MariaDB Foreign Key Best Practices: Choosing the Right Data Type for Referenced Columns

• Foreign keys are a relational database concept that enforces data integrity by linking tables together.
• A foreign key in one table (child table) references a column (primary key) in another table (parent table).
• This ensures that data in the child table always corresponds to valid entries in the parent table, preventing inconsistencies.

Error Cause:

In MariaDB 10.4.24, this error typically occurs when you attempt to create a foreign key that references a column of type TEXT (unbounded string). Here's the breakdown:

1. Index Requirement: Foreign keys rely on indexes in the child table to efficiently check references.
2. Index Size Limitation: MariaDB has a limit on the size of indexes for certain storage engines (like InnoDB with the default DYNAMIC row format and innodb_large_prefix=on). This limit is usually around 3072 bytes.
3. TEXT Column Issue: When the referenced column (in the child table) is TEXT, the index created for the foreign key might exceed the size limit due to the potentially large size of TEXT values.
4. Index Prefix Incompatibility: Foreign keys in MariaDB cannot use indexes with prefixes (specifying a subset of the column for indexing). This is because the entire referenced column needs to be compared for the foreign key constraint to work correctly.

Resolving the Error:

To fix this error, you need to modify the data type of the referenced column in the child table:

1. Change Column Type: Instead of TEXT, use a VARCHAR(n) data type, where n is a suitable maximum length for the strings you expect to store. This ensures the index size stays within the limit.
2. Example: If your TEXT column is named address, you could change it to VARCHAR(255). Adjust the maximum length (n) based on your actual data needs.

Additional Considerations:

• If you truly need unbounded strings, consider alternative approaches like:
  • Splitting the data into smaller, fixed-length columns.
  • Using a separate table for long text data and linking it to the main table using a foreign key on a shorter identifier column.

-- Create tables with incorrect foreign key definition (TEXT reference)
CREATE TABLE orders (
  order_id INT PRIMARY KEY,
  customer_id TEXT,  -- This will cause the error
  FOREIGN KEY (customer_id) REFERENCES customers(id)
);

CREATE TABLE customers (
  id INT PRIMARY KEY,
  name VARCHAR(255)
);

In this example, attempting to create the orders table with a FOREIGN KEY on a TEXT column (customer_id) will likely trigger the "10.4.24-MariaDB - Foreign key constraint is incorrectly formed" error because the resulting index may exceed the size limit for foreign keys.

Scenario 2: Fixing the Error with VARCHAR

-- Recreate tables with correct data type for foreign key reference
ALTER TABLE orders
  MODIFY customer_id VARCHAR(255);  -- Change TEXT to VARCHAR(255)

ALTER TABLE orders
  DROP FOREIGN KEY IF EXISTS FK_orders_customer_id;  -- Drop the existing (potentially invalid) constraint

ALTER TABLE orders
  ADD CONSTRAINT FK_orders_customer_id
  FOREIGN KEY (customer_id) REFERENCES customers(id);  -- Recreate the foreign key with VARCHAR reference

Here, we:

1. Modify the customer_id column in the orders table to VARCHAR(255). Adjust the maximum length (255) as needed.
2. Drop the existing foreign key constraint (if it exists) to avoid conflicts.
3. Recreate the foreign key constraint with the modified VARCHAR column as the reference, ensuring a valid configuration.

This approach ensures that the foreign key constraint is formed correctly and adheres to MariaDB's limitations.

Additional Notes:

• Remember to replace customers(id) with the actual table and column names in your specific database schema.
• Consider using shorter VARCHAR lengths if you know the maximum string size for customer_id to optimize storage.
• If you truly need unbounded string storage, explore alternative approaches as mentioned earlier.

• Break down the TEXT data into smaller, fixed-length columns that can fit within the foreign key index size limit.
• This approach is suitable if your long strings have a natural structure that can be logically divided.

Example:

CREATE TABLE orders (
  order_id INT PRIMARY KEY,
  customer_id TEXT,
  customer_first_name VARCHAR(255),  -- Split from TEXT for foreign key
  customer_last_name VARCHAR(255),   -- Split from TEXT for foreign key
  -- Other columns
);

CREATE TABLE customers (
  id INT PRIMARY KEY,
  name VARCHAR(255)
);

ALTER TABLE orders
  ADD FOREIGN KEY (customer_first_name, customer_last_name) REFERENCES customers(name);

In this example, we split the customer_id (assuming it contains first and last names) into separate customer_first_name and customer_last_name columns with a suitable VARCHAR length. Then, we create a foreign key that references the combined name column in the customers table.

Separate Table for Long Text Data:

• Create a separate table to store the long text data.
• Include a shorter identifier column (e.g., long_text_id) in this table as the primary key.
• In the main table (e.g., orders), have a foreign key referencing the long_text_id column.

CREATE TABLE long_text_data (
  id INT PRIMARY KEY AUTO_INCREMENT,  -- Use AUTO_INCREMENT for unique identifiers
  long_text TEXT
);

CREATE TABLE orders (
  order_id INT PRIMARY KEY,
  customer_id INT,  -- Reference the long_text_id
  FOREIGN KEY (customer_id) REFERENCES long_text_data(id)
);

Here, we create a long_text_data table to store the unbounded text data. The orders table has a foreign key referencing the id (primary key) of the long_text_data table. This approach keeps the foreign key index size manageable while allowing for long text storage.

Choosing the Right Method:

The best alternative method depends on your specific data structure and usage patterns. Consider the following factors:

• Data Structure: If your long strings have a natural division (like first and last names), splitting might be suitable.
• Query Patterns: If you frequently query based on the long text data, keeping it in the main table with a separate foreign key table might impact performance.
• Storage Efficiency: Splitting can save storage space if some columns are shorter, while a separate table might be more efficient for very large text blobs.