Comment by endymi0n

Also, while queuing systems are preferrable to a database for some of the properties mentioned by GP, they break down for some use cases. For example, for one operation, we needed a time based priority queue that deduplicates on some attributes and refreshes the timestamps. That's where Postgres really shines - and because it's not that many entries and throughput anyway, it was one system less to operate and works perfectly fine to this day.

Just keep in mind Postgres isn't made for high contention, deletion and mutation rates on really small tables (Uber "Schemaless" case)