How to Create Unique Constraints with Nullable Columns in PostgreSQL?

Managing Unique Constraints and Nullable Columns in PostgreSQL

PostgreSQL presents unique challenges when enforcing uniqueness constraints across columns that allow NULL values. Multiple rows might share identical non-NULL values while differing only in their NULL entries. This necessitates careful handling of NULLs within the constraint definition.

PostgreSQL 15 and Later: The NULLS NOT DISTINCT Solution

PostgreSQL 15 and subsequent versions simplify this process with the NULLS NOT DISTINCT clause. This clause, when added to a UNIQUE constraint, treats NULL values as equivalent. Therefore, only one row can exist with a given combination of non-NULL values, regardless of whether the nullable column contains NULL.

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

PostgreSQL 14 and Earlier: Partial Indexes as a Workaround

For older PostgreSQL versions, the recommended approach involves creating partial indexes. These indexes enforce uniqueness on subsets of the data, effectively handling NULLs separately. One index covers rows where the nullable column is NOT NULL, and another handles rows where it IS NULL.

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;

Important Considerations

Partial indexes, while effective, might limit functionality. They can restrict foreign key referencing and clustering options. If a complete index is essential, consider a UNIQUE constraint spanning a broader column set, including the nullable column.

Another option, although potentially unsuitable for all situations, is to assign a non-NULL default value to the nullable column. This simplifies constraint creation but could impact data integrity depending on the application's requirements.

Finally, consistent naming conventions are crucial. Using lowercase identifiers in PostgreSQL enhances readability and avoids potential issues.

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