in 改写优化案例

l凌晨1点开发迁移业务告知sql响应有问题,赶紧爬起来看看

6000w的大表,没走索引导致查询了50s,执行计划如下

 desc select a, b, c, d from table where FIND_IN_SET(a,'MD5value1,MD5value2');
+----+-------------+------------+------+---------------+------+---------+------+----------+-------------+
| id | select_type | table      | type | possible_keys | key  | key_len | ref  | rows     | Extra       |
+----+-------------+------------+------+---------------+------+---------+------+----------+-------------+
|  1 | SIMPLE      | table | ALL  | NULL          | NULL | NULL    | NULL | 60435142 | Using where |
+----+-------------+------------+------+---------------+------+---------+------+----------+-------------+
1 row in set (0.01 sec)

使用了in导致查询没有走索引改写为union all sql秒出结果

select a, b, c, d from table where a='md5value1'
union all
select a, b, c, d from table where a='MD5value2';　　　　

看下执行计划

+----+--------------+------------+-------+---------------+---------+---------+-------+------+-----------------+
| id | select_type  | table      | type  | possible_keys | key     | key_len | ref   | rows | Extra           |
+----+--------------+------------+-------+---------------+---------+---------+-------+------+-----------------+
|  1 | PRIMARY      | table | const | PRIMARY       | PRIMARY | 302     | const |    1 | NULL            |
|  2 | UNION        | table | const | PRIMARY       | PRIMARY | 302     | const |    1 | NULL            |
| NULL | UNION RESULT | <union1,2> | ALL   | NULL          | NULL    | NULL    | NULL  | NULL | Using temporary |
+----+--------------+------------+-------+---------------+---------+---------+-------+------+-----------------+
3 rows in set (0.03 sec)