web-dev-qa-db-ja.com

クラスタ化インデックススキャンを取り除く方法は？

C＃/ Linq-To-Entitiesコードによって生成されたクエリがあります。この種のクエリと同様に、見た目はひどく醜いので、ここに含めると役に立ちません。しかし、それをクエリプロファイラーで実行したところ、処理時間の50.5％が「ネストされたループ（内部結合）」ステップで発生していることがわかりました。このステップでは、_a2_AssessmentQuestion_（0.3％）および同じテーブルに対するクラスタ化インデックススキャン（13.1％）。

私はこの「ネストされたループ結合」が何をしているのか本当に理解していないと告白しますが、検索しているフィールドが別のインデックスを介して直接利用できる場合、クエリがインデックスシークの直後にテーブルをスキャンするのはなぜですか正確に必要なフィールド？そして、はい、私は_update statistics a2_AssessmentQuestion_を実行しましたが、効果はありません。

クエリプランを理解する専門家ではないので、関連する質問への回答を提供していただければ幸いです。あなたに十分な有用な情報を提供するのに十分なほど知らなかったかもしれません。

ありがとう！

enter image description here

編集： FWIW、ここにクエリがあります：

_SELECT 
[Project6].[ID] AS [ID], 
[Project6].[ID2] AS [ID1], 
[Project6].[ID1] AS [ID2], 
[Project6].[ID3] AS [ID3], 
[Project6].[ID5] AS [ID4], 
[Project6].[ID7] AS [ID5], 
[Project6].[ID6] AS [ID6], 
[Project6].[ID8] AS [ID7], 
[Project6].[ID9] AS [ID8], 
[Project6].[ID11] AS [ID9], 
[Project6].[ID10] AS [ID10], 
[Project6].[ID12] AS [ID11], 
[Project6].[ID13] AS [ID12], 
[Project6].[Number] AS [Number], 
[Project6].[ScholarAssessmentID] AS [ScholarAssessmentID], 
[Project6].[C1] AS [C1], 
[Project6].[GradingStyle] AS [GradingStyle], 
[Project6].[PartID] AS [PartID], 
[Project6].[C2] AS [C2], 
[Project6].[ID4] AS [ID13], 
[Project6].[Description] AS [Description], 
[Project6].[Version] AS [Version], 
[Project6].[DifficultyLevel] AS [DifficultyLevel], 
[Project6].[AssessmentQuestionID] AS [AssessmentQuestionID], 
[Project6].[GradingDueDate] AS [GradingDueDate], 
[Project6].[C3] AS [C3], 
[Project6].[AssessmentFormat] AS [AssessmentFormat], 
[Project6].[C4] AS [C4], 
[Project6].[Structure] AS [Structure], 
[Project6].[Title] AS [Title], 
[Project6].[C5] AS [C5], 
[Project6].[AssessmentFormat1] AS [AssessmentFormat1], 
[Project6].[C6] AS [C6], 
[Project6].[Label] AS [Label]
FROM ( SELECT 
    [Extent1].[ID] AS [ID], 
    [Extent1].[ScholarAssessmentID] AS [ScholarAssessmentID], 
    [Extent1].[AssessmentQuestionID] AS [AssessmentQuestionID], 
    [Join1].[ID1] AS [ID1], 
    [Join1].[ID2] AS [ID2], 
    [Extent4].[ID] AS [ID3], 
    [Extent4].[PartID] AS [PartID], 
    [Extent4].[Number] AS [Number], 
    [Extent4].[DifficultyLevel] AS [DifficultyLevel], 
    [Extent4].[Structure] AS [Structure], 
    [Extent4].[GradingStyle] AS [GradingStyle], 
    [Extent5].[ID] AS [ID4], 
    [Extent5].[Description] AS [Description], 
    [Extent5].[Version] AS [Version], 
    [Extent6].[ID] AS [ID5], 
    [Extent6].[GradingDueDate] AS [GradingDueDate], 
    [Extent6].[Title] AS [Title], 
    [Join6].[ID3] AS [ID6], 
    [Join6].[ID4] AS [ID7], 
    [Extent9].[ID] AS [ID8], 
    [Extent10].[ID] AS [ID9], 
    [Extent10].[AssessmentFormat] AS [AssessmentFormat], 
