问从开始日期开始，如何根据星期几计算下一次发生日期？

EN

startdate    enddate     nextdate
12/29/14     12/31/15    1/12/15
1/2/15       4/1/15      1/9/15
6/1/14       12/31/14    null
4/1/15       12/31/15    4/1/15

CREATE OR REPLACE FUNCTION nextdowinrange(startdate timestamp with time zone, enddate timestamp with time zone)
  RETURNS timestamp with time zone AS
$BODY$
    DECLARE
        loopday int;
        nextdate timestamp with time zone;
        BEGIN

        --RANGE PAST
        IF enddate < current_timestamp THEN
            RETURN null;
        --RANGE NOT STARTED
        ELSIF startdate > current_timestamp THEN
            RETURN startdate;
        --SAME DAY OF WEEK
        ELSIF extract(dow from startdate) = extract(dow from current_timestamp) THEN
            RETURN startdate;
        --FIND NEXT
        ELSE
            loopday:= 0;
            LOOP
                IF extract(dow from now() + (loopday || ' days')::interval) = extract(dow from startdate) THEN
                    nextdate:= now() + (loopday || ' days')::interval;
                END IF;
                EXIT WHEN extract(dow from now() + (loopday || ' days')::interval) = extract(dow from startdate);
                loopday:= loopday + 1;

            END LOOP;
            RETURN nextdate;
        END IF;

        END;
$BODY$
  LANGUAGE plpgsql VOLATILE
  COST 100;