This note is in Hasura.RQL.DDL.CustomTypes. It is referenced at:
It’s very convenient to be able to reference Postgres scalars in custom type definitions. For example, we might have a type like this:
type User {
id: uuid!
name: String!
location: geography
}
The uuid and geography types are Postgres scalars, not separately-defined GraphQL types. To support this, we have to take a few extra steps:
The set of Postgres base types is not fixed; extensions like PostGIS add new ones, and users can even define their own. Therefore, we fetch the currently defined base types from the @pg_catalog.pg_type@ system table as part of loading the metadata.
It’s possible for a custom type definition to use a type that doesn’t appear elsewhere in the GraphQL schema, so we record which base types were referenced while validating the custom type definitions and make sure to include them in the generated schema explicitly.
We currently have no plan to extend that functionality to other backends; if we do, we will probably choose to prefix such types with an explicit tag to avoid having to disambiguate type names across backends.