Postgres unique constraint not enforcing uniqueness

Question

Here is my constraint:

CREATE UNIQUE INDEX index_subscriptions_on_user_id_and_class_type_id_and_deleted_at
  ON subscriptions
  USING btree
  (user_id, class         


        

Answer 1

Unique indexes in Postgres are based on values being equal, but NULL is never equal to anything, including other NULLs. Therefore any row with a NULL deleted_at value is distinct from any other possible row - so you can insert any number of them.

One way around this is to create partial indexes, applying different rules to rows with and without NULLs:

 CREATE UNIQUE INDEX ... ON subscriptions
 (user_id, class_type_id) WHERE deleted_at IS NULL;

 CREATE UNIQUE INDEX ... ON subscriptions
 (user_id, class_type_id, deleted_at) WHERE deleted_at IS NOT NULL;