How to avoid auto-increment increase on unsuccessful request?

Question

PostgreSQL

The users table has two unique fields login and email.
If you try to add a record where the login or email is not unique, the request will fail, the record will not be added, but auto-increment will add one. And the next successful id will be one more. How to avoid it?

Answer 1

Check your username and email before proceeding. No => insert. Yes => error

Answer 2

No way.
The sequence is guaranteed to be unique and non-transactional. He does not say anything about the sequence of issuing values, and even more so without gaps.