root@mysqldb 22:12:  [xucl]> show create table t1\G
*************************** 1. row ***************************
       Table: t1
Create Table: CREATE TABLE `t1` (
  `id` varchar(255) DEFAULT NULL
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)


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






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);


  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
  Inspired loosely by William D. Clinger's paper "How to Read Floating
  Point Numbers Accurately" [Proc. ACM SIGPLAN '90, pp. 92-101].
   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).


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)


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






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.