Comment by peterhunt

The scalability problem is way overblown. Setting the correct isolation level and locking mode isn't that hard and a modern cloud-hosted PG/MySQL can push 10s of thousands of inserts/s no problem.

IMO, the downsides of hosting a queue inside your primary relational DB are very much outweighed by the downsides of 1) having to run a new piece of infra like rabbit and 2) having to coordinate consistency between your message queue and your relational DB(s)