Is there any nice way to handle sum types (aka tagged unions) in a PostgreSQL database? [...]
A third method would be to save all fields of all variants into a single table, with all fields being nullable.
So you'd have a nullable text field, nullable integer and nullable double precision field.
Yes, we do that. That's IMHO the only sane way to do it.
And if some of those alternatives are FKs (relations), that's the only choice.
You'd then need an additional tag field to indicate which variant of the union is used
No, you don't need it. That's implicit from the NULL'ability of the alternative columns.
If you want, you can have it as a generated column, thus read-only.
Worse, having it as an explicit column would make it denormalized, and possibly out of sync.
and you'd have to write check constraints for each variant to ensure that all the fields in that variant are not null and all the fields not in that variant are null.
Yes indeed.
This almost works, but has two major problems:
1. It wastes space. In Rust, an enum is only as big as its largest variant. Using this method, a table row would be as big as the sum of all the variants.
Not really, or not to a point it matters that much.
I don't know about the actual physical bytes on disk for PostgreSQL, but as an example in SQLite,
all columns have *at least* 1 "header" byte per value, and NULL values (and 0 and 1) have no "payload" bytes.
In PostgreSQL (which is more "rigidly typed" as DRH would say :)) you may waste space for primitive types,
but not for text and bytea, which is where it really matters IMHO.
2. Querying the data is very cumbersome, [...].
Sure, it's cumbersome. But I don't get your point here. NULL handling is part of SQL.
And sum-types (variants) implemented via exclusive NULL'ness is just a special case.
You "dispatch" to the proper column on writes. You read all alternatives and assign the one (if any) NOT NULL to the variant.
Both of these problems get bigger and bigger as you add more variants - it doesn't scale well.
ORMs cannot magically resolve the impedence mismatch between SQL and OO-based or sum-type based type systems a la Rust (and co).
If you need SQL, you need to design for SQL for the get go. Not shoehorn your Rust data model into SQL.
My $0.02.