Use un disparador de Postgres para registrar el JSON solo de los campos modificados

Su función de activación no puede funcionar bien, produce el error al insertar una fila:

Debes tratar los tres casos de INSERT , UPDATE y DELETE por separado:

create or replace function changelog_procedure() 
returns trigger as $$
declare
    json_new jsonb;
    json_old jsonb;
begin
    if tg_op = 'INSERT' then
        json_new:= to_jsonb(new);
    elsif tg_op = 'DELETE' then
        json_old:= to_jsonb(old);
    else
        select jsonb_object_agg(new_key, new_value), jsonb_object_agg(old_key, old_value)
        into json_new, json_old
        from jsonb_each(to_jsonb(new)) as n(new_key, new_value)
        join jsonb_each(to_jsonb(old)) as o(old_key, old_value) 
        on new_key = old_key and new_value <> old_value;
    end if;
    insert into tbl_changelog(tbl, op, new, old)
    values (tg_table_name, tg_op, json_new, json_old);
    return null;
end;
$$ language plpgsql;