how to pass tablename to a function

Hi, All

I like to pass the tablename to function but not using execute clause, here is my script

CREATE OR REPLACE FUNCTION functions.pgsql_event_unpack_batch(IN _tbl text, IN jobid bigint, IN jobtime timestamp with time zone, IN startid bigint, IN stopid bigint)
  RETURNS TABLE(events bigint, errors bigint[]) AS
$BODY$
BEGIN

RETURN QUERY
WITH
     unpacking (raw_id, time, userid, eventtype, pagename, userhost, application, status, error)
     AS (
        select
              id as raw_id,
              (up).time,
              (up).userid,
              coalesce((up).eventtype, ''),
              coalesce((up).pagename, ''),
              (up).userhostaddress as userhost,
              coalesce((up).application, ''),
              (up).status,
              (up).error
         from(
              select id,
                     functions.python_events_unpack(event_object) up
              from  _tbl
              where id between startid and stopid
         ) a
         where (up).userid is not NULL
      )


I want to pass the _tbl to the select query integrated in the unpacking(), how can I make it?

thanks

Alec