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Description

This error occurs when an SQL statement attempts to read the value of a system variable that is designated as write-only. Such variables are designed to accept a setting but do not expose their current value for retrieval or use in expressions.

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Error Message

Variable '%s' can only be set, not read

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Known Causes

3 known causes

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Attempting to SELECT a Write-Only Variable

This error occurs when an SQL query tries to retrieve the value of a system variable that is configured to be write-only.

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Using a Write-Only Variable in an Expression

The error triggers if a write-only variable is referenced within an SQL expression that requires its value to be evaluated.

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Misunderstanding Variable Properties

Arises from an incorrect assumption about the read/write capabilities or scope of a particular MariaDB system variable.

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Solutions

3 solutions available

1. Avoid Reading 'SET' Only Variables in Queries easy

Ensure you are not attempting to read variables that are designed solely for setting configuration.

1

Identify the variable mentioned in the error message. It will be a variable that can only be set using `SET` and cannot be queried with `SELECT` or used in expressions where its value is expected.

2

Review your SQL queries, stored procedures, or scripts. Look for instances where you are trying to retrieve the value of this variable. Common culprits are using it in `SELECT` statements, `WHERE` clauses, or as part of calculations.

SELECT @@variable_name;
SELECT column_name FROM table WHERE column_name = @@variable_name;

3

Remove or rephrase the part of the query that attempts to read the variable. If you need to use its value, set it to a user-defined variable first and then read that user-defined variable.

SET @my_var = @@global.some_set_only_variable;
SELECT @my_var;

2. Use User-Defined Variables for Temporary Storage easy

Store the desired value in a user-defined variable before attempting to use it.

1

When you encounter the error 'Variable '%s' can only be set, not read', note the variable name indicated by '%s'.

2

In your script or query, first set the value of a user-defined variable (prefixed with '@') from the 'SET' only variable.

SET @my_temp_value = @@global.variable_that_cannot_be_read;
-- Or for session variables:
SET @my_temp_value = @@session.variable_that_cannot_be_read;

3

Now, you can use the user-defined variable `@my_temp_value` in your subsequent queries or logic.

SELECT * FROM your_table WHERE some_column = @my_temp_value;

3. Consult MariaDB Documentation for Variable Scope and Usage medium

Understand which variables are read-only and their intended purpose.

1

Open your web browser and navigate to the official MariaDB documentation website.

2

Search for the specific variable name that caused the error (e.g., 'innodb_buffer_pool_size', 'max_connections').

3

Carefully read the documentation page for that variable. Pay close attention to sections describing its scope (GLOBAL, SESSION) and whether it can be read directly or only set.

4

Based on the documentation, adjust your SQL statements to align with the variable's intended usage. If it's a configuration variable that should only be set at startup or via `SET GLOBAL`, do not attempt to read it dynamically in your queries.

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Related Errors

5 related errors

MariaDB Error 1233: Variable Cannot Be Read