MySQL backed, in turn, by a log structured storage engine (RocksDB). But the they reason they do that is storage space efficiency rather than availability or performance.

I’ve been working in Ruby that whole time, and Postgres is like the right arm of the prototypical Ruby app. We did have a fad where Mongo was used at my company in the early ‘10s, and indeed we had little choice but to do rewrite that part. We actually had an amazing intern do that one year.