完全相同的查询 - 不同的性能

Question

完全相同的查询 - 不同的性能

Gar*_*rom 4 performance sql-server sql-server-2014 query-performance

数据库：SQL Server 12.0.5207

除了过滤条件之一的值外，我有几个查询在所有方面都完全相同。同一张表（不是另一台服务器上的模式副本），因此索引、资源等都是相同的。一切都是完全相同的。

此查询在一秒钟内运行：

SELECT 
         MAX(MessageID) AS [MaxID]
FROM BoothComm.UniversalMessageQueue
WHERE 
       MessagePlatform = 'linux'

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此查询在一秒钟内运行：

SELECT 
         MAX(MessageID) AS [MaxID]
FROM BoothComm.UniversalMessageQueue
WHERE 
       MessagePlatform = 'linux'
       AND
       MessageCategory = 'accounting'

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此查询在一秒钟内运行：

SELECT 
         MAX(MessageID) AS [MaxID]
FROM BoothComm.UniversalMessageQueue
WHERE 
       MessagePlatform = 'windows'

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那么为什么这个运行需要将近 30 秒呢？

SELECT 
         MAX(MessageID) AS [MaxID]
FROM BoothComm.UniversalMessageQueue
WHERE 
       MessagePlatform = 'windows'
       AND
       MessageCategory = 'accounting'

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我的一个同事在表中添加了另一个索引，解决了延迟的业务问题。该索引将 30 秒减少到 FULL 秒，同时将其他查询加速到瞬时。同样，执行计划完全相同：

（索引扫描应为 100%）。我听取了其他论坛的建议，并确保查询中的列顺序与它们存储在索引中的顺序相匹配...

CREATE NONCLUSTERED INDEX [MessageID and Platform and Category] ON [BoothComm].[UniversalMessageQueue]
(
    [MessageID] ASC,
    [MessagePlatform] ASC,
    [MessageCategory] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
GO

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如果有帮助，我还会提供表模式。

CREATE TABLE [BoothComm].[UniversalMessageQueue](
    [MessageQueueId] [bigint] IDENTITY(1,1) NOT NULL,
    [MessageID] [bigint] NOT NULL,
    [MessagePlatform] [nvarchar](50) NULL,
    [AssetNumber] [nvarchar](50) NOT NULL,
    [MessageState] [int] NULL,
    [MessageStateLabel] [nvarchar](50) NULL,
    [MessageType] [int] NULL,
    [MessageTypeLabel] [nvarchar](50) NULL,
    [MessageCategory] [nvarchar](50) NULL,
    [MessageSource] [int] NULL,
    [MessageSourceLabel] [nvarchar](50) NULL,
    [MessageSourceSerialNumber] [nvarchar](50) NULL,
    [MessageCreateDate] [datetime] NULL,
    [MessageTransmitDate] [datetime] NULL,
    [MessageReceivedDate] [datetime] NULL,
    [MessageStoredDate] [datetime] NULL,
    [XMLPayload] [nvarchar](max) NULL,
    [JSONPayload] [nvarchar](max) NULL,
    [SemanticXML] [nvarchar](max) NULL,
    [SemanticJSON] [nvarchar](max) NULL,
    [MessageSequenceNumber] [int] NULL,
    [ERPImportDate] [datetime] NULL,
    [ERPImportStatus] [int] NULL,
    [ERPMsg] [nvarchar](max) NULL,
    [NormalizationDate] [datetime] NULL,
    [NormalizationStatus] [int] NULL,
    [NormalizationDesc] [nvarchar](max) NULL,
    [SemanticDate] [datetime] NULL,
    [SemanticStatus] [int] NULL,
    [SemanticDesc] [nvarchar](max) NULL,
    [CreatedDate] [datetime] NOT NULL DEFAULT (getdate()),
    [CreatedBy] [nvarchar](50) NOT NULL DEFAULT ('ETL'),
    [UpdatedDate] [datetime] NOT NULL DEFAULT (getdate()),
    [UpdatedBy] [nvarchar](50) NOT NULL DEFAULT ('ETL'),
    [ETL_ID] [uniqueidentifier] NULL,
PRIMARY KEY CLUSTERED 
(
    [MessageQueueId] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY],
 CONSTRAINT [CK_ETL_Unique_MessageID_Platform] UNIQUE NONCLUSTERED 
(
    [MessageID] ASC,
    [MessagePlatform] ASC,
    [MessageType] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY] TEXTIMAGE_ON [PRIMARY]

