Question

其中一个查询（如下所示）执行时间超过90秒。它从一个相当大的表LogMessage返回~500行。如果ESCAPE N'~'从查询中删除，则会在几秒钟内执行。同样，如果删除了TOP (1000)，它会在几秒钟内执行。查询计划在第一种情况下显示Key Lookup (Clustered) PK_LogMessage, Index Scan (NonClustered) IX_LogMessage and Nested Loops (Inner Join)。删除子句ESCAPE N'~'或TOP (1000)后，查询计划会更改并显示Clustered Index Scan (Clustered) PK_LogMessage。虽然我们正在寻找添加更多索引（可能在ApplicationName上），但我们想了解当前的情况。

正在从Entity Framework生成查询，以防您想知道为什么以这种方式编写。此外，实际查询更复杂，但这是表现出相同行为的最短版本。

查询：

SELECT TOP (1000) 
    [Project1].[MessageID] AS [MessageID], 
    [Project1].[TimeGenerated] AS [TimeGenerated], 
    [Project1].[SystemName] AS [SystemName], 
    [Project1].[ApplicationName] AS [ApplicationName]
FROM
    (
        SELECT
            [Project1].[MessageID] AS [MessageID],
            [Project1].[TimeGenerated] AS [TimeGenerated],
            [Project1].[SystemName] AS [SystemName],
            [Project1].[ApplicationName] AS [ApplicationName]
        FROM
        (
            SELECT 
                [Extent1].[MessageID] AS [MessageID], 
                [Extent1].[TimeGenerated] AS [TimeGenerated], 
                [Extent1].[SystemName] AS [SystemName], 
                [Extent1].[ApplicationName] AS [ApplicationName]
            FROM
                [dbo].[LogMessage] AS [Extent1]
            INNER JOIN
                [dbo].[LogMessageCategory] AS [Extent2]
            ON
                [Extent1].[CategoryID] = [Extent2].[CategoryID]
            WHERE
                ([Extent1].[ApplicationName] LIKE N'%tier%' ESCAPE N'~')
        )  AS [Project1]
    )  AS [Project1]
ORDER BY
    [Project1].[TimeGenerated] DESC

表LogMessage：

CREATE TABLE [dbo].[LogMessage](
    [MessageID] [int] IDENTITY(1000001,1) NOT NULL,
    [TimeGenerated] [datetime] NOT NULL,
    [SystemName] [nvarchar](256) NOT NULL,
    [ApplicationName] [nvarchar](512) NOT NULL,
        [CategoryID] [int] NOT NULL,
 CONSTRAINT [PK_LogMessage] PRIMARY KEY CLUSTERED 
(
    [MessageID] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF,
    ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON, FILLFACTOR = 90) ON [PRIMARY]
) ON [PRIMARY]

ALTER TABLE [dbo].[LogMessage]  WITH CHECK ADD CONSTRAINT [FK_LogMessage_LogMessageCategory] FOREIGN KEY([CategoryID])
    REFERENCES [dbo].[LogMessageCategory] ([CategoryID])

ALTER TABLE [dbo].[LogMessage] CHECK CONSTRAINT [FK_LogMessage_LogMessageCategory]

ALTER TABLE [dbo].[LogMessage] ADD  DEFAULT ((100)) FOR [CategoryID]

CREATE NONCLUSTERED INDEX [IX_LogMessage] ON [dbo].[LogMessage] 
(
    [TimeGenerated] DESC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, SORT_IN_TEMPDB = OFF,
    IGNORE_DUP_KEY = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS  = ON,
    ALLOW_PAGE_LOCKS  = ON, FILLFACTOR = 90) ON [PRIMARY]

表LogMessageCategory：

CREATE TABLE [dbo].[LogMessageCategory](
    [CategoryID] [int] NOT NULL,
    [Name] [nvarchar](128) NOT NULL,
    [Description] [nvarchar](256) NULL,
 CONSTRAINT [PK_LogMessageCategory] PRIMARY KEY CLUSTERED 
(
    [CategoryID] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]

查询计划1（需要90多秒）

Query Plan 1 (takes 90+ seconds)

查询计划2（需要约3秒）

Query Plan 2 (takes ~3 seconds)

Answer 1

这看起来像是一个直接的参数嗅探问题。

如您所愿TOP 1000 TimeGenerated命令SQL Server可以扫描TimeGenerated上的索引并对基表进行查找以评估ApplicationName上的谓词并在找到第1000行时停止，或者可以执行聚簇索引扫描，找到与ApplicationName谓词匹配的所有行，然后执行TOP N种类型的行。

SQL Server维护有关字符串列的统计信息。如果第一个计划认为许多行最终会匹配ApplicationName谓词，那么第一个计划就更有可能被选择，但是这个计划并不适合作为参数化查询重新使用，因为它可能在灾难性方面效率低下。几行匹配的事件。如果小于1,000匹配，则肯定需要执行与表中的行一样多的键查找。

从测试结束开始，我无法找到添加或删除冗余ESCAPE更改的SQL Server基数估计值的任何情况。当然，更改参数化查询的文本意味着原始计划无法使用，但需要编译一个不同的计划，这可能更适合当前考虑的特定值。

Answer 2

为什么所有这些嵌套查询？下面的代码执行相同的工作

        SELECT TOP(1000)
            [Extent1].[MessageID] AS [MessageID], 
            [Extent1].[TimeGenerated] AS [TimeGenerated], 
            [Extent1].[SystemName] AS [SystemName], 
            [Extent1].[ApplicationName] AS [ApplicationName]
        FROM
            [dbo].[LogMessage] AS [Extent1]
        INNER JOIN
            [dbo].[LogMessageCategory] AS [Extent2]
        ON
            [Extent1].[CategoryID] = [Extent2].[CategoryID]
        WHERE
            ([Extent1].[ApplicationName] LIKE N'%tier%' ESCAPE N'~')
        ORDER BY [Extent1].[TimeGenerated] DESC

我也同意ESCAPE N'〜'可以省略，因为我没有理由使用它。

Answer 3

首先，我会简化@niktrs指定的查询。尽管执行计划似乎忽略了子查询，但它使它更加人性化，因此更容易操作和理解。

然后，你有一个INNER JOIN，在我看来它可能会消失。是否真的需要INNER JOIN LogMessage到LogMessageCategory？您可以使用以下内容快速检查..

SELECT LM.CategoryID AS FromLogMessage, LMC.CategoryID AS FromLogMessageCategory
FROM dbo.LogMessage AS LM
     FULL OUTER JOIN dbo.LogMessageCategory AS LMC ON LMC.CategoryID = LM.CategoryID
WHERE LM.CategoryID IS NULL OR LMC.CategoryID IS NULL

Answer 4

如果你这样做，它是如何运作的？

Select * 
FROM
 (your whole scary framework query with the escape N) a
LIMIT 1000 
(or the mssql alternative if mssql does not support the correct syntax -- )

因为如果滚动..你有可能继续使用该框架并从非常糟糕的sql中获得不错的性能（比如......这意味着你创建完整的rs然后只从中选择1k .. 。）。

使用TOP和ESCAPE更改查询计划和执行时间

4 个答案: