好吧，我已经解决了这个问题并提出了一些建议。
但首先让我们填充辅助表
CREATE TABLE dbo.Numbers(n INT NOT NULL PRIMARY KEY)
GO
DECLARE @i INT;
SET @i = 1;
INSERT INTO dbo.Numbers(n) SELECT 1;
WHILE @i<1024000 BEGIN
INSERT INTO dbo.Numbers(n)
SELECT n + @i FROM dbo.Numbers;
SET @i = @i * 2;
END;
GO
和测试数据，一年每分钟一分钟的广告，同年每分钟一个客户来电：
CREATE TABLE dbo.Commercials(
StartedAt DATETIME NOT NULL
CONSTRAINT PK_Commercials PRIMARY KEY,
EndedAt DATETIME NOT NULL,
CommercialName VARCHAR(30) NOT NULL);
GO
INSERT INTO dbo.Commercials(StartedAt, EndedAt, CommercialName)
SELECT DATEADD(minute, n - 1, '20080101')
,DATEADD(minute, n, '20080101')
,'Show #'+CAST(n AS VARCHAR(6))
FROM dbo.Numbers
WHERE n<=24*365*60;
GO
CREATE TABLE dbo.Calls(CallID INT
CONSTRAINT PK_Calls NOT NULL PRIMARY KEY,
AirTime DATETIME NOT NULL,
SomeInfo CHAR(300));
GO
INSERT INTO dbo.Calls(CallID,
AirTime,
SomeInfo)
SELECT n
,DATEADD(minute, n - 1, '20080101')
,'Call during Commercial #'+CAST(n AS VARCHAR(6))
FROM dbo.Numbers
WHERE n<=24*365*60;
GO
CREATE UNIQUE INDEX Calls_AirTime
ON dbo.Calls(AirTime) INCLUDE(SomeInfo);
GO
最初尝试选择年中三个小时的广告期间拨打的所有电话非常慢：
SET STATISTICS IO ON;
SET STATISTICS TIME ON;
GO
SELECT COUNT(*) FROM(
SELECT s.StartedAt, s.EndedAt, c.AirTime
FROM dbo.Commercials s JOIN dbo.Calls c
ON c.AirTime >= s.StartedAt AND c.AirTime < s.EndedAt
WHERE c.AirTime BETWEEN '20080701' AND '20080701 03:00'
) AS t;
SQL Server parse and compile time:
CPU time = 15 ms, elapsed time = 30 ms.
(1 row(s) affected)
Table 'Calls'. Scan count 1, logical reads 11, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Worktable'. Scan count 2, logical reads 3338264, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Commercials'. Scan count 2, logical reads 7166, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Worktable'. Scan count 0, logical reads 0, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
SQL Server Execution Times:
CPU time = 71704 ms, elapsed time = 36316 ms.
原因很简单：我们知道广告不重叠，所以一个电话
最多适合一个广告，但优化器不知道。
我们知道广告很短，但优化器也不知道。
这两个假设都可以作为约束强制执行，但优化器不会仍然这样做。
假设广告不超过 15 分钟，我们可以看出
到优化器，查询非常快：
SELECT COUNT(*) FROM(
SELECT s.StartedAt, s.EndedAt, c.AirTime
FROM dbo.Commercials s JOIN dbo.Calls c
ON c.AirTime >= s.StartedAt AND c.AirTime < s.EndedAt
WHERE c.AirTime BETWEEN '20080701' AND '20080701 03:00'
AND s.StartedAt BETWEEN '20080630 23:45' AND '20080701 03:00'
) AS t;
SQL Server parse and compile time:
CPU time = 15 ms, elapsed time = 15 ms.
(1 row(s) affected)
Table 'Worktable'. Scan count 1, logical reads 753, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Calls'. Scan count 1, logical reads 11, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Commercials'. Scan count 1, logical reads 4, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
SQL Server Execution Times:
CPU time = 31 ms, elapsed time = 24 ms.
假设广告不重叠，所以一个电话
最多适合一个广告，我们可以告诉
到优化器，查询又很快：
SELECT COUNT(*) FROM(
SELECT s.StartedAt, s.EndedAt, c.AirTime
FROM dbo.Calls c CROSS APPLY(
SELECT TOP 1 s.StartedAt, s.EndedAt FROM dbo.Commercials s
WHERE c.AirTime >= s.StartedAt AND c.AirTime < s.EndedAt
ORDER BY s.StartedAt DESC) AS s
WHERE c.AirTime BETWEEN '20080701' AND '20080701 03:00'
) AS t;
SQL Server parse and compile time:
CPU time = 0 ms, elapsed time = 7 ms.
(1 row(s) affected)
Table 'Commercials'. Scan count 181, logical reads 1327, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Calls'. Scan count 1, logical reads 11, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
SQL Server Execution Times:
CPU time = 31 ms, elapsed time = 31 ms.