Create unique constraint with null columns

I have a table with this layout:

CREATE TABLE Favorites (
  FavoriteId uuid NOT NULL PRIMARY KEY,
  UserId uuid NOT NULL,
  RecipeId uuid NOT NULL,
  MenuId uuid
);

I want to create a unique constraint similar to this:

ALTER TABLE Favorites
ADD CONSTRAINT Favorites_UniqueFavorite UNIQUE(UserId, MenuId, RecipeId);

However, this will allow multiple rows with the same (UserId, RecipeId), if MenuId IS NULL. I want to allow NULL in MenuId to store a favorite that has no associated menu, but I only want at most one of these rows per user/recipe pair.

The ideas I have so far are:

1. Use some hard-coded UUID (such as all zeros) instead of null.
   However, MenuId has a FK constraint on each user's menus, so I'd then have to create a special "null" menu for every user which is a hassle.

2. Check for existence of a null entry using a trigger instead.
   I think this is a hassle and I like avoiding triggers wherever possible. Plus, I don't trust them to guarantee my data is never in a bad state.

3. Just forget about it and check for the previous existence of a null entry in the middle-ware or in a insert function, and don't have this constraint.

I'm using Postgres 9.0. Is there any method I'm overlooking?

Postgres 15 or newer

Postgres 15 adds the clause NULLS NOT DISTINCT. The release notes:

• Allow unique constraints and indexes to treat NULL values as not distinct (Peter Eisentraut)

  Previously NULL values were always indexed as distinct values, but this can now be changed by creating constraints and indexes using UNIQUE NULLS NOT DISTINCT.

With this clause null is treated like just another value, and a UNIQUE constraint does not allow more than one row with the same null value. The task is simple now:

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

There are examples in the manual chapter "Unique Constraints".
The clause switches behavior for all keys of the same index. You can't treat null as equal for one key, but not for another.
NULLS DISTINCT remains the default (in line with standard SQL) and does not have to be spelled out.

The same clause works for a UNIQUE index, too:

CREATE UNIQUE INDEX favo_uni_idx
ON favorites (user_id, menu_id, recipe_id) NULLS NOT DISTINCT;

Note the position of the new clause after the key fields.

Postgres 14 or older

Create two 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 way, there can only be one combination of (user_id, recipe_id) where menu_id IS NULL, effectively implementing the desired constraint.

Possible drawbacks:

• You cannot have a foreign key referencing (user_id, menu_id, recipe_id). (It seems unlikely you'd want a FK reference three columns wide - use the PK column instead!)
• You cannot base CLUSTER on a partial index.
• Queries without a matching WHERE condition cannot use the partial index.

If you need a complete index, you can alternatively drop the WHERE condition from favo_3col_uni_idx and your requirements are still enforced.
The index, now comprising the whole table, overlaps with the other one and gets bigger. Depending on typical queries and the percentage of null values, this may or may not be useful. In extreme situations it may even help to maintain all three indexes (the two partial ones and a total on top).

This is a good solution for a single nullable column, maybe for two. But it gets out of hands quickly for more as you need a separate partial index for every combination of nullable columns, so the number grows binomially. For multiple nullable columns, see instead:

• Why doesn't my UNIQUE constraint trigger?

Aside: I advise not to use mixed case identifiers in PostgreSQL.