Explicite typecasting of functions

Hello,

I want to insert new data into a table with increasing data set ids.
The table has two separate "regions" of data: Those with Ids below
1000000 and other.  If I want to create a new Id in the "lower region"
I tried the following (simplified example):


CREATE TABLE Items ( Id    int DEFAULT NextItem()
) ;


/*  ERROR:  Function 'nextitem()' does not existUnable to identify a function that satisfies the given argument
typesYoumay need to add explicit typecasts*/
 

CREATE FUNCTION NextItem() RETURNS INT4   AS 'select max(Id)+1 from Items where Id < 1000000;'   LANGUAGE 'sql';


I did not found any trace of documentation how to do an explicit typecast
for the function.  Defining the function first fails because:
  ERROR:  Relation "items" does not exist

Any hint to solve this kind of chicken-egg-problem?

Kind regards
      Andreas.