MySQL权限系统(三).权限表 Grant Tables

The mysql system database includes several grant tables that contain information about user accounts and the privileges held by them. This section describes those tables. For information about other tables in the system database, see Section 6.3, “The mysql System Database”.mysql系统数据库包括几个授权表，其中包含有关用户帐户及其拥有的权限的信息。 本节描述这些表。 有关系统数据库中其他表的信息，请参见第6.3节“mysql系统数据库”。

Normally, to manipulate the contents of grant tables, you modify them indirectly by using account-management statements such as CREATE USER, GRANT, and REVOKE to set up accounts and control the privileges available to each one. See Section 14.7.1, “Account Management Statements”. The discussion here describes the underlying structure of the grant tables and how the server uses their contents when interacting with clients.通常，要操作授权表的内容，可以使用帐户管理语句（如CREATE USER，GRANT和REVOKE）间接修改它们，以设置帐户并控制每个帐户可用的权限。 请参见第14.7.1节“帐户管理语句”。 这里的讨论描述了权限表的底层结构以及服务器在与客户端交互时如何使用它们的内容。

Note
Direct modification of grant tables using statements such as INSERT, UPDATE, or DELETE is discouraged and done at your own risk. 
The server is free to ignore rows that become malformed as a result of such modifications.
不鼓励使用诸如INSERT，UPDATE或DELETE之类的语句直接修改授权表及风险自己承担。 服务器可以随意忽略由于这种修改而导致格式不正确的行。
As of MySQL 5.7.18, for any operation that modifies a grant table, the server checks whether the table has the expected structure
 and produces an error if not. mysql_upgrade must be run to update the tables to the expected structure.
从MySQL 5.7.18起，对于修改授权表的任何操作，服务器都会检查表是否具有预期结构，如果没有，则会产生错误。 必须运行mysql_upgrade才能将表更新为预期结构。

These mysql database tables contain grant information: mysql数据库表包含的授权信息:

Each grant table contains scope columns and privilege columns:每个授权表包含范围列和特权列：

The server uses the grant tables in the following manner:

The server uses the user and db tables in the mysql database at both the first and second stages of access control (see Section 7.2, “The MySQL Access Privilege System”). The columns in the user and db tables are shown here.

The user table plugin, Password, and authentication_string columns store authentication plugin and credential information. In MySQL 5.7.6, the Password column was removed and all credentials are stored in the authentication_string column.

If an account row names a plugin in the plugin column, the server uses it to authenticate connection attempts for the account. It is up to the plugin whether it uses the Password and authentication_string column values.

As of MySQL 5.7.2, the plugin column must be nonempty.

Before MySQL 5.7.2, the plugin column for an account row is permitted to be empty. In this case, the server authenticates the account using the mysql_native_password or mysql_old_password plugin implicitly, depending on the format of the password hash in the Password column. If the Password value is empty or a 4.1 password hash (41 characters), the server uses mysql_native_password. If the password value is a pre-4.1 password hash (16 characters), the server uses mysql_old_password. (For additional information about these hash formats, see Section 7.1.2.4, “Password Hashing in MySQL”.) Clients must match the password in the Password column of the account row.

At startup, and at runtime when FLUSH PRIVILEGES is executed, the server checks user table rows. As of MySQL 5.7.2, for any row with an empty plugin column, the server writes a warning to the error log of this form:

[Warning] User entry ‘user_name‘@‘host_name‘ has an empty plugin
value. The user will be ignored and no one can login with this user
anymore.

To address this problem, see Section 7.5.1.3, “Migrating Away from Pre-4.1 Password Hashing and the mysql_old_password Plugin”.

The password_expired column permits DBAs to expire account passwords and require users to reset their password. The default password_expired value is ‘N‘, but can be set to ‘Y‘ with the ALTER USER statement. After an account‘s password has been expired, all operations performed by the account in subsequent connections to the server result in an error until the user issues an ALTER USER statement (for MySQL 5.7.6 and up) or SET PASSWORD statement (before MySQL 5.7.6) to establish a new account password.

