can i use a string in where clause such as "id in (str)"

hi every body.

i have  a pl/pgsql function (i want to form it to run correctly) :

 

CREATE OR REPLACE FUNCTION "public"."ftest" () RETURNS boolean AS
$body$
declare
       f boolean;
       str varchar(100);
begin
     str := '1,5,10';
     if exists(    select 1 from tb_categories where cat_id in ( str )    ) then
        return true;
     else
         return false;
     end if;
end;
$body$
LANGUAGE 'plpgsql' VOLATILE RETURNS NULL ON NULL INPUT SECURITY INVOKER;

 

there is a "10" value for cat_id(int4) column which should make function return true.

 

but, str is seen as a string value as whole, so it doesnt return true.

 

i achived this by using execute command on string.

but i dont want to use execute, because there is no prepared plan for it(i am not sure, but it may slow down procedure).

 

how can i achive this, are there any solutions?

 

thanks for any help.