mysql - sql 优化问题，between比in好？

问题描述

问 题

看到网上的资料说：

in 和 not in 也要慎用，否则会导致全表扫描，如：

select id from t where num in(1,2,3)

对于连续的数值，能用 between 就不要用 in 了：

select id from t where num between 1 and 3

解决方案

连续数值当然between好了 减少解析 并且in的范围默认超过一定数目就会走全表 9个还是多少忘了

针对楼下评论再补充一下答案：
in走全表是分情况的，上面的解答只是凭记忆大概说了一下，楼下小伙伴比较认真，这里就再解释一下，这个不是个数而应该是个比例，大概25%-35%左右，你要再问到底多少不好意思水平有限不读源码没办法确认。然后这个30左右的比例也并不是说一定走全表扫描，因为mysql还有一个索引扫描，就是说如果select的内容在你的索引里面就能找到的话当然不会去扫全表了，比如下面的例子中select id from ttt where id in (..);和select * from ttt where id in (...);前面这个肯定是走主键扫描，即使你in了所有id值他也是走主键，而后面的情况就是这种百分比的情况了，具体看下面示例，欢迎指正^_^
额，然后还想说一下between的情况，为什么好是因为除了索引段上连续存取减少解析以外，还有一个情况就是在磁盘寻址检索数据的时候，会默认读取第一次取值附近的部分数据（有这么一个概率算法说的是当一个数据被检索到的时候，他附近的数据也很大概率会被用到）所以就有了这么一个一次性取出冗余数据避免多次寻址的情况，这时候使用between的连续取值就正适用了

mysql> select * from ttt;
+----+-------------------+
| id | name              |
+----+-------------------+
|  1 | I17021234001      |
|  2 | IC17031234002     |
|  3 | C17041234003      |
|  4 | IAsEw1234001      |
|  5 | I17021234001A2    |
|  6 | IC17031234002A2   |
|  7 | C17041234003A2    |
|  8 | IAsEw1234001A2    |
|  9 | I17021234001A2    |
| 10 | IC17031234002A2   |
| 11 | C17041234003A2    |
| 12 | IAsEw1234001A2    |
| 13 | I17021234001A2A2  |
| 14 | IC17031234002A2A2 |
| 15 | C17041234003A2A2  |
| 16 | IAsEw1234001A2A2  |
| 17 | I17021234001A2    |
| 18 | IC17031234002A2   |
| 19 | C17041234003A2    |
| 20 | IAsEw1234001A2    |
+----+-------------------+
20 rows in set (0.00 sec)

mysql> show create table ttt;
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                                             |
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ttt   | CREATE TABLE `ttt` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` char(32) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=32 DEFAULT CHARSET=utf8 |
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> explain select * from ttt where id in (1,2,3,4,5,6);
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | ttt   | ALL  | PRIMARY       | NULL | NULL    | NULL |   20 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

mysql> explain select * from ttt where id in (1,2,3,4,5);
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
|  1 | SIMPLE      | ttt   | range | PRIMARY       | PRIMARY | 4       | NULL |    5 | Using where |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
1 row in set (0.00 sec)

mysql> explain select * from ttt where id in (1,2,3);
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
|  1 | SIMPLE      | ttt   | range | PRIMARY       | PRIMARY | 4       | NULL |    3 | Using where |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
1 row in set (0.00 sec)

mysql> explain select * from ttt where id in (1,2,3,4,5,6,7);
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | ttt   | ALL  | PRIMARY       | NULL | NULL    | NULL |   20 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

mysql> explain select id from ttt where id in (1,2,3,4,5,6,7);
+----+-------------+-------+-------+---------------+---------+---------+------+------+--------------------------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref  | rows | Extra                    |
+----+-------------+-------+-------+---------------+---------+---------+------+------+--------------------------+
|  1 | SIMPLE      | ttt   | index | PRIMARY       | PRIMARY | 4       | NULL |   20 | Using where; Using index |
+----+-------------+-------+-------+---------------+---------+---------+------+------+--------------------------+
1 row in set (0.00 sec)

mysql> explain select name from ttt where id in (1,2,3,4,5,6,7);
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | ttt   | ALL  | PRIMARY       | NULL | NULL    | NULL |   20 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

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