I accidentally built an in-memory database that now lives prominently in our production stack. It works great, its incredibly performant, the codebase is relatively simple (it makes heavy use of code-generation), it will scale very well - but not a day goes by I don't think what if I had just taken the time to adapt an existing solution to the problem set.

There are just so many free things you get with SQL and established RDBMS that deeply impact application features, quality, stability, operations, and much, much, more. I've had to write a custom mongo-db like interface for querying, as well as a fair number of hacky bits to effectively cover the surface area of SQL in an inferior way.

I've learned tremendously, but I just wish people don't follow in my exact footsteps because that's probably wasted time.