feat: Support Postgres json_agg/json_build_object aggregate functions

[wilkinson4]

Introduce json_build_object/1 with literal-map input and json_agg/1,2 with aggregate-local ordering.

These functions make database-side JSON construction composable in Ecto queries without handwritten SQL fragments. Aggregating related rows into JSON can reduce repeated parent data transferred from the database and avoid application-side grouping and object assembly.

Sometimes it makes sense not to use preloads for performance reasons. It is nice to not have to use fragments when we don't have to. In these situations, the combined usage of json_agg + json_build_object Postgres aggregate functions has been a nice solution for the grouping problem when trying to build an array of objects for each row in the results.

Example:

from(a in Appointment
  as: :appointment,
  select: %{
    id: a.id,
    participants:
      fragment(
        """
          select json_agg(json_build_object('primary' , sub.primary , 'name' , sub.full_name))
          from (
            select p.primary
              , u.full_name
            from appointment_participants p
              join users u on u.id = p.user_id
            where p.appointment_id = ?
            order by p.primary desc
        ) sub
        """,
        a.id
      )
  }
)
|> Repo.all()

[josevalim]

Thanks for the PR but we will likely not go down this path given it will lead to jsonb_agg, json_object_agg and so on. :)