MySQL令人大跌眼镜的隐式转换

MySQL的隐式转换

一、问题描述

show create table t1\G*************************** 1. row ***************************       Table: t1Create Table: CREATE TABLE `t1` (  `id` varchar(255) DEFAULT NULL) ENGINE=InnoDB DEFAULT CHARSET=utf81 row in set (0.00 sec)  select * from t1;+--------------------+| id                 |+--------------------+| 204027026112927605 || 204027026112927603 || 2040270261129276   || 2040270261129275   || 100                || 101                |+--------------------+6 rows in set (0.00 sec)

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

二、源码解释

堆栈调用关系如下所示：

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;}

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);}

/*  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).*/

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)

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

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的隐式转换

一、问题描述

二、源码解释

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