Static-language clients need to create a class (or collection of classes) for each GraphQL query, or forgo type safety (typically just null-safety, but I suppose you could treat each result as a dictionary type). Creating just the schema-level types, with relationships modeled separately, does throw out some of the benefits ("just the data you need") but simplifies the client, moreso if the client may perform multiple complex queries on the same data.

You can do that in Postgres, either return nested json structures, or nested arrays.

For an example see: https://shusson.info/post/building-nested-json-objects-with-...