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
IDEA 链接Mysql数据库并执行查询操作的完整代码
May 20 MySQL
MySQL中使用or、in与union all在查询命令下的效率对比
May 26 MySQL
MySQL 常见的数据表设计误区汇总
Jun 07 MySQL
MySQL里面的子查询的基本使用
Aug 02 MySQL
Node-Red实现MySQL数据库连接的方法
Aug 07 MySQL
MySQL中的引号和反引号的区别与用法详解
Oct 24 MySQL
记一次Mysql不走日期字段索引的原因小结
Oct 24 MySQL
SQL优化老出错,那是你没弄明白MySQL解释计划用法
Nov 27 MySQL
mysql sum(if())和count(if())的用法说明
Jan 18 MySQL
如何创建一个创建MySQL数据库中的datetime类型
Mar 21 MySQL
Windows 64位 安装 mysql 8.0.28 图文教程
Apr 19 MySQL
SQL中去除重复数据的几种方法汇总(窗口函数对数据去重)
May 08 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
《APMServ 5.1.2》使用图解
2006/10/23 PHP
PHP 日期时间函数的高级应用技巧
2009/10/10 PHP
php页面消耗内存过大的处理办法
2013/03/18 PHP
PHP时间戳格式全部汇总 (获取时间、时间戳)
2016/06/13 PHP
PHP长连接实现与使用方法详解
2018/02/11 PHP
PHP扩展类型及安装方式解析
2020/04/27 PHP
javascript获取和判断浏览器窗口、屏幕、网页的高度、宽度等
2014/05/08 Javascript
JS数组的常见用法实例
2015/02/10 Javascript
JavaScript中诡异的delete操作符
2015/03/12 Javascript
javascript实现十秒钟后注册按钮可点击的方法
2015/05/13 Javascript
Bootstrap每天必学之弹出框(Popover)插件
2016/04/25 Javascript
Bootstrap和Angularjs配合自制弹框的实例代码
2016/08/24 Javascript
JS实现简单的tab切换选项卡效果
2016/09/21 Javascript
docker中编译nodejs并使用nginx启动
2017/06/23 NodeJs
详谈innerHTML innerText的使用和区别
2017/08/18 Javascript
浅谈node.js 命令行工具(cli)
2018/05/10 Javascript
搭建vue开发环境
2018/07/19 Javascript
layui点击弹框页面 表单请求的方法
2019/09/21 Javascript
[02:15]2014DOTA2国际邀请赛 专访LGD.lin小兔子是大腿
2014/07/14 DOTA
[44:47]Ti4 循环赛第三日 iG vs NaVi
2014/07/12 DOTA
[42:00]完美世界DOTA2联赛PWL S3 Phoenix vs INK ICE 第一场 12.13
2020/12/17 DOTA
Python多线程爬虫简单示例
2016/03/04 Python
Python基础教程之内置函数locals()和globals()用法分析
2018/03/16 Python
对python中raw_input()和input()的用法详解
2018/04/22 Python
Django数据库迁移常见使用方法
2020/11/12 Python
使用纯 CSS 创作一个脉动 loader效果的源码
2018/09/28 HTML / CSS
优衣库英国官网:UNIQLO英国
2016/12/25 全球购物
美国木工工具和用品商店:Woodcraft
2019/10/30 全球购物
电气个人求职信范文
2014/02/04 职场文书
办公室主任岗位竞聘书
2015/09/15 职场文书
教师理论学习心得体会
2016/01/21 职场文书
《飘》英文读后感五篇
2019/10/11 职场文书
只用40行Python代码就能写出pdf转word小工具
2021/05/31 Python
解决SpringCloud Feign传对象参数调用失败的问题
2021/06/23 Java/Android
MySQL 聚合函数排序
2021/07/16 MySQL
css布局巧妙技巧之css三角示例的运用
2022/03/16 HTML / CSS