Question

I have an InnoDB table containing users, like this:

+--------------+-----------------------+
| user_id      | name                  |
+--------------+-----------------------+
| 1            | Bob                   |
| 1            | Marry                 |
| 2            | Bob                   |
| 1            | John                  |
| 3            | Bob                   |
| 2            | Marry                 |
+--------------+-----------------------+

On each insert I increment user_id by 1 so for example, the next user_id for Bob will be 4

I use this query to do that:

INSERT INTO users (user_id, name)
SELECT 1 + coalesce((SELECT max(user_id) FROM users WHERE name='Bob'), 0), 'Bob';

Now I need to make sure that two different users don’t add a ‘Bob’ at the same time. I don’t want any two Bobs with user_id 4. Each of Bob’s user ids must be different.

Is is possible to write and update lock all of Bob’s rows when the above insert query is run? I cannot lock the whole table because many other users still need full access to their rows. Also, all rows should be readable at all times. How would I do this?

Answer 1

Assuming InnoDB here, you can use SELECT … LOCK IN SHARE MODE to lock the rows when you select them.