How to Detect and Fix Foreign Key Violations in InnoDB Tables?

Detecting Foreign Key Violations in InnoDB Tables

When manipulating InnoDB tables with foreign key constraints, it's essential to verify data integrity after modifications. However, simply re-enabling foreign key checks may not be sufficient to detect violations.

Configuring Foreign Key Checking

To prevent data inconsistencies during table loading, you can temporarily disable foreign key checks before inserting new rows and re-enable them after the process is complete:

SET FOREIGN_KEY_CHECKS=0;
-- Insert new rows
SET FOREIGN_KEY_CHECKS=1;

Checking Foreign Key Integrity

However, after re-enabling foreign key checks, there may still be undetected violations due to the deferred nature of constraint checking in InnoDB. To proactively detect and fix these issues, you can use the following stored procedure:

DELIMITER $$

CREATE PROCEDURE `ANALYZE_INVALID_FOREIGN_KEYS`(
  checked_database_name VARCHAR(64), 
  checked_table_name VARCHAR(64), 
  temporary_result_table ENUM('Y', 'N'))
LANGUAGE SQL
NOT DETERMINISTIC
READS SQL DATA

BEGIN
  DECLARE DONE INT DEFAULT 0;
  -- Iterate over foreign keys
  DECLARE foreign_key_cursor CURSOR FOR
    SELECT
      `TABLE_SCHEMA`,
      `TABLE_NAME`,
      `COLUMN_NAME`,
      `CONSTRAINT_NAME`,
      `REFERENCED_TABLE_SCHEMA`,
      `REFERENCED_TABLE_NAME`,
      `REFERENCED_COLUMN_NAME`
    FROM 
      INFORMATION_SCHEMA.KEY_COLUMN_USAGE 
    WHERE 
      `CONSTRAINT_SCHEMA` LIKE checked_database_name AND
      `TABLE_NAME` LIKE checked_table_name AND
      `REFERENCED_TABLE_SCHEMA` IS NOT NULL;

  DECLARE CONTINUE HANDLER FOR NOT FOUND SET DONE = 1;

  -- Create temporary result table for invalid foreign keys
  IF temporary_result_table = 'Y' THEN
    DROP TEMPORARY TABLE IF EXISTS INVALID_FOREIGN_KEYS;
    CREATE TEMPORARY TABLE INVALID_FOREIGN_KEYS(...);
  ELSE
    DROP TABLE IF EXISTS INVALID_FOREIGN_KEYS;
    CREATE TABLE INVALID_FOREIGN_KEYS(...);
  END IF;

  -- Check foreign key violations and insert into result table
  OPEN foreign_key_cursor;
  foreign_key_cursor_loop: LOOP
    FETCH foreign_key_cursor INTO 
      TABLE_SCHEMA_VAR, 
      TABLE_NAME_VAR, 
      COLUMN_NAME_VAR, 
      CONSTRAINT_NAME_VAR, 
      REFERENCED_TABLE_SCHEMA_VAR, 
      REFERENCED_TABLE_NAME_VAR, 
      REFERENCED_COLUMN_NAME_VAR;
    IF DONE THEN
      LEAVE foreign_key_cursor_loop;
    END IF;

    SET @from_part = "...";
    SET @full_query = "...";
    PREPARE stmt FROM @full_query;

    EXECUTE stmt;
    IF @invalid_key_count > 0 THEN
      INSERT INTO 
        INVALID_FOREIGN_KEYS 
      SET 
        `TABLE_SCHEMA` = TABLE_SCHEMA_VAR, 
        `TABLE_NAME` = TABLE_NAME_VAR, 
        `COLUMN_NAME` = COLUMN_NAME_VAR, 
        `CONSTRAINT_NAME` = CONSTRAINT_NAME_VAR, 
        `REFERENCED_TABLE_SCHEMA` = REFERENCED_TABLE_SCHEMA_VAR, 
        `REFERENCED_TABLE_NAME` = REFERENCED_TABLE_NAME_VAR, 
        `REFERENCED_COLUMN_NAME` = REFERENCED_COLUMN_NAME_VAR, 
        `INVALID_KEY_COUNT` = @invalid_key_count,
        `INVALID_KEY_SQL` = "...";
    END IF;
    DEALLOCATE PREPARE stmt; 
  END LOOP foreign_key_cursor_loop;
END$$

DELIMITER ;

Usage

To use the stored procedure, execute it with the desired database and table names:

CALL ANALYZE_INVALID_FOREIGN_KEYS('%', '%', 'Y');
DROP PROCEDURE IF EXISTS ANALYZE_INVALID_FOREIGN_KEYS;

SELECT * FROM INVALID_FOREIGN_KEYS;

The INVALID_FOREIGN_KEYS table will contain details of any invalid foreign keys, including the referring and referenced tables, columns, and an SQL query that can be used to identify the specific violating rows.

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