Understanding 'Recursion Limit Exceeded' in Non-Recursive MySQL Procedures

• Recursion is a programming technique where a function (or stored procedure in this case) calls itself. This creates a nested execution where the function keeps calling itself until a base case is reached, stopping the recursion.

Recursion in MySQL Stored Procedures:

• MySQL allows recursive stored procedures, meaning they can call themselves. However, recursion is disabled by default to prevent potential infinite loops and excessive resource usage.

The Error Message:

• The error "Recursion limit exceeded in non-recursive procedure" indicates that you're encountering a recursion limit even though your stored procedure might not be explicitly written to be recursive.

Possible Causes:

1. Accidental Recursion: There might be unintended logic in your stored procedure that's causing it to call itself indirectly. This could be due to loops that iterate through data in a way that triggers a recursive behavior.
2. Triggers or Cascading Operations: MySQL triggers are procedures that execute automatically when certain events occur on a table. If a trigger inadvertently calls your stored procedure, it can create a recursive chain if not handled carefully.
3. External Dependencies: If your stored procedure interacts with other procedures or functions that are recursive, this might also lead to the recursion limit being exceeded.

Resolving the Issue:

1. Review Your Stored Procedure Logic: Carefully examine your code for any loops or conditional statements that might be causing an implicit recursive call.
2. Analyze Triggers: If you're using triggers, make sure they're not accidentally invoking your stored procedure in a recursive manner. Test disabling triggers temporarily to isolate the issue.
3. Inspect Dependencies: If your stored procedure interacts with other recursive procedures or functions, consider refactoring them to avoid unnecessary recursion.

Additional Considerations:

• You can check the current recursion limit with the MySQL command SHOW VARIABLES LIKE '%recur%';.
• If your use case genuinely requires recursion, you can temporarily increase the limit by setting the max_sp_recursion_depth system variable within your stored procedure or at the session level. However, use caution and only raise the limit as much as needed, as excessive recursion can impact performance and resource consumption.
• In some cases, it might be possible to achieve the desired outcome without recursion by employing iterative approaches (using loops) or breaking down the logic into smaller, non-recursive procedures.

CREATE PROCEDURE flawed_procedure(id INT)
BEGIN
  DECLARE child_id INT;

  SELECT child_id FROM some_table WHERE parent_id = id;

  /* This loop might inadvertently cause recursion if there are circular references in the data */
  WHILE child_id IS NOT NULL DO
    CALL flawed_procedure(child_id);  SELECT child_id FROM some_table WHERE parent_id = child_id;
  END WHILE;

  -- Rest of the procedure logic
END;

In this example, the loop iterates through parent-child relationships in a table. If there are circular references (child A points to parent B, and parent B points back to child A), the loop could keep calling the procedure for the same records, exceeding the recursion limit.

Trigger Causing Recursion:

Trigger:

CREATE TRIGGER update_trigger BEFORE UPDATE ON some_table
FOR EACH ROW
BEGIN
  UPDATE some_table SET modified = NOW() WHERE id = OLD.id;  CALL some_stored_procedure(OLD.id);
END;

Stored Procedure:

CREATE PROCEDURE some_stored_procedure(id INT)
BEGIN
  -- Logic of the procedure
END;

Here, the update_trigger is fired before an update on the some_table. If the some_stored_procedure also updates the same table, it might create a loop where the trigger fires again due to the updated data, causing recursion.

External Dependency:

Recursive Function:

CREATE FUNCTION is_descendant(parent_id INT, child_id INT) RETURNS INT
BEGIN
  DECLARE is_descendant INT DEFAULT 0;
  SELECT 1 INTO is_descendant WHERE some_table.parent_id = child_id;
  IF is_descendant THEN
    RETURN 1;
  ELSE
    RETURN is_descendant(parent_id, child_id);  END IF;
END;

CREATE PROCEDURE some_stored_procedure(id INT)
BEGIN
  DECLARE result INT;
  SELECT is_descendant(id, another_id) INTO result;  -- Rest of the procedure logic
END;

This example demonstrates how a stored procedure can hit the recursion limit if it calls a recursive function that might have a deep nesting level.

• Instead of relying on a recursive call to process hierarchical data or iterate through nested structures, you can often achieve the same result using loops (like WHILE or FOR) combined with conditional statements (IF, ELSE).
• This approach can be more efficient for large datasets as it avoids the overhead of function calls and potential stacking of execution contexts.

Example:

CREATE PROCEDURE iterative_procedure(id INT)
BEGIN
  DECLARE child_id INT;
  DECLARE done BOOL DEFAULT FALSE;

  SET child_id = id;

  WHILE NOT done DO
    -- Process data for current child_id
    SELECT child_id FROM some_table WHERE parent_id = child_id;

    SET done = (child_id IS NULL);  -- Check if loop should end
    IF NOT done THEN
      SET child_id = child_id;  -- Update child_id for next iteration
    END IF;
  END WHILE;

  -- Rest of the procedure logic
END;

Non-Recursive CTEs (Common Table Expressions):

• Starting with MySQL 8.0, you can leverage non-recursive CTEs to simulate hierarchical queries that might have traditionally been handled with recursion.
• CTEs allow you to define temporary result sets within a single query, enabling you to break down complex logic into smaller, more manageable steps.

Example (assuming MySQL 8.0 or later):

CREATE PROCEDURE cte_procedure(id INT)
BEGIN
  WITH hierarchy AS (
    SELECT id, parent_id
    FROM some_table
    WHERE id = ?  -- Starting point (replace ? with parameter)
    UNION ALL
    SELECT c.id, c.parent_id
    FROM some_table c
    INNER JOIN hierarchy h ON c.parent_id = h.id
  )
  SELECT * FROM hierarchy;
END;

Breaking Down Logic into Smaller Procedures:

• If your stored procedure involves intricate logic for processing hierarchical or nested data, consider refactoring it into a series of smaller, non-recursive procedures.
• This modular approach can improve readability, maintainability, and potentially enhance performance by avoiding the potential pitfalls of deep recursion.

CREATE PROCEDURE process_level_1(id INT);
CREATE PROCEDURE process_level_2(id INT);

CREATE PROCEDURE non_recursive_procedure(id INT)
BEGIN
  CALL process_level_1(id);
  SELECT * FROM some_table WHERE parent_id = id;  -- Identify children
  FOR EACH child_row IN SELECTOR * FROM (SELECT child_id FROM some_table WHERE parent_id = id) DO
    CALL process_level_2(child_row.child_id);
  END FOR;
END;