Hi,
Is there any nice way to handle sum types (aka tagged unions) in a PostgreSQL database? I've searched far and wide and have not reached any satisfying answer.
As a (somewhat contrived) example, say I have the following enum in Rust:
enum TaggedUnion {
Variant1(String),
Variant2(i32),
Variant3(f64),
}
How might I best save this data in a PostgreSQL database? There is to my knowledge unfortunately no way to "natively" handle sum types like this.
One method would be to have 3 different tables, one for each variant. This is not a great solution as you can't (as far as I know) easily query for all variants at once (for instance, to serialize into a Vec<TaggedUnion> on the Rust side).
Another method would be to use PostgreSQL table inheritance. This has the same problem as the above workaround but also has the issue that you could put something into the supertable without having a corresponding entry in any of the subtables (basically a value not inhabiting any of the variants, which is nonsense).
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. You'd then need an additional tag field to indicate which variant of the union is used 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. 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.
2. Querying the data is very cumbersome, as there is no way to indicate (beyond check constraints) that, given a specific tag, certain other fields must be filled while certain other fields must not be. For instance, the nullability of fields can be used to serialize into the Option type in Rust. There is no "nice" way to tell the host language that the nullability of the variant fields is hinged on the value of the extra tag field.
Both of these problems get bigger and bigger as you add more variants - it doesn't scale well.
Does anyone know of better methods? I realize the example enum is contrived but this kind of thing comes up a lot in all kinds of places. For instance different kinds of messages that all have various different associated data. Or different events that record different additional data. Sum types are ubiquitous in languages like Rust and Haskell.
If there are no good methods of handling this, is there any way PostgreSQL could be extended with capabilities for this? I have no idea how this would be done in practice though. Perhaps SQL itself is just unsuited for data of this kind? I don't really see why it should be though.
Thanks,
Victor Nordam Suadicani
