Hacker News Reader

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.

View on HN

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.

This is not true in postgres. When the second transaction tries to update the row, it will wait for the first transaction to commit first and then recheck the WHERE.

https://www.postgresql.org/docs/current/transaction-iso.html...

Reply on news.ycombinator.com