How to Enforce Unique Constraints in PostgreSQL with NULL Columns?

PostgreSQL Unique Constraints and NULL Columns: A Practical Guide

Handling unique constraints in PostgreSQL when dealing with nullable columns requires careful consideration. Imagine a table with UserId, MenuId, and RecipeId columns, where UserId and RecipeId are non-null. A standard unique constraint on these columns would permit duplicate entries if MenuId differs, including NULL values.

Here are effective strategies to address this:

PostgreSQL 15 and Later Versions

PostgreSQL 15 introduced the NULLS NOT DISTINCT clause, providing a straightforward solution. This clause treats NULL values as equal when enforcing uniqueness within constraints and indexes:

ALTER TABLE favorites
ADD CONSTRAINT favo_uni UNIQUE NULLS NOT DISTINCT (user_id, menu_id, recipe_id);

PostgreSQL 14 and Earlier Versions

For older PostgreSQL versions, the recommended approach involves creating partial indexes:

CREATE UNIQUE INDEX favo_3col_uni_idx ON favorites (user_id, menu_id, recipe_id)
WHERE menu_id IS NOT NULL;

CREATE UNIQUE INDEX favo_2col_uni_idx ON favorites (user_id, recipe_id)
WHERE menu_id IS NULL;

This effectively enforces uniqueness by creating separate indexes for rows where menu_id is NULL and where it's NOT NULL.

Important Considerations for Partial Indexes

Using partial indexes introduces some limitations:

• Foreign key constraints cannot be directly applied to the three-column combination (user_id, menu_id, recipe_id).
• Clustering based on partial indexes is not supported.
• Queries lacking matching WHERE clauses will not utilize the partial indexes.

Best Practices:

It's advisable to use lowercase identifiers (e.g., favorites) in PostgreSQL for consistency and to avoid potential issues. Choosing the appropriate method depends on your PostgreSQL version and specific needs. The NULLS NOT DISTINCT clause offers a cleaner solution in newer versions, while partial indexes provide a functional alternative for older versions.

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