php - 为什么mysqli_fetch_fields()给我不正确的结果？

length的结果给了我不准确的结果。
MySQL>mysqli_fetch_fields()

Field     Type               Null     Key     Default     Extra
Id        int(10) unsigned   NO       PRI     (NULL)      auto_increment
UserId    int(10) unsigned   NO       MUL     (NULL)
Title     char(255)          NO               (NULL)
Name      char(255)          NO               (NULL)
Summary   text               NO               (NULL)

然而，

class Db {
    public function exec($sql) {
        if (!$this->link) {
            $this->open();
        }
        if (($this->result = mysqli_query($this->link, $sql)) === false) {
            throw new Exception('<b>' . __METHOD__ . '</b>: ' . $sql . ': ' . mysqli_error($this->link));
        }
        return true;
    }
    public function field_info($table) {
        $sql = 'SELECT * FROM `' . $table . '` LIMIT 1';
        $this->exec($sql);
        return $this->field_info = $this->result->fetch_fields();
    }
}

$a  = $db->field_info('Notices');
print_r($a);

产生以下输出：

// Note: "max_length" is the maximum existing length in the result set,
// while "length" is the set maximum length in the table definition.

Array
(
    [0] => stdClass Object
        (
            [name] => Id
            [orgname] => Id
            [table] => Notices
            [orgtable] => Notices
            [def] =>
            [max_length] => 1
            [length] => 10
            [charsetnr] => 63
            [flags] => 49699
            [type] => 3
            [decimals] => 0
        )

    [1] => stdClass Object
        (
            [name] => UserId
            [orgname] => UserId
            [table] => Notices
            [orgtable] => Notices
            [def] =>
            [max_length] => 1
            [length] => 10
            [charsetnr] => 63
            [flags] => 53289
            [type] => 3
            [decimals] => 0
        )

    [2] => stdClass Object
        (
            [name] => Title
            [orgname] => Title
            [table] => Notices
            [orgtable] => Notices
            [def] =>
            [max_length] => 27
            [length] => 765
            [charsetnr] => 33
            [flags] => 4097
            [type] => 254
            [decimals] => 0
        )

    [3] => stdClass Object
        (
            [name] => Name
            [orgname] => Name
            [table] => Notices
            [orgtable] => Notices
            [def] =>
            [max_length] => 25
            [length] => 765
            [charsetnr] => 33
            [flags] => 4097
            [type] => 254
            [decimals] => 0
        )

    [4] => stdClass Object
        (
            [name] => Summary
            [orgname] => Summary
            [table] => Notices
            [orgtable] => Notices
            [def] =>
            [max_length] => 26
            [length] => 196605
            [charsetnr] => 33
            [flags] => 4113
            [type] => 252
            [decimals] => 0
        )
)

如果您查看，您将看到DESCRIBE Notices;字段的报告长度不匹配。CHAR给我DESCRIBE，而255给我fetch_fields()。为什么？
（如果您想知道，这个想法是为765标记生成一个maxlength属性。）

最佳答案

字段长度为255个字符，但报告的长度以字节为单位。
由于MySQL使用3 bytes per character进行UTF-8排序，因此得到255*3=765。