使用 TOP 条件估计的行数有误

Question

我想这是一个非常基本的问题，但我无法理解它。我对一列有完美的统计数据，但是在使用 TOP 条件时，估计的行数总是非常接近于使用的数字对于 TOP 运营商给我一个完全错误的数字。知道应用程序表上的InterviewerID 的统计信息确切地知道ID 12868 行有超过3K 行，对此有什么简单的解释吗？为什么告诉我 13.83？

使用的版本：Microsoft SQL Server 2008 R2 (SP3) - 10.50.6000.34 (X64)

谢谢你们！

我怎么称呼SP？EXEC agy.sp_searchTest3 @interviewerId = 12868,@searchTerm = 'aar'

导致问题的查询：

ALTER PROCEDURE [agy].[sp_searchTest3] (
    @interviewerId INT
    ,@searchTerm VARCHAR(50) = NULL
    ,@searchApplicationId INT = NULL
    )
AS
BEGIN
    IF @searchApplicationId IS NOT NULL
        SELECT TOP 10 u.userId
            ,u.firstName + ' ' + u.lastName AS 'fullname'
            ,app.applicationId
            ,u.loginId AS email
            ,adi.URI AS imageurl
        FROM [app].[application] AS app
        INNER JOIN [app].[applicant] AS a ON a.applicantId = app.applicantId
        INNER JOIN [usr].[user] AS u ON u.userId = a.userId
        LEFT JOIN upl.applicationDocuments AS ud ON ud.applicationId = app.applicationId
            AND ud.documentTypeId = 5 -- (Portrait pic)
            AND ud.documentStateId NOT IN (
                3
                ,5
                ) -- (Rejected, Deleted)
        LEFT JOIN upl.applicationDocumentImages AS adi ON adi.documentId = ud.documentId
        WHERE app.interviewerId = @interviewerId
            AND u.isActive = 1
            AND app.applicationId = @searchApplicationId


    IF @searchTerm IS NOT NULL
        SELECT 
        TOP 15 -- everytime I change the top value the estimated number of rows change as well
        u.userId
            ,u.firstName + ' ' + u.lastName AS 'fullname'
            ,app.applicationId
            ,u.loginId AS email
            ,adi.URI AS imageurl
        FROM [app].[application] AS app
        INNER JOIN [app].[applicant] AS a ON a.applicantId = app.applicantId
        INNER JOIN [usr].[user] AS u ON u.userId = a.userId
        LEFT JOIN upl.applicationDocuments AS ud ON ud.applicationId = app.applicationId
            AND ud.documentTypeId = 5 -- (Portrait pic)
            AND ud.documentStateId NOT IN (
                3
                ,5
                ) -- (Rejected, Deleted)
        LEFT JOIN upl.applicationDocumentImages AS adi ON adi.documentId = ud.documentId
        WHERE app.interviewerId = @interviewerId
            AND u.isActive = 1
            AND (
                (u.firstName LIKE @searchTerm)
                OR (u.lastName LIKE @searchTerm)
                OR (u.loginId LIKE @searchTerm)
                )
END

Answer 1

13.83是它认为满足您要求的 15 行所需的行数。它猜测在那之后它可以停止。

Paul White 在他的文章Inside Optimizer Row Goals in Depth 中对此进行了更多的讨论。

由于他在此站点上花费了大量时间，您可能会看到一个广泛的答案，其中详细介绍了为什么是 13.83 而不是 13.84。;)

我建议您阅读以下博客

• 为什么 Top1 扼杀了我的表现您还会看到为什么拥有行目标会造成伤害的一些原因。
• 每个人都错过的 SSIS 调优技巧