MySQL隐式类型的转换陷阱和规则_MySQL

相信大家都知道隐式类型转换有无法命中索引的风险，在高并发、大数据量的情况下，命不中索引带来的后果非常严重。将数据库拖死，继而整个系统崩溃，对于大规模系统损失惨重。所以下面通过本文来好好学习下MySQL隐式类型的转换陷阱和规则。

1. 隐式类型转换实例

今天生产库上突然出现MySQL线程数告警，IOPS很高，实例会话里面出现许多类似下面的sql：(修改了相关字段和值)

SELECT f_col3_id,f_qq1_id FROM d_dbname.t_tb1 WHERE f_col1_id=1226391 and f_col2_id=1244378 and 
f_qq1_id in (12345,23456,34567,45678,56789,67890,78901,89012,90123,901231,901232,901233)

用 explain 看了下扫描行数和索引选择情况：

mysql>explain SELECT f_col3_id,f_qq1_id FROM d_dbname.t_tb1 WHERE f_col1_id=1226391 
and f_col2_id=1244378 and f_qq1_id in (12345,23456,34567,45678,56789,67890,78901,89012,90123,901231,901232,901233);
+------+---------------+---------+--------+--------------------------------+---------------+------------+--------+--------+------------------------------------+
| id | select_type | table | type | possible_keys     | key   | key_len | ref | rows | Extra        |
+------+---------------+---------+--------+--------------------------------+---------------+------------+--------+--------+------------------------------------+
| 1 | SIMPLE  | t_tb1 | ref | uid_type_frid,idx_corpid_qq1id | uid_type_frid | 8   | const | 1386 | Using index condition; Using where |
+------+---------------+---------+--------+--------------------------------+---------------+------------+--------+--------+------------------------------------+
共返回 1 行记录,花费 11.52 ms.

t_tb1 表上有个索引uid_type_frid(f_col2_id,f_type) 、idx_corp_id_qq1id(f_col1_id,f_qq1_id) ，而且如果选择后者时，f_qq1_id的过滤效果应该很佳，但却选择了前者。当使用 hint use index(idx_corp_id_qq1id)时：

mysql>explain extended SELECT f_col3_id,f_qq1_id FROM d_dbname.t_tb1 use index(idx_corpid_qq1id) WHERE f_col1_id=1226391 and f_col2_id=1244378 and f_qq1_id in (12345,23456,34567,45678,56789,67890,78901,89012,90123,901231,901232,901233);
+------+---------------+--------+--------+---------------------+------------------+------------+----------+-------------+------------------------------------+
| id | select_type | table | type | possible_keys  | key    | key_len | ref  | rows  | Extra        |
+------+---------------+--------+--------+---------------------+------------------+------------+----------+-------------+------------------------------------+
| 1 | SIMPLE  | t_tb1 | ref | idx_corpid_qq1id | idx_corpid_qq1id | 8   | const | 2375752  | Using index condition; Using where |
+---- -+---------------+--------+--------+---------------------+------------------+------------+----------+-------------+------------------------------------+
共返回 1 行记录,花费 17.48 ms.
mysql>show warnings;
+-----------------+----------------+-----------------------------------------------------------------------------------------------------------------------+
| Level   | Code   | Message                            |
+-----------------+----------------+-----------------------------------------------------------------------------------------------------------------------+
| Warning   |   1739 | Cannot use range access on index 'idx_corpid_qq1id' due to type or collation conversion on field 'f_qq1_id'   |
| Note   |   1003 | /* select#1 */ select `d_dbname`.`t_tb1`.`f_col3_id` AS `f_col3_id`,`d_dbname`.`t_tb1`.`f_qq1_id` AS `f_qq1_id` from `d_dbname`.`t_tb1` USE INDEX (`idx_corpid_qq1id`) where |
|     |    | ((`d_dbname`.`t_tb1`.`f_col2_id` = 1244378) and (`d_dbname`.`t_tb1`.`f_col1_id` = 1226391) and (`d_dbname`.`t_tb1`.`f_qq1_id` in |
|     |    | (12345,23456,34567,45678,56789,67890,78901,89012,90123,901231,901232,901233)))          |
+-----------------+----------------+-----------------------------------------------------------------------------------------------------------------------+
共返回 2 行记录,花费 10.81 ms.

