This seems really cool and I'd be happy to help (I'm currently a pgtyped + Kysely user and community contributor), and I see how this solves n+1 from promise pipelining when fetched "nested" data with a similar approach as Cap'n Web, but I don't we've solved the map problem.

If I run, in client side Cap'n Web land (from the post): ``` let friendsWithPhotos = friendsPromise.map(friend => { return {friend, photo: api.getUserPhoto(friend.id))}; } ```

And I implement my server class naively, the server side implementation will still call `getUserPhoto` on a materialized friend returned from the database (with a query actually being run) instead of an intermediate query builder.

@kentonv, I'm tempted to say that in order for a query builder like typegres to do a good job optimizing these RPC calls, the RpcTarget might need to expose the pass by reference control flow so the query builder can decide to never actually run "select id from friends" without the join to the user_photos table, or whatever.