    [Join10].[ID5] AS [ID10], 
    [Join10].[ID6] AS [ID11], 
    [Extent13].[ID] AS [ID12], 
    [Extent14].[ID] AS [ID13], 
    [Extent14].[AssessmentFormat] AS [AssessmentFormat1], 
     CAST( [Extent4].[Points] AS bigint) AS [C1], 
     CAST( [Extent1].[PointsAwarded] AS decimal(19,0)) AS [C2], 
     CAST( [Extent6].[DueDate] AS datetime2) AS [C3], 
     CAST( [Extent4].[Number] AS bigint) AS [C4], 
    [Join14].[Label] AS [Label], 
    CASE WHEN ([Extent4].[RubricWeight] IS NOT NULL) THEN  CAST( [Extent4].[RubricWeight] AS int) ELSE 0 END AS [C5], 
    CASE WHEN ([Join14].[ScholarAssessmentAnswerID] IS NULL) THEN CAST(NULL AS int) ELSE 1 END AS [C6]
    FROM            [dbo].[a2_ScholarAssessmentAnswer] AS [Extent1]
    INNER JOIN  (SELECT [Extent2].[ID] AS [ID1], [Extent2].[AssessmentID] AS [AssessmentID], [Extent3].[ID] AS [ID2]
        FROM  [dbo].[a2_ScholarAssessment] AS [Extent2]
        LEFT OUTER JOIN [dbo].[a2_ScholarFNPAssessment] AS [Extent3] ON [Extent2].[ID] = [Extent3].[ID] ) AS [Join1] ON [Extent1].[ScholarAssessmentID] = [Join1].[ID1]
    INNER JOIN [dbo].[a2_AssessmentQuestion] AS [Extent4] ON [Extent1].[AssessmentQuestionID] = [Extent4].[ID]
    LEFT OUTER JOIN [dbo].[a2_AssessmentQuestionType] AS [Extent5] ON [Extent4].[QuestionTypeID] = [Extent5].[ID]
    INNER JOIN [dbo].[a2_AssessmentPart] AS [Extent6] ON [Extent4].[PartID] = [Extent6].[ID]
    INNER JOIN  (SELECT [Extent7].[ID] AS [ID3], [Extent7].[AssessmentID] AS [AssessmentID], [Extent8].[ID] AS [ID4]
        FROM  [dbo].[a2_ScholarAssessment] AS [Extent7]
        LEFT OUTER JOIN [dbo].[a2_ScholarFNPAssessment] AS [Extent8] ON [Extent7].[ID] = [Extent8].[ID] ) AS [Join6] ON [Extent1].[ScholarAssessmentID] = [Join6].[ID3]
    INNER JOIN [dbo].[a2_Assessment] AS [Extent9] ON [Join6].[AssessmentID] = [Extent9].[ID]
    LEFT OUTER JOIN [dbo].[a2_AssessmentType] AS [Extent10] ON [Extent9].[AssessmentTypeID] = [Extent10].[ID]
    INNER JOIN  (SELECT [Extent11].[ID] AS [ID5], [Extent11].[AssessmentID] AS [AssessmentID], [Extent12].[ID] AS [ID6]
        FROM  [dbo].[a2_ScholarAssessment] AS [Extent11]
        LEFT OUTER JOIN [dbo].[a2_ScholarFNPAssessment] AS [Extent12] ON [Extent11].[ID] = [Extent12].[ID] ) AS [Join10] ON [Extent1].[ScholarAssessmentID] = [Join10].[ID5]
    INNER JOIN [dbo].[a2_Assessment] AS [Extent13] ON [Join10].[AssessmentID] = [Extent13].[ID]
    LEFT OUTER JOIN [dbo].[a2_AssessmentType] AS [Extent14] ON [Extent13].[AssessmentTypeID] = [Extent14].[ID]
    LEFT OUTER JOIN  (SELECT [Extent15].[ScholarAssessmentAnswerID] AS [ScholarAssessmentAnswerID], [Extent16].[Label] AS [Label]
