Sql server 2005 使用变量可以增加扫描和逻辑读取

我不明白。我有一个查询，如果我在查询中为EntityType硬编码一个数字，这就是IO统计信息

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.
Table 'FranchiseAgreement'. Scan count 1, logical reads 157, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Address'. Scan count 1, logical reads 5485, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'County'. Scan count 1, logical reads 126, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'State'. Scan count 0, logical reads 505, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Country'. Scan count 0, logical reads 488, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'City'. Scan count 0, logical reads 630, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'ARTaxMasterfile'. Scan count 4, logical reads 36, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'ELMasterfile'. Scan count 1, logical reads 205, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

没什么不好的。现在我要做的就是用一个变量替换硬编码的数字，以提高可读性

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.
Table 'ELMasterfile'. Scan count 1, logical reads 205, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
***Table 'FranchiseAgreement'. Scan count 539932, logical reads 1750032, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Address'. Scan count 1, logical reads 5485, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'County'. Scan count 0, logical reads 481, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'State'. Scan count 0, logical reads 505, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Country'. Scan count 0, logical reads 488, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'City'. Scan count 0, logical reads 630, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'ARTaxMasterfile'. Scan count 4, logical reads 36, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

DECLARE @BalanceIdEL SMALLINT
SET @BalanceIdEL = 14

PRINT 'Version 5'
SELECT Stores.*, Tax.ARTaxMasterFileId
FROM
(   -- Get The Addres of Each Lease
    SELECT 
        E.LeaseNum
    ,   E.LeaseDate
    ,   A.CountryCode
    ,   A.StateProvCode
    ,   A.County
    ,   A.City
    FROM 
        ELMasterFile E
    INNER JOIN
        FranchiseAgreement F
        ON  E.EntityId = F.FranchiseID
        AND E.EntityType = 7
    INNER JOIN
        Address A
        ON  F.FranchiseNum = A.EntityId
        AND F.SatelliteNum = A.SatelliteNum
        AND A.EntityType = 5 -- Store
        AND A.AddressType = 3 -- Store Address  
    )  Stores
INNER JOIN 
    -- Get the Taxes at each Level, Country, State, County and City
    (SELECT A.*, C.CountryCode, '' AS StateProvCode, '' AS CountyName, '' AS CityName
    FROM ARTaxMasterFile A
    INNER JOIN
        COMMON.dbo.Country C
        ON A.ARTaxLevelTypeId = C.CountryId 
    where BalanceId = @BalanceIdEL
    and ARTaxLevelType = 1
    UNION ALL
    SELECT A.*, S.CountryCode, S.StateProvCode AS StateProvCode, '' AS CountyName, '' AS CityName
    FROM ARTaxMasterFile A
    INNER JOIN
        COMMON.dbo.State S
        ON A.ARTaxLevelTypeId = S.StateId 
    where BalanceId = @BalanceIdEL
    and ARTaxLevelType = 2
    UNION ALL
    SELECT A.*, C.CountryCode, C.StateProvCode AS StateProvCode,  C.CountyName AS CountyName, '' AS CityName
    FROM ARTaxMasterFile A
    INNER JOIN
        COMMON.dbo.County C
        ON A.ARTaxLevelTypeId = C.CountyId 
    where BalanceId = @BalanceIdEL
    and ARTaxLevelType = 3
    UNION ALL
    SELECT A.*, Country.CountryCode, State.StateProvCode AS StateProvCode,  County.CountyName AS CountyName, City.CityName AS CityName
    FROM ARTaxMasterFile A
    INNER JOIN
        COMMON.dbo.City City
        ON A.ARTaxLevelTypeId = City.CityId 
    INNER JOIN
        COMMON.dbo.County County
        ON City.CountyId = County.CountyId
    INNER JOIN
        COMMON.dbo.State State
        ON City.StateId = State.StateId
    LEFT OUTER JOIN
        COMMON.dbo.Country Country
        ON City.CountryId = Country.CountryId       
    where BalanceId = @BalanceIdEL
    and ARTaxLevelType = 4) Tax
    ON  (Stores.CountryCode = Tax.CountryCode
        AND Tax.StateProvCode = ''
        AND Tax.CountyName = ''
        AND Tax.CityName = '')
    OR 
        (Stores.CountryCode = Tax.CountryCode
        AND Stores.StateProvCode = Tax.StateProvCode 
        AND Tax.CountyName = ''
        AND Tax.CityName = '')
    OR 
        (Stores.CountryCode = Tax.CountryCode
        AND Stores.StateProvCode = Tax.StateProvCode 
        AND Stores.County = Tax.CountyName 
        AND Tax.CityName = '')
    OR 
        (Stores.CountryCode = Tax.CountryCode
        AND Stores.StateProvCode = Tax.StateProvCode 
        AND Stores.County = Tax.CountyName 
        AND Stores.City =Tax.CityName)
WHERE Tax.StartDate <= GETDATE()
AND Tax.StartDate > Stores.LeaseDate 

最有趣的是，该表上没有使用参数

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如果我取出变量BalanceIdEL，并用14替换它，查询速度会快2倍。

我认为查询优化器没有意识到您的变量实际上是一个常量，因此通过表扫描而不是索引查找来选择查询计划，因为它认为值是未知的。查询计划是怎么说的？也许索引提示会迫使它做正确的事情

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出于这个原因，我基本上已经停止使用变量作为常量了——我现在使用带有注释的常量，我也看到了这一点，并且不会像这样使用@variables，除非它们是必需的，因为它们会发生变化。这有点像参数嗅探，但变量不是参数。