使用函数改进SQL查询

Question

我试图通过使用SQL Server Express 2008中的函数来提高查询可读性。 下面是我正在尝试执行的一个示例。

我有一个表，我们存储最大温度每一天的时间阅读，然后我想选择所有天凡8-10AM之间的最高温度比之间的最高温度更大12-2PM

因此，这里是如何像：

DECLARE @TableA TABLE ([Date] DATE, [Time] TIME(0), HighTemp DECIMAL(6,2)); 

INSERT @TableA VALUES 
('2011-09-10','08:00:00',38.15), 
('2011-09-10','09:00:00',38.32), 
('2011-09-10','10:00:00',38.17), 
('2011-09-10','11:00:00',38.10), 
('2011-09-10','12:00:00',38.05), 
('2011-09-10','13:00:00',38.15), 
('2011-09-10','14:00:00',38.30), 

('2011-09-11','08:00:00',38.12), 
('2011-09-11','09:00:00',38.09), 
('2011-09-11','10:00:00',38.07), 
('2011-09-11','11:00:00',38.15), 
('2011-09-11','12:00:00',38.13), 
('2011-09-11','13:00:00',38.11), 
('2011-09-11','14:00:00',38.10), 

('2011-09-12','08:00:00',38.30), 
('2011-09-12','09:00:00',38.33), 
('2011-09-12','10:00:00',38.35), 
('2011-09-12','11:00:00',38.30), 
('2011-09-12','12:00:00',38.25), 
('2011-09-12','13:00:00',38.23), 
('2011-09-12','14:00:00',38.20) 

select distinct [DATE] from @TableA maintbl 
where 
-- Select the high temp between 08:00:00-10:00:00 
(select MAX(HighTemp) from @TableA tmptbl where tmptbl.Time >= '08:00:00' and tmptbl.Time <= '10:00:00' and maintbl.Date = tmptbl.Date) 
> 
-- Select the high between 12:00:00-14:00:00 
(select MAX(HighTemp) from @TableA tmptbl where tmptbl.Time >= '12:00:00' and tmptbl.Time <= '14:00:00' and maintbl.Date = tmptbl.Date) 

查询运行良好（快）和上面的查询结果应该是： 2011-09-10 2011-09-12

现在，我试图简化查询Ÿ通过使用检索某一天和时间段的最大tempreture的功能，所以查询更容易阅读，就像这样：

select distinct [DATE] from @TableA maintbl 
where GetPeriodHigh(maintbl.Date, '08:00:00', '10:00:00') > GetPeriodHigh(maintbl.Date, '12:00:00', '14:00:00') 

而且功能就像这样：

CREATE FUNCTION [dbo].[GetPeriodHigh] 
(
    @Date date, 
    @From time, 
    @To time 
) 
RETURNS decimal(6,2) 
AS 
BEGIN 

    declare @res decimal(6,2) 

    select @res = MAX(high) from MyTable 
    where Time >= @from and Time <= @to and Date = @Date 

    return @res 
END 

问题是运行查询使用该函数需要LONG时间，实际上我从来没有看到它完成，看起来像是在某种无限循环...

任何想法，为什么是这样的，而且有什么我可以为了简化我的查询吗？

Thx。

Answer 1

为了提高性能，请尝试重写UDF价值作为价值UDF内嵌表的标量。

一些链接：

http://sqlblog.com/blogs/alexander_kuznetsov/archive/2008/05/23/reuse-your-code-with-cross-apply.aspx

http://sqlblog.com/blogs/alexander_kuznetsov/archive/2008/04/21/not-all-udfs-are-bad-for-performance.aspx

Answer 2

做数据访问的标量函数通常很糟糕，最好避免。它们不会被优化器扩展出来，并且基本上强制函数查询作为嵌套循环连接的内部，而不管其是否适合。

使事情变得更糟糕，您可能没有正确的索引来评估函数内的Time >= @from and Time <= @to and Date = @Date谓词，这意味着对于外部查询中的每一行，您将通过函数调用强制实施2个表扫描。

缺乏指标也是在源例子的情况下，它可以与在线版本，查询优化器能够有效地改写这个两个MAX/GROUP BY查询不同WHERE条款中可以看出然后合并连接的结果一起。当逻辑是标量UDF时，目前没有考虑这种转换。

你可以尝试另一种方法是

SELECT [Date] 
FROM @TableA 
WHERE Time BETWEEN '08:00:00' AND '10:00:00' 
     OR Time BETWEEN '12:00:00' AND '14:00:00' 
GROUP BY [Date] 
HAVING MAX(CASE 
       WHEN Time BETWEEN '08:00:00' AND '10:00:00' THEN HighTemp END) > 
     MAX(CASE 
       WHEN Time BETWEEN '12:00:00' AND '14:00:00' THEN HighTemp END)