Recursive query using plpgsql

Question

I'm trying to write a plpgsql function that recursively returns a set of columns from records in a tree structure.

I have a data table and a table to link the data together:

DATATABLE
-----------
id integer
value text
info text

LINKTABLE
-----------
link integer
parent integer

My thought was to do like in the following function:

CREATE OR REPLACE FUNCTION my_function(itemID integer)
  RETURNS TABLE(id integer, value text) AS
$BODY$
BEGIN    
    RETURN QUERY SELECT my_function(A.link) FROM linktable A, datatable B 
        WHERE A.parent = B.id AND B.id = itemID) C;

    RETURN QUERY SELECT id, value FROM datatable WHERE id = itemID;            
    RETURN;
END;
$BODY$
  LANGUAGE plpgsql VOLATILE
  COST 100;

But that doesn't work, I get an error in my first query:

ERROR: structure of query does not match function result type

My Just-In-Brain compiler detects no problems, so what am I doing wrong here?