Оптимизация запроса

-- if object_id('Days') is not null drop table Days
IF object_id('Days') IS NULL BEGIN
   CREATE TABLE Days (DAY INT)
   INSERT Days SELECT NUMBER FROM master..spt_values WHERE TYPE='P' AND NUMBER BETWEEN 1 AND 31
   SELECT * FROM Days
END
GO
 
ALTER 
FUNCTION [dbo].[Q_days](
   @PEAK REAL, @TODAY datetime, @WORKDAYS_BEFORE_TODAY REAL, @demandSoFarThisMonth INT,
   @Day_1 INT, @Day_2 INT, @Day_3 INT, @Day_4 INT, @Day_5 INT, @Day_6 INT, @Day_7 INT, @Day_8 INT, @Day_9 INT, @Day_10 INT,
   @Day_11 INT, @Day_12 INT, @Day_13 INT, @Day_14 INT, @Day_15 INT, @Day_16 INT, @Day_17 INT, @Day_18 INT, @Day_19 INT, @Day_20 INT,
   @Day_21 INT, @Day_22 INT, @Day_23 INT, @Day_24 INT, @Day_25 INT, @Day_26 INT, @Day_27 INT, @Day_28 INT, @Day_29 INT, @Day_30 INT, @Day_31 INT)
RETURNS REAL /*char(6)*/ AS
BEGIN
   /*
   select @WORKDAYS_BEFORE_TODAY=isnull(nullif(count(*),0),0.001)
   from days d
   left join Holidays h on 
      h.ondate between dateadd(day,1-day(@TODAY),@TODAY) and @TODAY
      and day(ondate)=d.day
   where d.day<=day(@TODAY) and h.ondate is null
   */
   DECLARE @koeff REAL
   SELECT @koeff=@PEAK*@demandSoFarThisMonth/@WORKDAYS_BEFORE_TODAY
   
   DECLARE @rv REAL /*char(6)*/
   SELECT @rv=SUM(CASE
      WHEN d.day=01 AND @Day_1 < @Koeff THEN 1 
      WHEN d.day=02 AND @Day_2 < @Koeff THEN 1 
      WHEN d.day=03 AND @Day_3 < @Koeff THEN 1 
      WHEN d.day=04 AND @Day_4 < @Koeff THEN 1 
      WHEN d.day=05 AND @Day_5 < @Koeff THEN 1 
      WHEN d.day=06 AND @Day_6 < @Koeff THEN 1 
      WHEN d.day=07 AND @Day_7 < @Koeff THEN 1 
      WHEN d.day=08 AND @Day_8 < @Koeff THEN 1 
      WHEN d.day=09 AND @Day_9 < @Koeff THEN 1 
      WHEN d.day=10 AND @Day_10< @Koeff THEN 1 
      WHEN d.day=11 AND @Day_11< @Koeff THEN 1 
      WHEN d.day=12 AND @Day_12< @Koeff THEN 1 
      WHEN d.day=13 AND @Day_13< @Koeff THEN 1 
      WHEN d.day=14 AND @Day_14< @Koeff THEN 1 
      WHEN d.day=15 AND @Day_15< @Koeff THEN 1 
      WHEN d.day=16 AND @Day_16< @Koeff THEN 1 
      WHEN d.day=17 AND @Day_17< @Koeff THEN 1 
      WHEN d.day=18 AND @Day_18< @Koeff THEN 1 
      WHEN d.day=19 AND @Day_19< @Koeff THEN 1 
      WHEN d.day=20 AND @Day_20< @Koeff THEN 1 
      WHEN d.day=21 AND @Day_21< @Koeff THEN 1 
      WHEN d.day=22 AND @Day_22< @Koeff THEN 1 
      WHEN d.day=23 AND @Day_23< @Koeff THEN 1 
      WHEN d.day=24 AND @Day_24< @Koeff THEN 1 
      WHEN d.day=25 AND @Day_25< @Koeff THEN 1 
      WHEN d.day=26 AND @Day_26< @Koeff THEN 1 
      WHEN d.day=27 AND @Day_27< @Koeff THEN 1 
      WHEN d.day=28 AND @Day_28< @Koeff THEN 1 
      WHEN d.day=29 AND @Day_29< @Koeff THEN 1 
      WHEN d.day=30 AND @Day_30< @Koeff THEN 1 
      WHEN d.day=31 AND @Day_31< @Koeff THEN 1 
      ELSE 0 END)
   FROM days d
   LEFT JOIN Holidays h ON 
      h.ondate BETWEEN dateadd(DAY,1-DAY(@TODAY),@TODAY) AND @TODAY
      AND DAY(ondate)=d.day
   WHERE d.day<=DAY(@TODAY) AND h.ondate IS NULL
   
   RETURN @rv
END
GO