Mysql優化----一條SQL百倍提高之旅

在實現業務邏輯的時候，有些複雜一點邏輯會用數據庫子查詢去實現，可是sql用子查詢會帶來性能問題，下面就一個例子來講明，怎麼優化子查詢，來提高查詢速度

mysql> desc update t_student_info a 
    ->  set a.exstudentid='test01' 
    ->  where a.studentID in 
    ->  (select studentID from (select studentID from t_student_info where stdTYPE='8' and state=2 limit 10000,100) b);
+----+--------------------+----------------+------------+----------------+---------------+-------------+---------+------+--------+----------+-------------+
| id | select_type        | table          | partitions | type           | possible_keys | key         | key_len | ref  | rows   | filtered | Extra       |
+----+--------------------+----------------+------------+----------------+---------------+-------------+---------+------+--------+----------+-------------+
|  1 | UPDATE             | a              | NULL       | index          | NULL          | PRIMARY     | 24      | NULL | 221058 |   100.00 | Using where |
|  2 | DEPENDENT SUBQUERY | <derived3>     | NULL       | index_subquery | <auto_key0>   | <auto_key0> | 24      | func |    221 |   100.00 | Using index |
|  3 | DERIVED            | t_student_info | NULL       | ALL            | NULL          | NULL        | NULL    | NULL | 221058 |     1.00 | Using where |
+----+--------------------+----------------+------------+----------------+---------------+-------------+---------+------+--------+----------+-------------+
3 rows in set (0.00 sec)複製代碼

能夠看到這個Update語句的執行計劃，用的是DEPENDENT SUBQUERY，這樣就須要循環的去執行這個只查詢，效率會慢，能不能把這個只查詢改一下，改爲join查詢呢，下面就是優化以後的sql寫法

update t_student_info a set a.exstudentid='test01' where a.studentID in (select studentID from (select studentID from t_student_info where stdTYPE='8' and state=2 limit 10000,100) b)

mysql> desc update t_student_info a 
    ->        inner join 
    ->        (select studentID from t_student_info where stdTYPE='8' and state=2 limit 10000,100) b 
    ->        on a.studentID=b.studentID 
    ->        set a.exstudentid='test01';
+----+-------------+----------------+------------+--------+---------------+---------+---------+-------------+--------+----------+-------------+
| id | select_type | table          | partitions | type   | possible_keys | key     | key_len | ref         | rows   | filtered | Extra       |
+----+-------------+----------------+------------+--------+---------------+---------+---------+-------------+--------+----------+-------------+
|  1 | PRIMARY     | <derived2>     | NULL       | ALL    | NULL          | NULL    | NULL    | NULL        |   2210 |   100.00 | NULL        |
|  1 | UPDATE      | a              | NULL       | eq_ref | PRIMARY       | PRIMARY | 24      | b.studentID |      1 |   100.00 | NULL        |
|  2 | DERIVED     | t_student_info | NULL       | ALL    | NULL          | NULL    | NULL    | NULL        | 221058 |     1.00 | Using where |
+----+-------------+----------------+------------+--------+---------------+---------+---------+-------------+--------+----------+-------------+
3 rows in set (0.00 sec)複製代碼

能夠從執行計劃中看到執行計劃已經從DEPENDENT SUBQUERY變成了DERIVED，以驅動表去關聯查詢了，下面來看看實際執行效果

mysql> update t_student_info a set a.exstudentid='test01' where a.studentID in (select studentID from (select studentID from t_student_info where stdTYPE='8' and state=2 limit 10000,100) b);
Query OK, 0 rows affected (0.37 sec)
Rows matched: 100  Changed: 0  Warnings: 0

mysql> update t_student_info a set a.exstudentid='test01' where a.studentID in (select studentID from (select studentID from t_student_info where stdTYPE='8' and state=2 limit 10000,100) b);
Query OK, 0 rows affected (0.39 sec)
Rows matched: 100  Changed: 0  Warnings: 0

mysql> update t_student_info a inner join (select studentID from t_student_info where stdTYPE='8' and state=2 limit 10000,100) b on a.studentID=b.studentID set a.exstudentid='test01';
Query OK, 0 rows affected (0.07 sec)
Rows matched: 100  Changed: 0  Warnings: 0

mysql> update t_student_info a inner join (select studentID from t_student_info where stdTYPE='8' and state=2 limit 10000,100) b on a.studentID=b.studentID set a.exstudentid='test01';
Query OK, 0 rows affected (0.07 sec)
Rows matched: 100  Changed: 0  Warnings: 0複製代碼

爲了排除由於物理讀致使的干擾，沒條sql都連續執行2遍，從執行結果能夠看到，使用子查詢的sql平均執行時間在370毫秒，而用inner join的sql平均執行時間在70毫秒，效率提高了5倍多，優化效果仍是很明顯的，小夥伴可能會以爲，纔有5倍提高，其實優化以後的語句耗費時間的在limit 10000,100這裏，若是改外limit 1,100你們再來看看對比效果

mysql> update t_student_info a inner join (select studentID from t_student_info where stdTYPE='8' and state=2 limit 1,100) b on a.studentID=b.studentID set a.exstudentid='test01';
Query OK, 0 rows affected (0.00 sec)
Rows matched: 100  Changed: 0  Warnings: 0

mysql> update t_student_info a inner join (select studentID from t_student_info where stdTYPE='8' and state=2 limit 1,100) b on a.studentID=b.studentID set a.exstudentid='test01';
Query OK, 0 rows affected (0.00 sec)
Rows matched: 100  Changed: 0  Warnings: 0

mysql> update t_student_info a set a.exstudentid='test01' where a.studentID in (select studentID from (select studentID from t_student_info where stdTYPE='8' and state=2 limit 1,100) b);
Query OK, 0 rows affected (0.31 sec)
Rows matched: 100  Changed: 0  Warnings: 0

mysql> update t_student_info a set a.exstudentid='test01' where a.studentID in (select studentID from (select studentID from t_student_info where stdTYPE='8' and state=2 limit 1,100) b);
Query OK, 0 rows affected (0.31 sec)
Rows matched: 100  Changed: 0  Warnings: 0複製代碼

inner join的執行時間已是幾毫秒了，而子查詢仍是在310毫秒，這效果就分明顯，提高了100多倍，這種方法優化，不只適合in,仍是適合exists的優化

喜歡的同窗能夠關注個人公衆號（db_arch）（Mysql數據庫運維與架構設計）

喜歡的同窗能夠關注個人公衆號（db_arch）（Mysql數據庫運維與架構設計）