Using %rowtype when returning in a PostgreSQL function

Question

If I have a function that returns only one row with some columns from a table do I need to add %rowtype in the function return declaration?

CREATE OR REPLACE FUNCTION test(int n)
RETURNS tableName%rowtype AS
$BODY$
DECLARE 
    r tableName%rowtype;    
BEGIN   
        select a,b,c into r from tableName where d=n;
        return r;
$BODY$
END;

Solution

About %ROWTYPE

The %ROWTYPE construct is only good for portability. Rarely useful, since PL/pgSQL functions are hardly portable to begin with.
If you are going to use it, it's only meant for variable declaration inside PL/pgSQL function, not to declare the RETURN type, which is actually part of the outer SQL syntax.

Per documentation:

(Since every table has an associated composite type of the same name, it actually does not matter in PostgreSQL whether you write %ROWTYPE or not. But the form with %ROWTYPE is more portable.)

Answer

This would achieve what you seem to be trying:

CREATE OR REPLACE FUNCTION test_plpgsql(_n int)
  RETURNS tbl AS
$func$
BEGIN   
  RETURN (SELECT t FROM tbl t where tbl_id = _n); -- selecting the whole row
END
$func$ LANGUAGE plpgsql;

Call:

SELECT * FROM test_plpgsql(1);

But if it's as simple as that, use a simpler SQL function instead:

CREATE OR REPLACE FUNCTION test_sql(_n int)
  RETURNS SETOF tbl AS
$func$
   SELECT * FROM tbl WHERE tbl_id = _n;  -- Requires Postgres 9.3; or use $1
$func$ LANGUAGE sql;

Call:

SELECT * FROM test_sql(1);

Your original example was twisted and incorrect in too many places. Search for more plpgsql examples to get a grasp on basic syntax.