With Postgres you also need to worry about high churn, especially since you are creating/locking/deleting rows constantly. This can be alleviated through a variety of means, of which personally I would use per-day table partitioning and truncate older partitions on a cron, not to mention the sharp increase in database connections to the host now required.
Ignoring the literal elephant in the room of synced writes to the store. Redis can be used quite effectively in a blocking manner with RPOPLPUSH/LMOVE(6.2+) for a reliable queue, allowing an item to not be lost because atomically the pop and push from two different lists are done together.
[1] https://github.com/que-rb/que [2] https://gist.github.com/chanks/7585810