Thank you Brett ! I didn't know that a unique constrant automatically creates an (invisible) unique index in PostgreSQL! This would work perfectly for my use case (though I still wonder what about other databases...)

Although it's probably better if Hibernate gives a INFO/WARN log when the app tries to create an @Index in a @Column(unique=true) on a PostgreSQL... since it would be redundant and useless.

Feel free to take the issue to the direction you want to go. My specific problem is solved.