帮助查找没有谓词的连接

Question

很像swasheck 的一个相关问题，我有一个历史上曾遭受性能问题的查询。我正在查看 SSMS 上的查询计划并注意到Nested Loops (Inner Join)警告：

无连接谓词

根据一些仓促的研究（鼓舞人心的DBA和Brent Ozar 的信心），看起来这个警告告诉我我的查询中有一个隐藏的笛卡尔积。我已经检查了几次我的查询，但没有看到交叉连接。这是查询：

DECLARE @UserId INT; -- Stored procedure input
DECLARE @Now DATETIME2(7) = SYSUTCDATETIME();
;WITH AggregateStepData_CTE AS -- Considering converting this CTE into an indexed view
(
    SELECT 
        [UA].[UserId] -- FK to the UserId
        , [UA].[DeviceId] -- FK to the push device's DeviceId (int)
        , SUM(ISNULL([UA].[LatestSteps], 0)) AS [Steps]
    FROM [User].[UserStatus] [UA]
        INNER JOIN [User].[CurrentConnections] [M] ON 
           [M].[Monitored] = [UA].[UserId] AND [M].[Monitor] = @UserId
    WHERE
        [M].[ShareSteps] = 1 -- Only use step data if we are allowed to see.
        AND
        CAST([UA].[ReportedLocalTime] AS DATE) = 
          CAST(DATEADD(MINUTE, DATEPART(TZOFFSET, [UA].[ReportedLocalTime]), @Now) AS DATE)
          -- Aggregate the steps for today based on the device's time zone.         
    GROUP BY
        [UA].[UserId]
        , [UA].[DeviceId]
)
SELECT
    [UA].[UserId] -- FK to the UserId
    , [UA].[ReportedLocalTime]
    , CASE WHEN [M].[ShareLocation] = 1 THEN [UA].[Latitude] ELSE NULL END AS [Latitude]
    , CASE WHEN [M].[ShareLocation] = 1 THEN [UA].[Longitude] ELSE NULL END AS [Longitude]
    , CASE WHEN [M].[ShareLocation] = 1 THEN [UA].[LocationAccuracy] ELSE NULL END 
         AS [LocationAccuracy]
    , CASE WHEN [M].[ShareSteps] = 1 THEN ISNULL([SD].[Steps], 0) ELSE NULL END AS [Steps]
    , CASE WHEN [M].[ShareBattery] = 1 THEN [UA].[BatteryPercentage] ELSE NULL END 
         AS [BatteryPercentage]
    , CASE WHEN [M].[ShareBattery] = 1 THEN [UA].[IsDraining] ELSE NULL END 
         AS [IsDraining]
    , [PD].[DeviceName]
FROM [User].[LatestUserStatus] [UA]
    INNER JOIN [User].[CurrentConnections] [M] WITH (NOEXPAND) ON 
      [M].[Monitored] = [UA].[UserId] AND [M].[Monitor] = @UserId
    INNER JOIN [User].[PushDevice] [PD] ON [PD].[PushDeviceId] = [UA].[DeviceId]
    LEFT JOIN [AggregateStepData_CTE] [SD] ON 
      [M].[Monitored] = [SD].[UserId] AND [SD].[DeviceId] = [UA].[DeviceId]
ORDER BY        
    [UA].[UserId]
    , [UA].[ReportedLocalTime] DESC

查询计划见：https : //gist.github.com/anonymous/d6ac970b45eb75a88b99

或者我不应该像swasheck 问题中的结论那样害怕警告，毕竟估计的子树成本很低，只有 0.05？

---

这个答案似乎也很相关，这意味着这可能是 SQL Server 代表我进行的优化，因为它知道我可以删除连接。

---

这篇博客文章表明嵌套循环没有谓词问题可能是由连接列上的 UDF 引起的。我没有在此查询中引用任何 UDF。

---

这是CurrentConnections视图的定义：

CREATE VIEW [User].[CurrentConnections]
WITH SCHEMABINDING
AS 
    SELECT 
        [M].[Monitor] -- FK to the UserId
        , [M].[Monitored] -- FK to the UserId
        , [M].[MonitoringId]
        , [M].[ShareBattery]
        , [M].[ShareLocation]
        , [M].[ShareSteps]
        , [M].[ShowInSocialFeed]
        , [M].[Created] AS [RelationshipCreated]
        , [AT].[AlertThresholdId]
        , [AT].[EffectiveStartTime]
        , [AT].[EndTime]
        , [AT].[OverNightRedThreshold]
        , [AT].[SendBatteryAlerts]
        , [AT].[SendGeneralAlerts]
        , [AT].[StartTime]
        , [AT].[ThresholdInMinutes]
        , [AT].[Threshold]
        , [U_Monitored].[ProfilePhoto] AS [Monitored_ProfilePhoto]
        , [U_Monitored].[DisplayName] AS [Monitored_DisplayName]
        , [U_Monitored].[Fullname] AS [Monitored_FullName]
        , [U_Monitored].[PhoneNumber] AS [Monitored_PhoneNumber]
    FROM [User].[Monitoring] [M]
        INNER JOIN [User].[AlertThreshold] [AT] ON [AT].[MonitoringId] = [M].[MonitoringId]
        INNER JOIN [User].[User] [U_Monitored] ON [U_Monitored].[UserId] = [M].[Monitored]
    WHERE
        [M].[ArchivedOn] IS NULL
        AND
        [AT].[ArchivedOn] IS NULL

GO

CREATE UNIQUE CLUSTERED INDEX [IDX_User_CurrentConnections_Monitor_Monitored] ON 
   [User].[CurrentConnections]([Monitor], [Monitored]);
GO
CREATE NONCLUSTERED INDEX [IDX_User_CurrentConnections_Monitored] ON 
  [User].[CurrentConnections]([Monitored]) 
  INCLUDE ([Monitor], [ShareBattery], [ShareLocation], [ShareSteps]);

Answer 1

在我的 CTE 中，我缺少WITH (NOEXPAND)查询提示。添加此查询提示后，没有谓词的连接就从我的查询计划中消失了。

;WITH AggregateStepData_CTE AS
(
    SELECT
        [UA].[UserId]
        , [UA].[DeviceId]
        , SUM(ISNULL([UA].[LatestSteps], 0)) AS [Steps]
    FROM [User].[UserStatus] [UA]
        INNER JOIN [User].[CurrentConnections] [M] WITH (NOEXPAND) -- Added query hint here
          ON [M].[Monitored] = [UA].[UserId] AND [M].[Monitor] = @UserId
    WHERE
        [M].[ShareSteps] = 1 -- Only use step data if we are allowed to see.
        AND
        CAST([UA].[ReportedLocalTime] AS DATE) = 
          CAST(DATEADD(MINUTE, DATEPART(TZOFFSET, [UA].[ReportedLocalTime]), @Now) AS DATE) 
          -- Aggregate the steps for today based on the device's time zone.         
    GROUP BY
        [UA].[UserId]
        , [UA].[DeviceId]
)