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版本：SQL Server 2008 R2

数据库：来自http://msftdbprodsamples.codeplex.com/releases/view/55926的 AdventureWorks 2008R2

询问：

SELECT TOP 10
    *
FROM
    Person.Person --WITH (FORCESEEK)
WHERE
    LastName like 'Max%'
    OR EXISTS (
        SELECT
            1
        FROM
            Person.PersonPhone
        WHERE
            Person.PersonPhone.BusinessEntityID = Person.Person.BusinessEntityID
            AND Person.PersonPhone.PhoneNumber LIKE '122%'
    )
ORDER BY Person.Person.BusinessEntityID DESC


在没有任何查询提示的情况下，SQL Server 将使用聚集索引扫描，这是 IO 密集型的：

Table 'PersonPhone'. Scan count 14170, logical reads 28446, physical reads 15, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Person'. Scan count 1, logical reads 2844, physical reads 3, read-ahead reads 3215, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.


使用WITH (FORCESEEK)查询提示，SQL Server 将选择索引查找 + 键查找，完成速度更快，对 IO 友好 500 倍：

Table 'Person'. Scan count 1, logical reads 59, physical reads 22, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'PersonPhone'. Scan count 1, logical reads 2, physical reads 2, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.


我的问题是，有没有办法让 SQL Server 在没有任何查询提示的情况下使用更好的计划？也许添加索引？还是更改配置参数？还是 SQL Server 的查询优化器那么笨？

这是来自http://msdn.microsoft.com/en-us/library/bb510478.aspx的宝石：

当查询使用 IN 或 LIKE 作为搜索谓词时，查询优化器规则和较差的基数估计也会导致优化器执行表或索引扫描操作而不是索引查找。

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1 回答 1

5

这是一个展示“体面”的 IO 数据而不诉诸 FORCESEEK 的版本。有趣的是，这个看起来很糟糕的查询表现得“更好”。

免责声明：这对于 'Max%' 和 '122%' 可能会表现得更好，但是如果该计划被重复用于 'M%' 和 '%' 将如何表现则是另一回事......

SELECT TOP 10 *
FROM Person.Person P
INNER JOIN (
SELECT BusinessEntityID
FROM    Person.Person --WITH (FORCESEEK)
WHERE    LastName like 'Max%'    

UNION

SELECT BusinessEntityID
FROM    Person.Person --WITH (FORCESEEK)
WHERE    EXISTS (        SELECT            *        FROM        
    Person.PersonPhone        WHERE            Person.PersonPhone.BusinessEntityID = Person.Person.BusinessEntityID            AND Person.PersonPhone.PhoneNumber LIKE '122%'    )
) NOTNICE
ON NOTNICE.BusinessEntityID = P.BusinessEntityID
ORDER BY P.BusinessEntityID DESC
于 2011-05-10T06:46:36.097 回答