MySQL令人大跌眼镜的隐式转换


Posted in MySQL onAugust 23, 2021

MySQL的隐式转换

一、问题描述

show create table t1\G
*************************** 1. row ***************************
       Table: t1
Create Table: CREATE TABLE `t1` (
  `id` varchar(255) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)
 
 select * from t1;
+--------------------+
| id                 |
+--------------------+
| 204027026112927605 |
| 204027026112927603 |
| 2040270261129276   |
| 2040270261129275   |
| 100                |
| 101                |
+--------------------+
6 rows in set (0.00 sec)

奇怪的现象:

select * from t1 where id=204027026112927603;
+--------------------+
| id                 |
+--------------------+
| 204027026112927605 |
| 204027026112927603 |
+--------------------+
2 rows in set (0.00 sec)

什么鬼,明明查的是204027026112927603,为什么204027026112927605也出来了

二、源码解释

堆栈调用关系如下所示:

MySQL令人大跌眼镜的隐式转换

其中JOIN::exec()是执行的入口,Arg_comparator::compare_real()是进行等值判断的函数,其定义如下

int Arg_comparator::compare_real()
{
  /*
    Fix yet another manifestation of Bug#2338. 'Volatile' will instruct
    gcc to flush double values out of 80-bit Intel FPU registers before
    performing the comparison.
  */
  volatile double val1, val2;
  val1= (*a)->val_real();
  if (!(*a)->null_value)
  {
    val2= (*b)->val_real();
    if (!(*b)->null_value)
    {
      if (set_null)
        owner->null_value= 0;
      if (val1 < val2)  return -1;
      if (val1 == val2) return 0;
      return 1;
    }
  }
  if (set_null)
    owner->null_value= 1;
  return -1;
}

比较步骤如下图所示,逐行读取t1表的id列放入val1,而常量204027026112927603存在于cache中,类型为double类型(2.0402702611292762E+17),所以到这里传值给val2后val2=2.0402702611292762E+17。

MySQL令人大跌眼镜的隐式转换

当扫描到第一行时,204027026112927605转成doule的值为2.0402702611292762e17,等式成立,判定为符合条件的行,继续往下扫描,同理204027026112927603也同样符合

MySQL令人大跌眼镜的隐式转换

如何检测string类型的数字转成doule类型是否溢出呢?这里经过测试,当数字超过16位以后,转成double类型就已经不准确了,例如20402702611292711会表示成20402702611292712(如图中val1)

MySQL令人大跌眼镜的隐式转换

MySQL令人大跌眼镜的隐式转换

MySQL string转成double的定义函数如下:

{
  char buf[DTOA_BUFF_SIZE];
  double res;
  DBUG_ASSERT(end != NULL && ((str != NULL && *end != NULL) ||
                              (str == NULL && *end == NULL)) &&
              error != NULL);
 
  res= my_strtod_int(str, end, error, buf, sizeof(buf));
  return (*error == 0) ? res : (res < 0 ? -DBL_MAX : DBL_MAX);
}

真正转换函数my_strtod_int位置在dtoa.c(太复杂了,简单贴个注释吧)

/*
  strtod for IEEE--arithmetic machines.
 
  This strtod returns a nearest machine number to the input decimal
  string (or sets errno to EOVERFLOW). Ties are broken by the IEEE round-even
  rule.
 
  Inspired loosely by William D. Clinger's paper "How to Read Floating
  Point Numbers Accurately" [Proc. ACM SIGPLAN '90, pp. 92-101].
 
  Modifications:
 
   1. We only require IEEE (not IEEE double-extended).
   2. We get by with floating-point arithmetic in a case that
     Clinger missed -- when we're computing d * 10^n
     for a small integer d and the integer n is not too
     much larger than 22 (the maximum integer k for which
     we can represent 10^k exactly), we may be able to
     compute (d*10^k) * 10^(e-k) with just one roundoff.
   3. Rather than a bit-at-a-time adjustment of the binary
     result in the hard case, we use floating-point
     arithmetic to determine the adjustment to within
     one bit; only in really hard cases do we need to
     compute a second residual.
   4. Because of 3., we don't need a large table of powers of 10
     for ten-to-e (just some small tables, e.g. of 10^k
     for 0 <= k <= 22).
*/

既然是这样,我们测试下没有溢出的案例

select * from t1 where id=2040270261129276;
+------------------+
| id               |
+------------------+
| 2040270261129276 |
+------------------+
1 row in set (0.00 sec)
 
 select * from t1 where id=101;
+------+
| id   |
+------+
| 101  |
+------+
1 row in set (0.00 sec)

结果符合预期,而在本例中,正确的写法应当是

select * from t1 where id='204027026112927603';
+--------------------+
| id                 |
+--------------------+
| 204027026112927603 |
+--------------------+
1 row in set (0.01 sec)

三、结论

避免发生隐式类型转换,隐式转换的类型主要有字段类型不一致、in参数包含多个类型、字符集类型或校对规则不一致等
隐式类型转换可能导致无法使用索引、查询结果不准确等,因此在使用时必须仔细甄别
数字类型的建议在字段定义时就定义为int或者bigint,表关联时关联字段必须保持类型、字符集、校对规则都一致

