COALESCE("A", 0 ) + 
   COALESCE("B", 0 ) + 
   COALESCE("C", 0 ) 

CREATE OR REPLACE VIEW "SumAll" AS
SELECT *,  COALESCE("A", 0 ) + 
   COALESCE("B", 0 ) + 
   COALESCE("C", 0 ) from "Table"

CREATE OR REPLACE FUNCTION "SumP" ("ID" integer) RETURNS double precision AS
$body$
declare 
 SumAll real;
begin
 select (
COALESCE("A", 0 ) + 
   COALESCE("B", 0 ) + 
   COALESCE("C", 0 ) 
 ) into SumAll from "Table" where "PID"="ID";
 return SumAll;
end;
$body$
LANGUAGE 'plpgsql' IMMUTABLE RETURNS NULL ON NULL INPUT SECURITY INVOKER;

select *, "SumPredmetNagruzka"("PID")  from "Table"