There are a ton of job/queue systems out there that are based on SQL DBs. GoodJob and SupaBase Queues are two examples.

It’s not usable for high scale processing but most applications just need a simple queue with low depth and low complexity. If you’re already managing PSQL and don’t want to add more management to your stack (and managed services aren’t an option), this pattern works just fine. Go back 10-15yrs and it was more common, especially in Ruby shops, as teams willing to adopt Kafka/Cassandra/etc were more rare.

And there are a ton that aren’t.