rows列达到200w行，但问题也发现了：select_type应该是 range 才对，key_len看出来只用到了idx_corpid_qq1id索引的第一列。上面explain使用了 extended，所以show warnings;可以很明确的看到 f_qq1_id 出现了隐式类型转换：f_qq1_id是varchar，而后面的比较值是整型。

解决该问题就是避免出现隐式类型转换(implicit type conversion)带来的不可控：把f_qq1_id in的内容写成字符串：

mysql>explain SELECT f_col3_id,f_qq1_id FROM d_dbname.t_tb1 WHERE f_col1_id=1226391 and f_col2_id=1244378 and 
f_qq1_id in ('12345','23456','34567','45678','56789','67890','78901','89012','90123','901231');
+-------+---------------+--------+---------+--------------------------------+------------------+-------------+---------+---------+------------------------------------+
| id | select_type | table | type | possible_keys     | key    | key_len  | ref  | rows | Extra        |
+-------+---------------+--------+---------+--------------------------------+------------------+-------------+---------+---------+------------------------------------+
| 1  | SIMPLE  | t_tb1 | range | uid_type_frid,idx_corpid_qq1id | idx_corpid_qq1id | 70   |   | 40  | Using index condition; Using where |
+-------+---------------+--------+---------+--------------------------------+------------------+-------------+---------+---------+------------------------------------+
共返回 1 行记录,花费 12.41 ms.

扫描行数从1386减少为40。

类似的还出现过一例：

SELECT count(0) FROM d_dbname.t_tb2 where f_col1_id= '1931231' AND f_phone in(098890);
| Warning | 1292 | Truncated incorrect DOUBLE value: '1512-98464356'

优化后直接从扫描rows 100w行降为1。

借这个机会，系统的来看一下mysql中的隐式类型转换。

2. mysql隐式转换规则

2.1 规则

下面来分析一下隐式转换的规则：

a. 两个参数至少有一个是 NULL 时，比较的结果也是 NULL，例外是使用 <=> 对两个 NULL 做比较时会返回 1，这两种情况都不需要做类型转换

b. 两个参数都是字符串，会按照字符串来比较，不做类型转换

c. 两个参数都是整数，按照整数来比较，不做类型转换

d. 十六进制的值和非数字做比较时，会被当做二进制串

e. 有一个参数是 TIMESTAMP 或 DATETIME，并且另外一个参数是常量，常量会被转换为 timestamp

f. 有一个参数是 decimal 类型，如果另外一个参数是 decimal 或者整数，会将整数转换为 decimal 后进行比较，如果另外一个参数是浮点数，则会把 decimal 转换为浮点数进行比较

g. 所有其他情况下，两个参数都会被转换为浮点数再进行比较

mysql> select 11 + '11', 11 + 'aa', 'a1' + 'bb', 11 + '0.01a'; 
+-----------+-----------+-------------+--------------+
| 11 + '11' | 11 + 'aa' | 'a1' + 'bb' | 11 + '0.01a' |
+-----------+-----------+-------------+--------------+
|  22 |  11 |   0 |  11.01 |
+-----------+-----------+-------------+--------------+
1 row in set, 4 warnings (0.00 sec)
mysql> show warnings;
+---------+------+-------------------------------------------+
| Level | Code | Message         |
+---------+------+-------------------------------------------+
| Warning | 1292 | Truncated incorrect DOUBLE value: 'aa' |
| Warning | 1292 | Truncated incorrect DOUBLE value: 'a1' |
| Warning | 1292 | Truncated incorrect DOUBLE value: 'bb' |
| Warning | 1292 | Truncated incorrect DOUBLE value: '0.01a' |
+---------+------+-------------------------------------------+
4 rows in set (0.00 sec)
mysql> select '11a' = 11, '11.0' = 11, '11.0' =