Tackling MySQL Error 1238 (ER_INCORRECT_GLOBAL_LOCAL_VAR): Setting the Right Variable Scope

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MySQL Error 1238 – SQLSTATE: HY000 (ER_INCORRECT_GLOBAL_LOCAL_VAR) is a common issue that arises when there’s a mismatch between the variable’s scope you are trying to set and the scope that the variable actually supports. MySQL variables can be either global or session (local) scoped, and understanding this distinction is key to resolving the error. This guide will provide insight into diagnosing and fixing Error 1238, ensuring you apply the correct scope to your system variables.

Understanding Error 1238

Error 1238 occurs when you attempt to set a system variable at a scope that it does not support. For instance, trying to set a global variable as a session variable, or vice versa, will trigger this error. The %s in the error message will indicate the variable name and its incorrect scope.

Diagnosing the Error

The error message will specify the variable in question and the scope you’ve incorrectly tried to apply. For example:

Variable 'innodb_buffer_pool_size' is a GLOBAL variable.

Fixing the Error

To resolve this error, you need to set the variable at the correct scope. Here are multiple scenarios that might cause Error 1238 and how to address them:

1. Setting a Global Variable as Session

If you try to set a global-only variable at the session level, MySQL will raise Error 1238:

SET SESSION innodb_buffer_pool_size = 536870912;

The correct way to set a global variable is:

SET GLOBAL innodb_buffer_pool_size = 536870912;

Remember that some global variables may require a server restart and should be set in the configuration file.

2. Setting a Session Variable as Global

Conversely, attempting to set a session-only variable globally will also result in Error 1238:

SET GLOBAL sort_buffer_size = 1048576;

To set a session variable correctly:

SET SESSION sort_buffer_size = 1048576;

Or simply:

SET sort_buffer_size = 1048576;

Since SET without GLOBAL or SESSION defaults to session scope.

3. Confusion Between Global and Session Variables

Some variables can be set at both global and session scope. However, confusion can arise when you try to set them without specifying the scope:

SET autocommit = 0; -- This is correct as autocommit can be both GLOBAL and SESSION

Always specify the scope if you want to be explicit:

SET GLOBAL autocommit = 0;

Or:

SET SESSION autocommit = 0;

4. Read-Only Variables

Attempting to set a read-only global variable at runtime will trigger Error 1238:

SET GLOBAL read_only_variable = 'value';

For read-only variables, changes must be made in the MySQL configuration file (my.cnf or my.ini), and the server must be restarted for the changes to take effect.

Preventive Measures

To prevent Error 1238:

  • Familiarize yourself with the scope of system variables by consulting the MySQL documentation.
  • Use SHOW VARIABLES to check the current values and scope of variables:
SHOW VARIABLES LIKE 'variable_name';
  • When in doubt, specify the scope explicitly to avoid ambiguity.

Conclusion

Error 1238 in MySQL is a clear indication of a scope mismatch when setting system variables. By understanding the difference between global and session variables and ensuring you apply the correct scope, you can avoid this error. Accurate variable management is essential for the smooth operation of your MySQL server. For complex configurations or persistent issues, don’t hesitate to consult with a database administrator for expert assistance.

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