Using MySQL unique index with good atomicity without raising errors

Question

I have this table:

User
    id INT PK
    login VARCHAR UNIQUE

I want to know the best way to handle UNIQUE index insertion (ex: new user creation)

1) I can simply INSERT and when a "duplicate entry is raised" and I catch the exception and handle it in the application

=> I'm not sure this is the best solution to do that

2) I can SELECT ... WHERE login = ... then INSERT when no record found or display an error if the select found something

=> This is not atomic, an insert can happen between SELECT and INSERT

3) I can start a transaction, SELECT FOR UPDATE, INSERT then COMMIT when no records found or ROLLBACK and display an error if the user already exists

=> This simply won't work since MySQL doesn't lock no existing lines... So it will certainly result in a deadlock

So what is the best way to handle this very simple study case?

Answer 1

You can do an

INSERT IGNORE ...

MySQL manual INSERT

Or

REPLACE ...

MySQL manual REPLACE

Answer 2

The way I see it, you have two options:

1) Use Lock Tables and SELECT then Insert. Locking the table will ensure you don't end up with a race condition.

2) If you're concerned about the table lock, you can do the first option that you listed: INSERT or better yet, INSERT IGNORE. If you use INSERT IGNORE then the duplicate key exception will instead be returned as a warning.

Personally, I'd try the table lock and see if it works. Your SELECT statement should run quickly since it has a unique key, so I wouldn't worry about locks slowing you down too much.