I have modified my function, but I have problems with declaring variables. I use Postgres 8.4.

    CREATE OR REPLACE FUNCTION requestcounterid(_mindate timestamptz, _maxdate timestamptz) 
      RETURNS TABLE (kategorien text, requestcounter int) AS
    $func$  
    DECLARE
    _minid bigint;
    _maxid bigint;
    
    BEGIN 
    
    SELECT id  INTO _minid from tablename
    where starttime >= $1 ORDER BY starttime ASC LIMIT 1; 
    SELECT id  INTO _maxid from tablename
    where starttime < $2 ORDER BY starttime DESC LIMIT 1; 
    
    SELECT CASE WHEN duration <= 10000000 THEN '00-01 sec'::text
                WHEN duration <= 40000000 THEN '01-04 sec'
                WHEN duration <= 100000000 THEN '04-10 sec' 
                WHEN duration <= 300000000 THEN '10-30 sec' 
                WHEN duration <= 600000000 THEN '30-60 sec' 
                ELSE 'more than 60 sec' END  
         , count(*)::int                     
    FROM   tablename
    WHERE  id >= _minid and id <= _maxid
    GROUP  BY 1                              
    ORDER  BY 1; 
    
    END; 
    $func$ LANGUAGE plpgsql;

Error: 

>     ERROR:  query has no destination for result data
>     HINT:  If you want to discard the results of a SELECT, use PERFORM instead.
>     CONTEXT:  PL/pgSQL function "requestcounterid" line 12 at SQL statement

How to fix this?