最后贴一下官网对于隐式类型转换的说明吧

1、If one or both arguments are NULL, the result of the comparison is NULL, except for the NULL-safe
<=> equality comparison operator. For NULL <=> NULL, the result is true. No conversion is needed.
2、If both arguments in a comparison operation are strings, they are compared as strings.
3、If both arguments are integers, they are compared as integers.
4、Hexadecimal values are treated as binary strings if not compared to a number.
5、If one of the arguments is a TIMESTAMP or DATETIME column and the other argument is a
constant, the constant is converted to a timestamp before the comparison is performed. This is
done to be more ODBC-friendly. This is not done for the arguments to IN(). To be safe, always
use complete datetime, date, or time strings when doing comparisons. For example, to achieve best
results when using BETWEEN with date or time values, use CAST() to explicitly convert the values to
the desired data type.
A single-row subquery from a table or tables is not considered a constant. For example, if a subquery
returns an integer to be compared to a DATETIME value, the comparison is done as two integers.
The integer is not converted to a temporal value. To compare the operands as DATETIME values,
use CAST() to explicitly convert the subquery value to DATETIME.
6、If one of the arguments is a decimal value, comparison depends on the other argument. The
arguments are compared as decimal values if the other argument is a decimal or integer value, or as
floating-point values if the other argument is a floating-point value.
7、In all other cases, the arguments are compared as floating-point (real) numbers.

以上就是MySQL令人大跌眼镜的隐式转换的详细内容,更多关于MySQL的隐式转换的资料请关注三水点靠木其它相关文章!,希望大家以后多多支持三水点靠木!

MySQL 相关文章推荐
MySQL分库分表与分区的入门指南
Apr 22 MySQL
详解MySQL数据库千万级数据查询和存储
May 18 MySQL
MySQL时间盲注的五种延时方法实现
May 18 MySQL
mysql在项目中怎么选事务隔离级别
May 25 MySQL
解决Navicat for MySQL 连接 MySQL 报2005错误的问题
May 29 MySQL
MySQL REVOKE实现删除用户权限
Jun 18 MySQL
Mysql binlog日志文件过大的解决
Oct 05 MySQL
详解MySQL中timestamp和datetime时区问题导致做DTS遇到的坑
Dec 06 MySQL
MySQL中一条update语句是如何执行的
Mar 16 MySQL
MySql分区类型及创建分区的方法
Apr 13 MySQL
聊聊mysql都有哪几种分区方式
Apr 13 MySQL
MySQL的存储过程和相关函数
Apr 26 MySQL
SQL IDENTITY_INSERT作用案例详解
Aug 23 #MySQL
MySQL非空约束(not null)案例讲解
Aug 23 #MySQL
MySQL外键约束(FOREIGN KEY)案例讲解
Aug 23 #MySQL
MySQL 1130异常,无法远程登录解决方案详解
Aug 23 #MySQL
Node-Red实现MySQL数据库连接的方法
Aug 07 #MySQL
MySQL如何解决幻读问题
Aug 07 #MySQL
浅谈MySQL之select优化方案
Aug 07 #MySQL
You might like
调试一段PHP程序时遇到的三个问题
2012/01/17 PHP
PHP中抽象类和抽象方法概念与用法分析
2016/05/24 PHP
PHP simplexml_load_string()函数实例讲解
2019/02/03 PHP
php模拟实现斗地主发牌
2020/04/22 PHP
PHP设计模式之 策略模式Strategy详解【对象行为型】
2020/05/01 PHP
php判断IP地址是否在多个IP段内
2020/08/18 PHP
JavaScript窗口功能指南之在窗口中书写内容
2006/07/21 Javascript
jquery 全局AJAX事件使用代码
2010/11/05 Javascript
解决火狐浏览器下JS setTimeout函数不兼容失效不执行的方法
2012/11/14 Javascript
基于javascript实现单选及多选的向右和向左移动实例
2015/07/25 Javascript
JavaScript解决浮点数计算不准确问题的方法分析
2018/07/09 Javascript
JavaScript oncopy事件用法实例解析
2020/05/13 Javascript
原生JS生成指定位数的验证码
2020/10/28 Javascript
vue实现登录、注册、退出、跳转等功能
2020/12/23 Vue.js
Python常用内置函数总结
2015/02/08 Python
Python程序中用csv模块来操作csv文件的基本使用教程
2016/03/03 Python
python实现斐波那契数列的方法示例
2017/01/12 Python
Python使用Scrapy爬虫框架全站爬取图片并保存本地的实现代码
2018/03/04 Python
python正向最大匹配分词和逆向最大匹配分词的实例
2018/11/14 Python
Python 从相对路径下import的方法
2018/12/04 Python
python实现多张图片拼接成大图
2019/01/15 Python
Python常见的pandas用法demo示例
2019/03/16 Python
python字符串反转的四种方法详解
2019/12/02 Python
Python任务调度利器之APScheduler详解
2020/04/02 Python
如何使用css3实现一个类在线直播的队列动画的示例代码
2020/06/17 HTML / CSS
罗马尼亚购物网站:Vivantis.ro
2019/07/20 全球购物
办公室主任先进事迹
2014/01/18 职场文书
班级德育工作实施方案
2014/02/21 职场文书
关于安全的标语
2014/06/10 职场文书
学生干部培训方案
2014/06/12 职场文书
接待员岗位职责
2015/02/13 职场文书
初中毕业感言300字
2015/07/31 职场文书
PHP中strval()函数实例用法
2021/06/07 PHP
PostgreSQL事务回卷实战案例详析
2022/03/25 PostgreSQL
MySQL创建管理RANGE分区
2022/04/13 MySQL
详解Android中的TimePickerView(时间选择器)的用法
2022/04/30 Java/Android