create or replace function wcost_average_sf (numeric[],  numeric, numeric)
returns numeric[] as 
$Body$
	declare 
		astate numeric[];
	begin 
		astate := $1;
		astate[1] := astate[1] +  $2;
		astate[2] := astate[2] + $2 * $3;
		return astate;
	end;
$Body$
LANGUAGE 'plpgsql' VOLATILE;

create or replace function wcost_average_ff (numeric[] )
returns numeric as 
$Body$
	begin 
		return $1[2]/ $1[1];
	end;
$Body$
LANGUAGE 'plpgsql' VOLATILE;

create aggregate wcost_average (numeric, numeric)(
sfunc = wcost_average_sf, 
stype = numeric[],
initcond = '{0,0}',
finalfunc = wcost_average_ff
);