MySQL令人咋舌的隐式转换


Posted in MySQL onApril 05, 2021

一、问题描述

root@mysqldb 22:12:  [xucl]> 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)

root@mysqldb 22:19:  [xucl]> select * from t1;
+--------------------+
| id                 |
+--------------------+
| 204027026112927605 |
| 204027026112927603 |
| 2040270261129276   |
| 2040270261129275   |
| 100                |
| 101                |
+--------------------+
6 rows in set (0.00 sec)

奇怪的现象:

root@mysqldb 22:19:  [xucl]> select * from t1 where id=204027026112927603;
+--------------------+
| id                 |
+--------------------+
| 204027026112927605 |
| 204027026112927603 |
+--------------------+
2 rows in set (0.00 sec)

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

 

二、源码解释

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。

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

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 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

root@mysqldb 23:30:  [xucl]> select * from t1 where id=2040270261129276;
+------------------+
| id               |
+------------------+
| 2040270261129276 |
+------------------+
1 row in set (0.00 sec)

root@mysqldb 23:30:  [xucl]> 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

root@mysqldb 22:19:  [xucl]> select * from t1 where id='204027026112927603';
+--------------------+
| id                 |
+--------------------+
| 204027026112927603 |
+--------------------+
1 row in set (0.01 sec)

三、结论

  1. 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

  2. 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

  3. 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

  4. 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.

MySQL 相关文章推荐
MySQL infobright的安装步骤
Apr 07 MySQL
MySQL安装后默认自带数据库的作用详解
Apr 27 MySQL
Mysql服务添加 iptables防火墙策略的方案
Apr 29 MySQL
Mysql实现主从配置和多主多从配置
Jun 02 MySQL
mysql外连接与内连接查询的不同之处
Jun 03 MySQL
如何使用分区处理MySQL的亿级数据优化
Jun 18 MySQL
mysql事务隔离级别详情
Oct 24 MySQL
Mysql中有关Datetime和Timestamp的使用总结
Dec 06 MySQL
mysql中数据库覆盖导入的几种方式总结
Mar 25 MySQL
MySQL RC事务隔离的实现
Mar 31 MySQL
深入理解mysql事务隔离级别和存储引擎
Apr 12 MySQL
以MySQL5.7为例了解一下执行计划
Apr 13 MySQL
MySQL基础(一)
Apr 05 #MySQL
MySQL基础(二)
MySQL学习总结-基础架构概述
MySQL锁机制
mysql知识点整理
Apr 05 #MySQL
浅析InnoDB索引结构
Apr 05 #MySQL
MySQL入门命令之函数-单行函数-流程控制函数
Apr 05 #MySQL
You might like
php self,$this,const,static,-&amp;gt;的使用
2009/10/22 PHP
一个严格的PHP Session会话超时时间设置方法
2014/06/10 PHP
php命令行(cli)下执行PHP脚本文件的相对路径的问题解决方法
2015/05/25 PHP
PHP检测用户语言的方法
2015/06/15 PHP
javascript document.compatMode兼容性
2010/02/23 Javascript
网站导致浏览器崩溃的原因总结(多款浏览器) 推荐
2010/04/15 Javascript
图片无缝滚动代码(向左/向下/向上)
2013/04/10 Javascript
基于jquery的文章中所有图片width大小批量设置方法
2013/08/01 Javascript
使用javascript做的一个随机点名程序
2014/02/13 Javascript
用html5 js实现点击一个按钮达到浏览器全屏效果
2014/05/28 Javascript
jQuery实现的登录浮动框效果代码
2015/09/26 Javascript
基于JS如何实现给字符加千分符(65,541,694,158)
2016/08/03 Javascript
JS文件上传神器bootstrap fileinput详解
2021/01/28 Javascript
JavaScript中的call和apply的用途以及区别
2017/01/11 Javascript
ajax+node+request爬取网络图片的实例(宅男福利)
2017/08/28 Javascript
vue axios整合使用全攻略
2018/05/24 Javascript
vue组件创建的三种方式小结
2020/02/03 Javascript
[50:04]DOTA2上海特级锦标赛D组小组赛#2 Liquid VS VP第二局
2016/02/28 DOTA
复制粘贴功能的Python程序
2008/04/04 Python
python socket 超时设置 errno 10054
2014/07/01 Python
Django自定义认证方式用法示例
2017/06/23 Python
Python基于回溯法子集树模板实现图的遍历功能示例
2017/09/05 Python
轻量级的Web框架Flask 中模块化应用的实现
2017/09/11 Python
Python for循环中的陷阱详解
2018/07/13 Python
python 爬取古诗文存入mysql数据库的方法
2020/01/08 Python
浅析Python 责任链设计模式
2020/09/11 Python
CSS3 函数技巧 用css 实现js实现的事情(clac Counters Tooltip)
2017/08/15 HTML / CSS
IE9对HTML5中部分属性不支持的原因分析
2014/10/15 HTML / CSS
预订全球最佳旅行体验:Viator
2018/03/30 全球购物
如何写毕业求职自荐信
2013/11/06 职场文书
机关单位人员学雷锋心得体会
2014/03/10 职场文书
2014学习全国两会精神心得体会2000字
2014/03/11 职场文书
庐山导游词
2015/02/03 职场文书
大学副班长竞选稿
2015/11/21 职场文书
Django使用redis配置缓存的方法
2021/06/01 Redis
Android studio 简单计算器的编写
2022/05/20 Java/Android