When administering or developing for MySQL, you might come across Error 1233 – SQLSTATE: HY000 (ER_VAR_CANT_BE_READ), which indicates an attempt to read a variable that can only be set and not read. This error can be confusing, especially if you’re not aware of which system variables are read-only. In this blog post, we’ll explore how to diagnose this issue and what steps to take to resolve it.
Understanding Error 1233
MySQL has various system variables that control its operation. Some of these variables are read-only, meaning they can be set at server startup or in configuration files but cannot be queried with SHOW VARIABLES or other commands. Trying to read such a variable results in Error 1233.
Diagnosing Error 1233
The error message will include the name of the variable causing the issue, represented by %s. The first step in diagnosing this error is to identify the variable you’re trying to access and confirm whether it’s indeed a read-only variable.
Fixing the Error
Here are some scenarios that can lead to Error 1233 and how to address them:
Example 1: Attempting to Read a Write-Only Variable
Scenario:
SHOW VARIABLES LIKE 'innodb_fast_shutdown';If innodb_fast_shutdown is a write-only variable, this command will trigger Error 1233.
Fix:
Consult the MySQL documentation to determine if the variable is read-only and to understand its correct usage. If it’s a write-only variable, remove any attempts to read it from your scripts or application.
Example 2: Misunderstanding Variable Scope
Scenario:
SELECT @@local.innodb_fast_shutdown;If innodb_fast_shutdown is a global variable, trying to access it with a session scope prefix (@@local.) might cause issues.
Fix:
Use the correct scope for the variable:
SELECT @@global.innodb_fast_shutdown;Example 3: Incorrectly Set Variables in Configuration
Scenario:
You’ve set a read-only variable in your my.cnf or my.ini file and are trying to read it back within MySQL.
Fix:
Remove the attempt to read the variable within MySQL. If you need to confirm the variable’s value, check the configuration file or the MySQL server startup logs.
Example 4: Using Deprecated or Removed Variables
Scenario:
MySQL might deprecate or remove variables in newer versions. Attempting to read a variable that no longer exists will result in an error.
Fix:
Check the MySQL release notes for any mention of deprecated or removed variables. If the variable has been deprecated or removed, adjust your configuration and queries accordingly.
Example 5: Confusion Between System Variables and Status Variables
Scenario:
SHOW VARIABLES LIKE 'Threads_connected';Threads_connected is a status variable, not a system variable.
Fix:
Use the correct command to query status variables:
SHOW STATUS LIKE 'Threads_connected';Conclusion
MySQL Error 1233 occurs when there’s an attempt to read a variable that is set-only. To resolve this error, ensure you’re aware of the nature of the system variables you’re working with—whether they’re read-only, write-only, or have a specific scope. Always refer to the most current MySQL documentation for accurate information about system variables and their proper usage. By understanding the characteristics of MySQL variables and adhering to best practices, you can avoid Error 1233 and maintain smooth database operations.