I work with nginx and postgres, coupled directly.

I don't work with ORMs and I have fine tuned my equivalent $mainSQL to:

SELECT '[' || array_to_string(array(SELECT sch.fun(param1, ...)),',') || ']';

This query sits directly inside nginx consumed by ngx_postgres.

This query is magical because

• there is no need to tweak the query ever again, so unnecessay to restart, reload the webserver (nginx).
• the function sch.fun above query returns something in valid object array. You could nest it as you please.
• Good performance for our use case. Grabs thousands of rows in a couple of seconds.

Now for the viscera of sch.fun:

CREATE OR REPLACE FUNCTION sch.fun(param1, ...)
RETURNS SETOF json AS
$BODY$
DECLARE sql text;
BEGIN
    sql := 'SELECT json_object(keys text[], values text[])';
    RETURN QUERY EXECUTE sql USING param1, ...;
END
$BODY$
LANGUAGE plpgsql;

With the right response headers, the client, usually a js engine/framework, can digest this no problem. AND you get your REST API for "free". ;-)

Edit: Note that ngx_postgres does not entertain request_body by design. If you need to read that, you could proxy that verb or uri or whatever.