        FROM  [dbo].[a2_ScholarAssessmentAnswerSelection] AS [Extent15]
        INNER JOIN [dbo].[a2_AssessmentAnswers] AS [Extent16] ON [Extent15].[AssessmentAnswerID] = [Extent16].[ID] ) AS [Join14] ON [Extent1].[ID] = [Join14].[ScholarAssessmentAnswerID]
    WHERE (1538 = [Join1].[AssessmentID]) AND ( NOT EXISTS (SELECT 
        1 AS [C1]
        FROM ( SELECT 
            [Extent18].[ID] AS [ID]
            FROM  [dbo].[a2_AssessmentQuestion] AS [Extent17]
            INNER JOIN [dbo].[a2_AssessmentQuestion] AS [Extent18] ON [Extent17].[PartID] = [Extent18].[PartID]
            WHERE [Extent1].[AssessmentQuestionID] = [Extent17].[ID]
        )  AS [Project1]
        WHERE ( NOT EXISTS (SELECT 
            1 AS [C1]
            FROM [dbo].[a2_ScholarAssessmentAnswer] AS [Extent19]
            WHERE ([Extent1].[ScholarAssessmentID] = [Extent19].[ScholarAssessmentID]) AND ([Extent19].[AssessmentQuestionID] = [Project1].[ID])
        )) OR (CASE WHEN ( EXISTS (SELECT 
            1 AS [C1]
            FROM [dbo].[a2_ScholarAssessmentAnswer] AS [Extent20]
            WHERE ([Extent1].[ScholarAssessmentID] = [Extent20].[ScholarAssessmentID]) AND ([Extent20].[AssessmentQuestionID] = [Project1].[ID])
        )) THEN cast(1 as bit) WHEN ( NOT EXISTS (SELECT 
            1 AS [C1]
            FROM [dbo].[a2_ScholarAssessmentAnswer] AS [Extent21]
            WHERE ([Extent1].[ScholarAssessmentID] = [Extent21].[ScholarAssessmentID]) AND ([Extent21].[AssessmentQuestionID] = [Project1].[ID])
        )) THEN cast(0 as bit) END IS NULL)
    ))
)  AS [Project6]
ORDER BY [Project6].[ID] ASC, [Project6].[ID2] ASC, [Project6].[ID1] ASC, [Project6].[ID3] ASC, [Project6].[ID5] ASC, [Project6].[ID7] ASC, [Project6].[ID6] ASC, [Project6].[ID8] ASC, [Project6].[ID9] ASC, [Project6].[ID11] ASC, [Project6].[ID10] ASC, [Project6].[ID12] ASC, [Project6].[ID13] ASC, [Project6].[ID4] ASC, [Project6].[C6] ASC
_

実行計画を含めようとしましたが、30,000文字を超えてしまいました。

奇妙なことに、クエリで最も有用な制限情報であるAssessmentID = 1538を指定していますが、実行プランはこの事実をほとんど無視しており、ほとんどすべての他の結合テーブルをスキャンし、AssessmentIDの種類でのみフィルターします後付けとして...

sql-server-2012indexoptimizationindex-tuning
2
Shaul Behr

ネストされたループはこの部分で発生しており、a2_AssessmentQuestionをそれ自体に結合しています。

SELECT
   [Extent18].[ID] AS [ID]
FROM
   [dbo].[a2_AssessmentQuestion] AS [Extent17]
   INNER JOIN [dbo].[a2_AssessmentQuestion] AS [Extent18]
      ON [Extent17].[PartID] = [Extent18].[PartID]
WHERE
   [Extent1].[AssessmentQuestionID] = [Extent17].[ID]

投稿したクエリ全体からわかるように、where条件はこの部分には適用されません。

ネストされたループとクラスター化インデックススキャンの述語と出力を見て、a2_AssessmentQuestion.PartIDに非クラスター化インデックスを作成することをお勧めします。

Nested loop details from the execution planClustered index scan details from the execution plan

2
Blaž Dakskobler