create temp sequence counter_seq;
select nextval('counter_seq') as "count", * from ...
drop sequence counter_seq;

CREATE OR REPLACE FUNCTION reset_counter() RETURNS void AS
  $$ $rowcounter =  0 ; return; $$ LANGUAGE plperl VOLATILE;
CREATE OR REPLACE FUNCTION get_counter() RETURNS int AS
  $$ return ++$rowcounter; $$ LANGUAGE plperl VOLATILE;

select reset_counter();
select get_counter() as "count", * from ...