TBH it's a bit more verbose and less performant than need be, but hey, if it works, rock on! But don't use 'select *' in production code.

Consider next time something like

    if exists (select from sometable t1
      join sometable t2 on 
        t1.resource_id = t2.resource_id 
        and t1.res_id <> t2.res_id
        and tstzrange(t1.start, t1.end) && tstzrange(t2.start, t2.end)
      where t1.res_id = new.res_id )

then ... raise exception