為什么這個SQL導致索引掃描而不是索引搜索？

Question

有人可以幫我調整這個SQL查詢嗎？

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')

。

以下是執行計划的樣子：

。

我不明白的是為什么我要為NPapplicants進行索引掃描。 我有一個涵蓋BuildingID和ApartmentID的索引。 不應該使用嗎？

Answer 1

這是因為期待從比賽中返回接近10K的記錄。 回到數據以使用10K密鑰檢索其他列相當於僅掃描100K記錄（至少）和使用哈希匹配過濾的性能。

至於對其他表的訪問，查詢優化器已經確定您的索引是有用的（可能是針對Offline或MA ），因此它正在尋找該索引以獲取連接鍵。

然后將這兩個HASH匹配用於交叉點以產生最終輸出。

Answer 2

B-Tree索引中的搜索是表掃描（每個記錄）的幾倍。

此外，應該在聚集索引中進行另一次搜索以檢索其他列的值。

如果預計大部分記錄匹配，則掃描聚簇索引會更便宜。

要確保優化器選擇了最佳方法，您可以運行以下命令：

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')

將index_name替換為索引的實際名稱，並比較執行時間和I/O操作數（如消息選項卡中所示）