rewrite in to exists? - Mailing list pgsql-performance

I'm on 7.3.4 and this query gets horrible performance.  Is there a way to rewrite it with an exists or some way to get
betterperformance? 

select code, id, name, date_of_service
  from tbl
where date_of_service in
      (select date_of_service
         from tbl
        where xxx >= '29800'
          and xxx <= '29909'
          and code = 'XX')
  and client_code = 'XX'
order by  id, date_of_service;

Thanks!