> I use a database that was designed by someone who understands one sometimes wants to run a migration. It’s not hard to add new fields.

We're not talking about adding attributes to a database. We're talking about one of the most basic cases in API design: changing a interface and serving two or more versions.

If the mapping step between getting data from the db and outputting it in a response is expected to be eliminated by getting the DB to dump JSON, how is this issue solved?

And how exactly is a full blown db migration preferable to mapping and serializing a DTO?

The way you are supposed to use Postgrest or Postgraphile is to provide only views and stored procedures as the API. These views and stored procedures are layered over your data schema and should be the only way to query or change the data. This way you can make API changes without requiring migrations.

The point is that instead of having queries that return tables that you then map to DTOs for serialization, you have queries that return JSON directly.

Another way to accomplish the same thing, if you don't want views or stored procedures, is to use a library like jOOQ that does much the same thing in your Java middleware.