Asked a question: what is the maximum length of varchar in MySQL? This is not a fixed number. This article briefly describes the restriction rules.
1. Restriction Rules
The restriction of a field has the following rules when the field is defined:
A) storage limits
The varchar field stores the actual content separately from the clustered index, and the content begins with 1 to 2 bytes representing the actual length (2 bytes in length over 255), so the maximum length cannot exceed 65535.
b) Encoding length limit
If the character type is GBK, each character can be up to 2 bytes, and the maximum length cannot exceed 32766.
If the character type is UTF8, each character can be up to 3 bytes and the maximum length cannot exceed 21845.
If the limit above is defined, the varchar field is forcibly converted to the text type and generates warning.
c) Limit of the length of the president
The length of a row definition is the limit of the varchar length in the actual application. MySQL requires that a row's definition length cannot exceed 65535. If the defined table length exceeds this value, the prompt
ERROR 1118 (42000): Row size too large. The maximum row size for the used table type, not counting BLOBs, is 65535. You have the to change some columns to TEXT or BLOBs.
2. Calculation Examples
Give two examples to illustrate the actual length of the calculation.
A) If a table has only one varchar type, as defined as CREATE TABLE t4 (c varchar (N)) CHARSET=GBK;
The maximum value for n here is (65535-1-2)/2= 32766.
The reason for minus 1 is that the actual row storage starts with the second byte ';
The reason for minus 2 is that the 2 bytes of the varchar header represent the length;
The reason for the addition of 2 is that the character encoding is GBK.
b) If a table is defined as CREATE table t4 (c int, C2 char (), C3 varchar (N)) Charset=utf8;
The maximum value of n here is (65535-1-2-4-30*3)/3=21812
Minus 1 and minus 2 are the same as in the previous example;
The reason for minus 4 is that the int type C accounts for 4 bytes;
The reason for reducing 30*3 is that char (30) occupies 90 bytes and the encoding is UTF8.
If the varchar exceeds the B rule above and is strongly turned to the text type, then each field occupies a defined length of 11 bytes, which is not "varchar", of course.
Original: http://dinglin.iteye.com/blog/914276