It is possible after password expiration to “reset” a password by setting it to its current value. As a matter of good policy, it is preferable to choose a different password.

password_last_changed (added in MySQL 5.7.4) is a TIMESTAMP column indicating when the password was last changed. The value is non-NULL only for accounts that use MySQL built-in authentication methods (accounts that use an authentication plugin of mysql_native_password, mysql_old_password, or sha256_password). The value is NULL for other accounts, such as those authenticated using an external authentication system.

password_last_changed is updated by the CREATE USER, ALTER USER, and SET PASSWORD statements, and by GRANT statements that create an account or change an account password.

password_lifetime (added in MySQL 5.7.4) indicates the account password lifetime, in days. If the password is past its lifetime (assessed using the password_last_changed column), the server considers the password expired when clients connect using the account. A value of N greater than zero means that the password must be changed every N days. A value of 0 disables automatic password expiration. If the value is NULL (the default), the global expiration policy applies, as defined by the default_password_lifetime system variable.

account_locked (added in MySQL 5.7.6) indicates whether the account is locked (see Section 7.3.10, “User Account Locking”).

During the second stage of access control, the server performs request verification to ensure that each client has sufficient privileges for each request that it issues. In addition to the user and db grant tables, the server may also consult the tables_priv and columns_priv tables for requests that involve tables. The latter tables provide finer privilege control at the table and column levels. They have the columns shown in the following table.

The Timestamp and Grantor columns are set to the current timestamp and the CURRENT_USER value, respectively, but are otherwise unused.

For verification of requests that involve stored routines, the server may consult the procs_priv table, which has the columns shown in the following table.

The Routine_type column is an ENUM column with values of ‘FUNCTION‘ or ‘PROCEDURE‘ to indicate the type of routine the row refers to. This column enables privileges to be granted separately for a function and a procedure with the same name.

The Timestamp and Grantor columns are unused.

The proxies_priv table records information about proxy accounts. It has these columns:

For an account to be able to grant the PROXY privilege to other accounts, it must have a row in the proxies_priv table with With_grant set to 1 and Proxied_host and Proxied_user set to indicate the account or accounts for which the privilege can be granted. For example, the ‘root‘@‘localhost‘ account created during MySQL installation has a row in the proxies_priv table that enables granting the PROXY privilege for ‘‘@‘‘, that is, for all users and all hosts. This enables root to set up proxy users, as well as to delegate to other accounts the authority to set up proxy users. See Section 7.3.9, “Proxy Users”.

Scope columns in the grant tables contain strings. The default value for each is the empty string. The following table shows the number of characters permitted in each column.

For access-checking purposes, comparisons of User, Proxied_user, Password, authentication_string, Db, and Table_name values are case sensitive. Comparisons of Host, Proxied_host, Column_name, and Routine_name values are not case sensitive.

The user and db tables list each privilege in a separate column that is declared as ENUM(‘N‘,‘Y‘) DEFAULT ‘N‘. In other words, each privilege can be disabled or enabled, with the default being disabled.

The tables_priv, columns_priv, and procs_priv tables declare the privilege columns as SET columns. Values in these columns can contain any combination of the privileges controlled by the table. Only those privileges listed in the column value are enabled.

Only the user table specifies administrative privileges, such as RELOAD and SHUTDOWN. Administrative operations are operations on the server itself and are not database-specific, so there is no reason to list these privileges in the other grant tables. Consequently, the server need consult only the user table to determine whether a user can perform an administrative operation.

The FILE privilege also is specified only in the user table. It is not an administrative privilege as such, but a user‘s ability to read or write files on the server host is independent of the database being accessed.

The server reads the contents of the grant tables into memory when it starts. You can tell it to reload the tables by issuing a FLUSH PRIVILEGES statement or executing a mysqladmin flush-privileges or mysqladmin reload command. Changes to the grant tables take effect as indicated in Section 7.2.6, “When Privilege Changes Take Effect”.

When you modify an account, it is a good idea to verify that your changes have the intended effect. To check the privileges for a given account, use the SHOW GRANTS statement. For example, to determine the privileges that are granted to an account with user name and host name values of bob and pc84.example.com, use this statement:

SHOW GRANTS FOR ‘bob‘@‘pc84.example.com‘;

To display nonprivilege properties of an account, use SHOW CREATE USER:

SHOW CREATE USER ‘bob‘@‘pc84.example.com‘;