Clarified Question

>
> How can I create a function that will take in two variables and return
> an integer, when one of the variables is the tablename ?!
>
> I have tried :
>
> create function tst_func(text, varchar(16))
> as
> 'BEGIN
> result=select max(history_id)+1 from $1
> where client_id = $2;
> return result;
> END;'
>

This kind of thing does work in pltcl but AFAIK you can't specify a table name
with a parameter in plpgsql. This may give you the idea:

create function myfunc(text,text) returns result as '
set res [spi_exec "select f1 from $1 where f2 = \'$2\']
return $res
' language 'pltcl';