Shulou(Shulou.com)05/31 Report--

This article mainly introduces MySQL implicit conversion example analysis, the article is very detailed, has a certain reference value, interested friends must read!

I. Description of the problem

root@mysqldb 22:12: [xucl]> 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)root@mysqldb 22:19: [xucl]> select * from t1;+--------------------+| id |+--------------------+| 204027026112927605 || 204027026112927603 || 2040270261129276 || 2040270261129275 || 100 || 101 |+--------------------+6 rows in set (0.00 sec)

Strange phenomena:

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

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

Second, the source code explanation

The stack call relationship is as follows:

where JOIN::exec() is the entry for execution, Arg_comparator::compare_real() is the function for equivalence judgment, which is defined 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 shown in the following figure. The id column of t1 table is read row by row and put into val1, while the constant 204027026112927603 exists in cache, and the type is double type (2.0402702611292762E+17), so val2=2.0402702611292762E+17 after passing the value to val2 here.

When scanning to the first row, 204027026112927605 is converted into a double value of 2.0402702611292762e17, the equation is true, and it is determined to be a row that meets the conditions. Continue scanning downward. Similarly, 204027026112927603 also meets the conditions.

How do I detect if a string number is converted to a double overflow? After testing here, when the number exceeds 16 digits, it is inaccurate to convert to double type. For example, 20402702611292711 will be expressed as 20402702611292712 (val1 in the figure).

MySQL string to double definition function 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);} 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 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)

III. Conclusion

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.

Implicit type conversions can lead to unusable indexes, inaccurate query results, and so on, so you must be careful when using them

It is suggested that the number type be defined as int or bigint when defining the field. When associating tables, the associated fields must maintain the same type, character set and collation rules.

Finally, post a description of the official website for implicit type conversion

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 aconstant, the constant is converted to a timestamp before the comparison is performed. This isdone to be more ODBC-friendly. This is not done for the arguments to IN(). To be safe, alwaysuse complete datetime, date, or time strings when doing comparisons. For example, to achieve bestresults when using BETWEEN with date or time values, use CAST() to explicitly convert the values tothe desired data type.A single-row subquery from a table or tables is not considered a constant. For example, if a subqueryreturns 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. Thearguments are compared as decimal values if the other argument is a decimal or integer value, or asfloating-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. Hope to share the content to help everyone, more relevant knowledge, welcome to pay attention to the industry information channel!

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