我有疑问：
WITH O AS ( SELECT ROW_NUMBER() OVER(ORDER BY Advertisers.advertiser_title asc) as 'num', ROW_NUMBER() OVER(ORDER BY spot_date asc) as 'num2',product_title,advertiser_title,brand_title,sector_title,channel_title,copy_title,prog_title,clipType_title,Spots_tv.spot_id,Spots_tv.spot_date,Spots_tv.time_start,Spots_tv.time_end,Spots_tv.clipppos_id,Spots_tv.prog_id,Spots_tv.channel_id,Spots_tv.copy_id,Spots_tv.duration,official_cost FROM Spots_tv
RIGHT JOIN Copies_tv ON Copies_tv.copy_id = Spots_tv.copy_id
RIGHT JOIN ClipPositions ON ClipPositions.clippos_id = Spots_tv.clipppos_id
RIGHT JOIN Programmes ON Programmes.prog_id = Spots_tv.prog_id
RIGHT JOIN Channels ON Channels.channel_id = Spots_tv.channel_id
RIGHT JOIN Products ON Products.product_id = Copies_tv.product_id
RIGHT JOIN ClipTypes ON ClipTypes.cliptype_id = Copies_tv.cliptype_id
RIGHT JOIN Advertisers ON Advertisers.advertiser_id = Products.advertiser_id
RIGHT JOIN Brands ON Brands.brand_id = Products.brand_id
RIGHT JOIN Classes ON Classes.class_id = Products.class_id
RIGHT JOIN Producers ON Producers.producer_id = Brands.producer_id
RIGHT JOIN Categories ON Categories.category_id = Classes.category_id
RIGHT JOIN Sectors ON Sectors.sector_id = Categories.sector_id
WHERE Advertisers.advertiser_id IN (1640311,1640315,1640330) AND Spots_tv.spot_date BETWEEN '2014-3-20' AND '2015-3-20' )
SELECT O.spot_id,O.channel_id,O.copy_id,O.duration,O.official_cost,product_title,O.spot_date,advertiser_title,brand_title,sector_title,channel_title,copy_title,prog_title,clipType_title,O.time_start,O.time_end FROM O
WHERE O.num2 BETWEEN 1 AND 15 ORDER BY O.num2 ASC
子查询，它工作得非常快，子查询中有11000多行，很快应该有超过百万行，完整查询需要不到一秒，但如果我改变
WHERE O.num2 BETWEEN 1 AND 15 ORDER BY O.num2 ASC
与
WHERE O.num BETWEEN 1 AND 15 ORDER BY O.num ASC
它需要超过14秒，为什么会有这么大的差异？我知道按字符串排序速度较慢但是，我该怎么做才能加快这个查询的速度呢？
抱歉英语不好