppings]> explain select u.num, u.name, u.nickname, u.settime, u.addr,c.content from login_userinfo as u inner join login_comment as c on u.comment_id=c.id where u.num>20010044 limit 10;
+------+-------------+-------+--------+----------------------------------------------------------------+---------+---------+------------------------+---------+-------------+
| id   | select_type | table | type   | possible_keys                                                  | key     | key_len | ref                    | rows    | Extra       |
+------+-------------+-------+--------+----------------------------------------------------------------+---------+---------+------------------------+---------+-------------+
|    1 | SIMPLE      | u     | range  | PRIMARY,Login_userinfo_comment_id_3d2eced3_fk_Login_comment_id | PRIMARY | 4       | NULL                   | 3638072 | Using where |
|    1 | SIMPLE      | c     | eq_ref | PRIMARY                                                        | PRIMARY | 4       | shoppings.u.comment_id | 1       |             |
+------+-------------+-------+--------+----------------------------------------------------------------+---------+---------+------------------------+---------+-------------+

我們先拿到主鍵num的值 ,再通過num 進行查詢。通過這兩次查詢 時間不到4秒。效率大大提升。