MySQL long text field selection

A certain field needs to store long text type data, with variable length and unclear range.

varcharWhat is the maximum length that can be stored? Under what circumstances is textit better to use?

The following content explores this issue:

<1>. First set the content field to varchar(255), then this field can only store up to 255 characters.

package main

import "fmt"

func main() {
    
    

 var str string

 for i := 1; i <= 255; i++ {
    
    
  str += "a"
 }

 fmt.Println(str)

 fmt.Println(len(str))

}

输出为:
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

255

Write the generated 255-bit str into the content field and save it successfully. It has nothing to do with whether the text is a number, English or Chinese, full-width or half-width. (Since MySQL 5.0)

But if longer data is written, it will start from the 255th bit, and all subsequent data will be discarded.

That is baaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa(still 255 bits, discarding the last a) can be written,

and

aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaac(256 bit), cannot be written to

<2>. varcharThe upper limit of the type is that it can store up to 65535 bytes of data (not characters; that is, 64KB), so its upper limit is related to the character set. If so latin1, it can represent the number of characters of 65532 lengths (variable length fields are additional Use 1 or 2 bytes to record the actual data length and whether it is a NULL flag. If the data table has only one varchar field and the field is DEFAULT NULL, then the maximum length of the varchar field is 65532 bytes, that is, 65535-2 -1=65532 bytes)

When the character set utf-8is , then the maximum number of characters can be stored.

Try to automatically change the content to varchar(21844), and the error is as follows:

ERROR 1118 (42000): Row size too large. The maximum row size for the used table type, not counting BLOBs, is 65535. You have to change some columns to TEXT or BLOBs。

This is because it is not only subject to storage restrictions and encoding length restrictions , but also to the MySQL single line length limit . The length of a MySQL line record must not exceed 65535. If the length of each field in the defined table exceeds this value, the above error will be prompted.

因为这张表还有一个int(11)类型的id,故而达不到21844这个长度.

将id字段删掉,只保留content这一个字段,在utf-8下,可成功设置为 varchar(21844),

如改为varchar(21845),则会继续报如上错误.