[RichieAHB]

This works fine as long as you’re happy to do the same task multiple times. I.e. the task is idempotent and cheap.

[kdps]

I don't get it :(. Why could the same task be executed more than once? From my understanding, if the UPDATE is atomic, only one worker will be able to set `used = 1`. If the update statement is not successful (affected != 1), then the worker should drop the task and do another select.

[klauserc]

With a transaction isolation level below SERIALIZABLE you can have two transactions that both read the old row (with `used = 0`) at the time they perform the update (but before they commit the transaction). In that case, both transactions will have performed an update (rows affected = 1).

Why would both transactions see `used = 0`? The DB server tries to isolate transactions and actively hides effects of other transactions that have not committed yet.