为什么这个SQL导致索引扫描而不是索引搜索？

Question

有人可以帮我调整一下这个SQL查询吗？

SELECT  a.BuildingID, a.ApplicantID, a.ACH, a.Address, a.Age, a.AgentID, a.AmenityFee, a.ApartmentID, a.Applied, a.AptStatus, a.BikeLocation, a.BikeRent, a.Children, 
        a.CurrentResidence, a.Email, a.Employer, a.FamilyStatus, a.HCMembers, a.HCPayment, a.Income, a.Industry, a.Name, a.OccupancyTimeframe, a.OnSiteID,
        a.Other, a.ParkingFee, a.Pets, a.PetFee, a.Phone, a.Source, a.StorageLocation, a.StorageRent, a.TenantSigned, a.WasherDryer, a.WasherRent, a.WorkLocation, 
        a.WorkPhone, a.CreationDate, a.CreatedBy, a.LastUpdated, a.UpdatedBy
FROM    dbo.NPapplicants AS a INNER JOIN
        dbo.NPapartments AS apt ON a.BuildingID = apt.BuildingID AND a.ApartmentID = apt.ApartmentID
WHERE   (apt.Offline = 0)
AND     (apt.MA = 'M')

以下是执行计划的内容：

我不明白为什么我要为NPapplicants进行索引扫描。我有一个涵盖BuildingID和ApartmentID的索引。不应该使用吗？

Answer 1

这是因为期待从比赛中返回接近10K的记录。回到数据以使用10K密钥检索其他列等同于仅扫描100K记录（至少）并使用散列匹配进行过滤的性能。

对于对其他表的访问，查询优化器已经确定您的索引很有用（可能针对Offline或MA），因此它正在寻找该索引以获取连接键。 / p>

然后将这两个HASH匹配用于交叉点以产生最终输出。

Answer 2

B-Tree索引中的搜索是表扫描（每条记录）的几倍。

此外，应该在聚集索引中进行另一次搜索以检索其他列的值。

如果预计大部分记录匹配，则扫描聚簇索引会更便宜。

要确保优化程序选择了最佳方法，您可以运行：

SET STATISTICS IO ON
SET STATSTICS TIME ON

SELECT  a.BuildingID, a.ApplicantID, a.ACH, a.Address, a.Age, a.AgentID, a.AmenityFee, a.ApartmentID, a.Applied, a.AptStatus, a.BikeLocation, a.BikeRent, a.Children, 
        a.CurrentResidence, a.Email, a.Employer, a.FamilyStatus, a.HCMembers, a.HCPayment, a.Income, a.Industry, a.Name, a.OccupancyTimeframe, a.OnSiteID,
        a.Other, a.ParkingFee, a.Pets, a.PetFee, a.Phone, a.Source, a.StorageLocation, a.StorageRent, a.TenantSigned, a.WasherDryer, a.WasherRent, a.WorkLocation, 
        a.WorkPhone, a.CreationDate, a.CreatedBy, a.LastUpdated, a.UpdatedBy
FROM    dbo.NPapplicants AS a INNER JOIN
        dbo.NPapartments AS apt ON a.BuildingID = apt.BuildingID AND a.ApartmentID = apt.ApartmentID
WHERE   (apt.Offline = 0)
AND     (apt.MA = 'M')

SELECT  a.BuildingID, a.ApplicantID, a.ACH, a.Address, a.Age, a.AgentID, a.AmenityFee, a.ApartmentID, a.Applied, a.AptStatus, a.BikeLocation, a.BikeRent, a.Children, 
        a.CurrentResidence, a.Email, a.Employer, a.FamilyStatus, a.HCMembers, a.HCPayment, a.Income, a.Industry, a.Name, a.OccupancyTimeframe, a.OnSiteID,
        a.Other, a.ParkingFee, a.Pets, a.PetFee, a.Phone, a.Source, a.StorageLocation, a.StorageRent, a.TenantSigned, a.WasherDryer, a.WasherRent, a.WorkLocation, 
        a.WorkPhone, a.CreationDate, a.CreatedBy, a.LastUpdated, a.UpdatedBy
FROM    dbo.NPapplicants WITH (INDEX (index_name)) AS a
INNER JOIN
        dbo.NPapartments AS apt ON a.BuildingID = apt.BuildingID AND a.ApartmentID = apt.ApartmentID
WHERE   (apt.Offline = 0)
AND     (apt.MA = 'M')

将index_name替换为索引的实际名称，并比较执行时间和I/O操作的数量（如消息选项卡中所示）