Question

SELECT ... WHERE COL IN(A,B) 

or 

SELECT ... WHERE (COL = A or COL = B)

我试图找出这两种结构之间的区别是什么？
如果用于接近100万标记的结果集，是否会有显着的性能提升？

Answer 1

mysql> describe select * from users where id = 1 or id = 2;
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
|  1 | SIMPLE      | users | range | PRIMARY       | PRIMARY | 4       | NULL |    2 | Using where |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
1 row in set (0.01 sec)

mysql> describe select * from users where id in (1,2);
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
|  1 | SIMPLE      | users | range | PRIMARY       | PRIMARY | 4       | NULL |    2 | Using where |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
1 row in set (0.00 sec)

从DESCRIBE陈述的输出来看 - 它们是相同的。

Answer 2

使用

之间没有性能差异

col = A or col = B

和

col IN (8,7,5,2,....)

mysql范围优化器通过计算（8,7,5,2，....）的排序列表来优化IN查询，然后使用它来构造相应的SEL_TREE。

所以没有性能差异，因为两者都会对索引进行范围扫描。

Answer 3

就像一个小例子：

Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 5
Server version: 5.1.41-community-log MySQL Community Server (GPL)

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> SET @qwe=5;
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT BENCHMARK(100000000, @qwe IN (10,5));
+--------------------------------------+
| BENCHMARK(100000000, @qwe IN (10,5)) |
+--------------------------------------+
|                                    0 |
+--------------------------------------+
1 row in set (3.52 sec)

mysql> SELECT BENCHMARK(100000000, @qwe = 10 OR @qwe = 5);
+---------------------------------------------+
| BENCHMARK(100000000, @qwe = 10 OR @qwe = 5) |
+---------------------------------------------+
|                                           0 |
+---------------------------------------------+
1 row in set (5.91 sec)

和

mysql> SELECT BENCHMARK(100000000, @qwe IN (10,1,9,2,8,3,7,4,6,5));
+------------------------------------------------------+
| BENCHMARK(100000000, @qwe IN (10,1,9,2,8,3,7,4,6,5)) |
+------------------------------------------------------+
|                                                    0 |
+------------------------------------------------------+
1 row in set (6.02 sec)

mysql> SELECT BENCHMARK(100000000, @qwe = 10 OR @qwe = 1 OR @qwe = 9
    ->   OR @qwe = 2 OR @qwe = 8 OR @qwe = 3 OR @qwe = 7 OR @qwe = 4
    ->   OR @qwe = 6 OR @qwe = 5) as result;
+--------+
| result |
+--------+
|      0 |
+--------+
1 row in set (20.20 sec)

注意括号中的持续时间。

哪个mysql构造更快？

3 个答案: