MySQL數據庫in 太多不走索引案例

https://dev.mysql.com/doc/refman/5.7/en/range-optimization.html

Limiting Memory Use for Range Optimization

To control the memory available to the range optimizer, use the range_optimizer_max_mem_size system variable:

• A value of 0 means “no limit.”

• With a value greater than 0, the optimizer tracks the memory consumed when considering the range access method. If the specified limit is about to be exceeded, the range access method is abandoned and other methods, including a full table scan, are considered instead. This could be less optimal. If this happens, the following warning occurs (where N is the current range_optimizer_max_mem_size value):

 

 

同樣的SQL語句，in 幾百個。在一台實例上走索引，在另一個實例上不走索引，第一感覺是和eq_range_index_dive_limit 之類的參數，調了幾次之后無法復現問題，在看幫助文檔時才注意到 range_optimizer_max_mem_size 參數也會引起全表掃描。

問題復現如下：

mysql> show create table t1;
+-------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                                                                                                                                   |
+-------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| t1    | CREATE TABLE `t1` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `od` int(11) DEFAULT NULL,
  `name` varchar(200) DEFAULT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `od` (`od`),
  KEY `idx_name` (`name`)
) ENGINE=InnoDB AUTO_INCREMENT=101 DEFAULT CHARSET=utf8 |
+-------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)


mysql> select * from t1;
+-----+------+------+
| id  | od   | name |
+-----+------+------+
|   1 |    1 | a    |
|   2 |    2 | b    |
|   3 |    3 | c    |
|   4 |    4 | d    |
| 100 |  100 | f    |
+-----+------+------+
5 rows in set (0.00 sec)

mysql> show variables like 'range_optimizer_max_mem_size';
+------------------------------+---------+
| Variable_name                | Value   |
+------------------------------+---------+
| range_optimizer_max_mem_size | 8388608 |
+------------------------------+---------+
1 row in set (0.00 sec)

mysql> explain select * from t1 where id in(1,3,5);
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | t1    | NULL       | range | PRIMARY       | PRIMARY | 4       | NULL |    3 |   100.00 | Using where |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)


mysql> set range_optimizer_max_mem_size=1;
Query OK, 0 rows affected (0.00 sec)

mysql> 
mysql> explain select * from t1 where id in(1,3,5);
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | t1    | NULL       | ALL  | PRIMARY       | NULL | NULL    | NULL |    6 |    50.00 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 2 warnings (0.00 sec)