Let's not get carried away. It is very much debatable which database is "most mature".

Here is some discussions about how Postgres compares with Oracle DB (if $ is not an issue), it is often said that Postgres is a Cessna whereas Oracle DB is like a Fighter Jet: https://news.ycombinator.com/item?id=24582937

I have been working with postgres for multiple years and in one project I needed to work with oracle. I actually ran into a bug in oracle db and they did not listen to us because we where a small company. We where working for a very large customer and asked that company to complain on our behalf at oracle. They they finally fixed it. Project was a large GIS project. I never had a say in the choice of database otherwise I would have chosen postgres. Oracle regularly crashed on complex spacial queries. At some point I left te company just to stop trying to fix that product. I think the only way of fixing the problem was a rewrite of the product that would use postgres. I hope I never work with oracle products in my life again.

I've never had the opportunity to test Oracle DB at large, just some minor playing around with 12c many many years ago so I don't have an accurate view. That said, given my experience with other Oracle products and the fighter jet analogy... De omnibus dibutandum est...

Fully agree with this. I once heard it stated by someone "I don't like writing C, but I don't mind at all writing Postgres C". It is definitely a quality codebase and the credit goes to a pretty small set of maintainers over the years.