Take a look at MySQL’s amazing implicit conversions

Mysql tutorial column introduces related implicit conversions

More related free learning Recommended: mysql tutorial(video)

1. Problem description

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)

Strange phenomenon:

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

#What the hell, the check is clearly 204027026112927603, why 204027026112927605 also came out

2. Source code explanation

The stack call relationship is as follows:

JOIN::exec( ) is the entry point for execution, Arg_comparator::compare_real() is a function for equivalence judgment, and its definition is as follows

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;
}

The comparison steps are as shown in the figure below, read line by line The id column of the t1 table is placed in val1, and the constant 204027026112927603 exists in the cache and its type is double (2.0402702611292762E 17), so after passing the value to val2 here, val2=2.0402702611292762E 17.

When the first row is scanned, the value of 204027026112927605 converted into doule is 2.0402702611292762e17. The equation is established and the row is determined to meet the conditions. Continue to scan down. Reason 204027026112927603 is also consistent with

How to detect whether the string type number converted to doule type overflows?After testing here, when the number exceeds 16 digits, it is converted into The double type is already inaccurate, for example, 20402702611292711 will be expressed as 20402702611292712 (val1 in the picture)

##MySQL string conversion The definition function into double is as follows:

{
  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);
}

The real conversion function

my_strtod_int is located in dtoa.c (it’s too complicated, just post a comment)

/*
  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).
*/

In this case, the result of our test without overflow

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)

is in line with expectations, and in this case, the correct writing method should be

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

3. Conclusion

1. Avoid implicit type conversion. The types of implicit conversion mainly include inconsistent field types, in parameters containing multiple types, inconsistent character set types or collation rules, etc.

2. Implicit type conversion may lead to the inability to use the index, inaccurate query results, etc., so you must carefully screen when using it

3. It is recommended that the numeric type be defined as int or bigint when defining the field. When the table is associated, the associated fields must maintain the same type, character set, and collation rules

4. Finally, please post the instructions on implicit type conversion from the official website

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.

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