Question

我有下面的T-SQL查询（由linq-to-sql生成，所以看起来有点奇怪。）当我删除前10名时，它运行时间不到1秒。当我离开前10名时，需要68秒。

如果没有前10名，总行数仅为19。

为什么排名前10的人在这种情况下会如此摧毁表现呢？

SELECT TOP (10) [t3].[value] AS [StartDate], [t3].[AH_TimeStamp4] AS [EndDate], [t3].[value2] AS [RunTime], [t3].[AH_Idnr] AS [RunNumber], [t3].[AH_Status] AS [StatusCode], [t3].[STATUS_DESC] AS [StatusDesc], [t3].[STATUS_TYPE] AS [StatusType]
FROM (
    SELECT COALESCE([t1].[AH_TimeStamp2],[t1].[AH_TimeStamp1]) AS [value], [t1].[AH_TimeStamp4], CONVERT(BigInt,(((CONVERT(BigInt,DATEDIFF(DAY, [t1].[AH_TimeStamp1], [t1].[AH_TimeStamp4]))) * 86400000) + DATEDIFF(MILLISECOND, DATEADD(DAY, DATEDIFF(DAY, [t1].[AH_TimeStamp1], [t1].[AH_TimeStamp4]), [t1].[AH_TimeStamp1]), [t1].[AH_TimeStamp4])) * 10000) AS [value2], [t1].[AH_Idnr], [t1].[AH_Status], [t2].[STATUS_DESC], [t2].[STATUS_TYPE], [t0].[OH_Name]
    FROM [dbo].[OH] AS [t0]
    INNER JOIN [dbo].[AH] AS [t1] ON ([t0].[OH_Idnr]) = [t1].[AH_OH_Idnr]
    INNER JOIN [dbo].[CHK_JOB_STATUS_CODE] AS [t2] ON [t1].[AH_Status] = ([t2].[STATUS_CODE])
    ) AS [t3]
WHERE [t3].[OH_Name] = @p0
ORDER BY [t3].[AH_Idnr] DESC

编辑 - ＆gt;根据要求，这是没有Take（10）

的linq生成的查询

SELECT [t3].[value] AS [StartDate], [t3].[AH_TimeStamp4] AS [EndDate], [t3].[value2] AS [RunTime], [t3].[AH_Idnr] AS [RunNumber], [t3].[AH_Status] AS [StatusCode], [t3].[STATUS_DESC] AS [StatusDesc], [t3].[STATUS_TYPE] AS [StatusType]
FROM (
    SELECT COALESCE([t1].[AH_TimeStamp2],[t1].[AH_TimeStamp1]) AS [value], [t1].[AH_TimeStamp4], CONVERT(BigInt,(((CONVERT(BigInt,DATEDIFF(DAY, [t1].[AH_TimeStamp1], [t1].[AH_TimeStamp4]))) * 86400000) + DATEDIFF(MILLISECOND, DATEADD(DAY, DATEDIFF(DAY, [t1].[AH_TimeStamp1], [t1].[AH_TimeStamp4]), [t1].[AH_TimeStamp1]), [t1].[AH_TimeStamp4])) * 10000) AS [value2], [t1].[AH_Idnr], [t1].[AH_Status], [t2].[STATUS_DESC], [t2].[STATUS_TYPE], [t0].[OH_Name]
    FROM [dbo].[OH] AS [t0]
    INNER JOIN [dbo].[AH] AS [t1] ON ([t0].[OH_Idnr]) = [t1].[AH_OH_Idnr]
    INNER JOIN [dbo].[CHK_JOB_STATUS_CODE] AS [t2] ON [t1].[AH_Status] = ([t2].[STATUS_CODE])
    ) AS [t3]
WHERE [t3].[OH_Name] = @p0
ORDER BY [t3].[AH_Idnr] DESC

编辑2 - ＆gt;这是LINQ查询 - 没什么特别的，只有3个连接

List<UC4Status> statusList = uc4DB.OHs.Where(o => o.OH_Name == jobName).Join(uc4DB.AHs, oh => oh.OH_Idnr, ah => ah.AH_OH_Idnr, (oh, ah) => ah)
                        .Join(uc4DB.CHK_JOB_STATUS_CODEs, ah => ah.AH_Status, job => job.STATUS_CODE, (a, s) =>
                            new UC4Status
                            {
                                StartDate = a.AH_TimeStamp2 ?? a.AH_TimeStamp1,
                                EndDate = a.AH_TimeStamp4,
                                RunTime = a.AH_TimeStamp4 - a.AH_TimeStamp1,
                                StatusType = s.STATUS_TYPE,
                                StatusDesc = s.STATUS_DESC,
                                StatusCode = a.AH_Status,
                                RunNumber = a.AH_Idnr
                            }).OrderByDescending(r => r.RunNumber).Take(maxResults).ToList();

Answer 1

比返回的行数更重要的是每个连接表的行数。如果不访问查询计划，很难说问题是什么。但是，问题可能是由于过时或缺少列统计信息以及缺少良好的索引导致查询优化器在使用top时做出非常糟糕的决定。例如，如果优化器错误地假设top语句仅对每个连接表中的少量行进行操作，则可以选择在每个连接表上使用内部循环连接和表扫描 - 对于大量记录来说可能是一个非常缓慢的操作（但是在处理少量记录时可以非常有效地工作）。在没有top的情况下，优化器可以正确地假设它必须扫描连接表中的大量记录并使用更适合这种操作的方法，例如来自表的哈希匹配连接扫描。

您可以尝试更新统计信息并添加其他索引以查看是否有帮助。

Answer 2

没有前10名的行数是多少？是否有任何索引实施？我猜这是由于t0，t1和t2的子查询连接... 你可以尝试一下吗，


select top 10 * from (
  select t3.value....
  from ( 
    select coalesce... )
  where ... order by ... 
)

为什么选择Top X Kill Performance

2 个答案: