It depends on your tolerance for the exact feature set that you get. The JSON type can be pressed into service to get something that represents that, in the sen

It depends on your tolerance for the exact feature set that you get. The JSON type can be pressed into service to get something that represents that, in the sense that what you are asking for is a subset of the valid JSON values that Postgres can store. If you just want to slam the values into some local tagged union in your language when querying them, then problem effectively solved. If you want to start operating on it in the database itself, you have a lot more quirky things to work through. If you want the semantics inside Postgres to be exactly what a tagged union/sum type looks like then I think you'd be looking at a custom data type at a minimum, and even then there are probably limits as to how close you could get inside.