We were building a payments system in the early 2000s and got a diktat to not use Oracle. The amount of things we had to build to satisfy the availability and durability requirements were so huge it consumed the first few years of work. We didn’t get to the business side of things until much later. Funny thing is we ended up giving up on MySQL and went back to oracle after all that work. The whole thing was scraped after a couple of years.

To get to the level of scale that oracle can handle we had to build sharding and cluster replication from scratch. It still didn’t get to even 1/10th of a single oracle node. Obviously we made a lot of poor architecture decisions as well - in hindsight, of course.