7.1.9.1 System Variable Privileges

A system variable can have a global value that affects server operation as a whole, a session value that affects only the current session, or both:

• For dynamic system variables, the SET statement can be used to change their global or session runtime value (or both), to affect operation of the current server instance. (For information about dynamic variables, see Section 7.1.9.2, “Dynamic System Variables”.)

• For certain global system variables, SET can be used to persist their value to the mysqld-auto.cnf file in the data directory, to affect server operation for subsequent startups. (For information about persisting system variables and the mysqld-auto.cnf file, see Section 7.1.9.3, “Persisted System Variables”.)

• For persisted global system variables, RESET PERSIST can be used to remove their value from mysqld-auto.cnf, to affect server operation for subsequent startups.

This section describes the privileges required for operations that assign values to system variables at runtime. This includes operations that affect runtime values, and operations that persist values.

To set a global system variable, use a SET statement with the appropriate keyword. These privileges apply:

• To set a global system variable runtime value, use the SET GLOBAL statement, which requires the SYSTEM_VARIABLES_ADMIN privilege (or the deprecated SUPER privilege).

• To persist a global system variable to the mysqld-auto.cnf file (and set the runtime value), use the SET PERSIST statement, which requires the SYSTEM_VARIABLES_ADMIN or SUPER privilege.

• To persist a global system variable to the mysqld-auto.cnf file (without setting the runtime value), use the SET PERSIST_ONLY statement, which requires the SYSTEM_VARIABLES_ADMIN and PERSIST_RO_VARIABLES_ADMIN privileges. SET PERSIST_ONLY can be used for both dynamic and read-only system variables, but is particularly useful for persisting read-only variables, for which SET PERSIST cannot be used.

• Some global system variables are persist-restricted (see Section 7.1.9.4, “Nonpersistible and Persist-Restricted System Variables”). To persist these variables, use the SET PERSIST_ONLY statement, which requires the privileges described previously. In addition, you must connect to the server using an encrypted connection and supply an SSL certificate with the Subject value specified by the persist_only_admin_x509_subject system variable.

To remove a persisted global system variable from the mysqld-auto.cnf file, use the RESET PERSIST statement. These privileges apply:

• For dynamic system variables, RESET PERSIST requires the SYSTEM_VARIABLES_ADMIN or SUPER privilege.

• For read-only system variables, RESET PERSIST requires the SYSTEM_VARIABLES_ADMIN and PERSIST_RO_VARIABLES_ADMIN privileges.

• For persist-restricted variables, RESET PERSIST does not require an encrypted connection to the server made using a particular SSL certificate.

If a global system variable has any exceptions to the preceding privilege requirements, the variable description indicates those exceptions. Examples include default_table_encryption and mandatory_roles, which require additional privileges. These additional privileges apply to operations that set the global runtime value, but not operations that persist the value.

To set a session system variable runtime value, use the SET SESSION statement. In contrast to setting global runtime values, setting session runtime values normally requires no special privileges and can be done by any user to affect the current session. For some system variables, setting the session value may have effects outside the current session and thus is a restricted operation that can be done only by users who have a special privilege:

• The privilege required is SESSION_VARIABLES_ADMIN.

  Note

  Any user who has SYSTEM_VARIABLES_ADMIN or SUPER effectively has SESSION_VARIABLES_ADMIN by implication and need not be granted SESSION_VARIABLES_ADMIN explicitly.

If a session system variable is restricted, the variable description indicates that restriction. Examples include binlog_format and sql_log_bin. Setting the session value of these variables affects binary logging for the current session, but may also have wider implications for the integrity of server replication and backups.

SESSION_VARIABLES_ADMIN enables administrators to minimize the privilege footprint of users who may previously have been granted SYSTEM_VARIABLES_ADMIN or SUPER for the purpose of enabling them to modify restricted session system variables. Suppose that an administrator has created the following role to confer the ability to set restricted session system variables:

CREATE ROLE set_session_sysvars;
GRANT SYSTEM_VARIABLES_ADMIN ON *.* TO set_session_sysvars;

Any user granted the set_session_sysvars role (and who has that role active) is able to set restricted session system variables. However, that user is also able to set global system variables, which may be undesirable.

By modifying the role to have SESSION_VARIABLES_ADMIN instead of SYSTEM_VARIABLES_ADMIN, the role privileges can be reduced to the ability to set restricted session system variables and nothing else. To modify the role, use these statements:

GRANT SESSION_VARIABLES_ADMIN ON *.* TO set_session_sysvars;
REVOKE SYSTEM_VARIABLES_ADMIN ON *.* FROM set_session_sysvars;

Modifying the role has an immediate effect: Any account granted the set_session_sysvars role no longer has SYSTEM_VARIABLES_ADMIN and is not able to set global system variables without being granted that ability explicitly. A similar GRANT/REVOKE sequence can be applied to any account that was granted SYSTEM_VARIABLES_ADMIN directly rather than by means of a role.