选修-2-Where does the error come from?

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1. Error的来源

  error实际来源于biasvariance.

2. Estimator

  我们用回归模型预测宝可梦的战斗力时,是不知道宝可梦实际计算函数的,我们只能使用自己估测的函数去尽可能靠近这个函数.
![image](https://img2023.cnblog

  那么一个Estimatorbiasvariance是指什么呢?以概率与统计来说明:

  假设预测一个变量的均值和方差.直接预测是不可能的,所以取\\(N\\)个样本点.\\(N\\)个样本点的均值\\(m\\)只有在\\(Sample\\)无穷多个点时,(大概率)才会与\\(\\mu\\)相同.我们进行多次实验\\(Sample\\),每次\\(m\\)都不同.但如果我们算\\(m\\)的期望值,这个值会恰好是\\(\\mu\\).
  每次实验计算\\(m\\)都与实际\\(\\mu\\)有偏差,这个偏差大小取决于\\(\\sigma^2\\).

  我们用\\(m\\)计算\\(s^2\\)来估测\\(\\sigma^2\\).注意右边的\\(s_i\\)都是\\(s_i^2\\),图里没标.\\(s^2\\)是一次采集的样本方差,\\(\\sigma^2\\)是实际方差,这两关系如下.

ERROR 1819 (HY000): Your password does not satisfy the current policy requirements

为了加强安全性,MySQL5.7为root用户随机生成了一个密码,在error log中,关于error log的位置,如果安装的是RPM包,则默认是/var/log/mysqld.log。

一般可通过log_error设置

mysql> select @@log_error;
+---------------------+
| @@log_error         |
+---------------------+
| /var/log/mysqld.log |
+---------------------+
1 row in set (0.00 sec)

可通过# grep "password" /var/log/mysqld.log 命令获取MySQL的临时密码

2016-01-19T05:16:36.218234Z 1 [Note] A temporary password is generated for [email protected]: waQ,qR%be2(5

用该密码登录到服务端后,必须马上修改密码,不然会报如下错误:

mysql> select user();
ERROR 1820 (HY000): You must reset your password using ALTER USER statement before executing this statement.

如果只是修改为一个简单的密码,会报以下错误:

mysql>  ALTER USER USER() IDENTIFIED BY ‘12345678‘;
ERROR 1819 (HY000): Your password does not satisfy the current policy requirements

这个其实与validate_password_policy的值有关。

validate_password_policy有以下取值:

PolicyTests Performed
0 or LOW Length
1 or MEDIUM Length; numeric, lowercase/uppercase, and special characters
2 or STRONG Length; numeric, lowercase/uppercase, and special characters; dictionary file

默认是1,即MEDIUM,所以刚开始设置的密码必须符合长度,且必须含有数字,小写或大写字母,特殊字符。

有时候,只是为了自己测试,不想密码设置得那么复杂,譬如说,我只想设置root的密码为123456。

必须修改两个全局参数:

首先,修改validate_password_policy参数的值

mysql> set global validate_password_policy=0;
Query OK, 0 rows affected (0.00 sec)

这样,判断密码的标准就基于密码的长度了。这个由validate_password_length参数来决定。

技术分享图片
mysql> select @@validate_password_length;
+----------------------------+
| @@validate_password_length |
+----------------------------+
|                          8 |
+----------------------------+
1 row in set (0.00 sec)
技术分享图片

validate_password_length参数默认为8,它有最小值的限制,最小值为:

validate_password_number_count
+ validate_password_special_char_count
+ (2 * validate_password_mixed_case_count)

其中,validate_password_number_count指定了密码中数据的长度,

validate_password_special_char_count指定了密码中特殊字符的长度,

validate_password_mixed_case_count指定了密码中大小字母的长度。

这些参数,默认值均为1,所以validate_password_length最小值为4,如果你显性指定validate_password_length的值小于4,尽管不会报错,但validate_password_length的值将设为4。如下所示:

技术分享图片
mysql> select @@validate_password_length;
+----------------------------+
| @@validate_password_length |
+----------------------------+
|                          8 |
+----------------------------+
1 row in set (0.00 sec)

mysql> set global validate_password_length=1;
Query OK, 0 rows affected (0.00 sec)

mysql> select @@validate_password_length;
+----------------------------+
| @@validate_password_length |
+----------------------------+
|                          4 |
+----------------------------+
1 row in set (0.00 sec)
技术分享图片

如果修改了validate_password_number_count,validate_password_special_char_count,validate_password_mixed_case_count中任何一个值,则validate_password_length将进行动态修改。

技术分享图片
mysql> select @@validate_password_length;
+----------------------------+
| @@validate_password_length |
+----------------------------+
|                          4 |
+----------------------------+
1 row in set (0.00 sec)

mysql> select @@validate_password_mixed_case_count;
+--------------------------------------+
| @@validate_password_mixed_case_count |
+--------------------------------------+
|                                    1 |
+--------------------------------------+
1 row in set (0.00 sec)

mysql> set global validate_password_mixed_case_count=2;
Query OK, 0 rows affected (0.00 sec)

mysql> select @@validate_password_mixed_case_count;
+--------------------------------------+
| @@validate_password_mixed_case_count |
+--------------------------------------+
|                                    2 |
+--------------------------------------+
1 row in set (0.00 sec)

mysql> select @@validate_password_length;
+----------------------------+
| @@validate_password_length |
+----------------------------+
|                          6 |
+----------------------------+
1 row in set (0.00 sec)
技术分享图片

当然,前提是validate_password插件必须已经安装,MySQL5.7是默认安装的。

那么如何验证validate_password插件是否安装呢?可通过查看以下参数,如果没有安装,则输出将为空。

技术分享图片
mysql> SHOW VARIABLES LIKE ‘validate_password%‘;
+--------------------------------------+-------+
| Variable_name                        | Value |
+--------------------------------------+-------+
| validate_password_dictionary_file    |       |
| validate_password_length             | 6     |
| validate_password_mixed_case_count   | 2     |
| validate_password_number_count       | 1     |
| validate_password_policy             | LOW   |
| validate_password_special_char_count | 1     |
+--------------------------------------+-------+
6 rows in set (0.00 sec)

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