GO

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这应该为您提供了足够的代码来重现问题......只需用大约 1100 万条记录填充表，您就可以看到问题！

由于它被提出了几次，我什至没想过要检查，我查看了有多少“windows”记录与“linux”记录。

SELECT COUNT(*) 
FROM BoothComm.UniversalMessageQueue 
WHERE 
    MessageCategory = 'Accounting'
    AND
    MessagePlatform = 'linux';
-- returned 1762461

SELECT COUNT(*) 
FROM BoothComm.UniversalMessageQueue 
WHERE 
    MessageCategory = 'Accounting'
    AND
    MessagePlatform = 'windows';
-- returned 11786

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所以......我猜记录计数不是问题？

Answer 1

ind*_*iri 8

现在您正在进行索引扫描，因为所需的所有列都在索引中，并且比扫描表更快。但是，它必须扫描，而不是搜索，因为索引上的第一列不在WHERE语句中并且不限制返回。您首先需要重新排列索引列MessagePlatform，因为它始终在您的WHERE语句中。

根据您的数据大小和所需的插入速度，您可能需要考虑两个索引。如果您只想要一个索引，我会采用以下方法：

CREATE NONCLUSTERED INDEX [MessageID and Platform and Category] 
ON [BoothComm].[UniversalMessageQueue]
(
    [MessagePlatform] ASC,
    [MessageID] ASC,
    [MessageCategory] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF,
 DROP_EXISTING = OFF, ONLINE = OFF, 
 ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
GO

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但如果你能负担得起两个，我会转向：

CREATE NONCLUSTERED INDEX [MessageID and Platform] 
ON [BoothComm].[UniversalMessageQueue]
(
    [MessagePlatform] ASC,
    [MessageID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF,
 DROP_EXISTING = OFF, ONLINE = OFF, 
 ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
GO

CREATE NONCLUSTERED INDEX [MessageID and Platform and Category] 
ON [BoothComm].[UniversalMessageQueue]
(
    [MessagePlatform] ASC
    [MessageCategory] ASC,
    [MessageID] ASC,
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF,
 DROP_EXISTING = OFF, ONLINE = OFF, 
 ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
GO

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它使用哪个索引将取决于 ifMessageCategory在您的WHERE语句中。

Linux 记录比 Windows 记录多很多（1,762,461 对 11,786）。索引现在的方式是，索引扫描从最大的开始，MessageID然后向下移动列表，直到找到匹配的MessagePlatform. 由于有更多的记录是 Linux，它会很快命中一个。由于 Windows 的数量少得多，因此它必须扫描得更远，花费的时间更长。

Answer 2

Eri*_*ing 8

只是为了提供一些额外的索引策略，这就是我想出的。

我不确定您表中的其余数据分布。我知道Accounting/Linux作为一个组合构成 1,762,461 行，并Accounting/Windows构成 11,786 行。假设还有其他部门和平台，我把这些数据贴在一张表中。

USE tempdb;

CREATE TABLE dbo.Whatever
(
    Id INT IDENTITY(1,1),
    MessageId INT NOT NULL,
    MessageCategory NVARCHAR(50),
    MessagePlatform NVARCHAR(50)
);

INSERT dbo.Whatever ( MessageId, MessageCategory, MessagePlatform )
SELECT TOP 1762461 x.n, 'Accounting', 'Linux'
FROM (SELECT ROW_NUMBER() OVER (ORDER BY @@ROWCOUNT) AS n
        FROM sys.messages AS m
        CROSS JOIN sys.messages AS m2) AS x

INSERT dbo.Whatever ( MessageId, MessageCategory, MessagePlatform )
SELECT TOP 11786 x.n, 'Accounting', 'Windows'
FROM (SELECT ROW_NUMBER() OVER (ORDER BY @@ROWCOUNT) AS n
        FROM sys.messages AS m
        CROSS JOIN sys.messages AS m2) AS x


INSERT dbo.Whatever ( MessageId, MessageCategory, MessagePlatform )
SELECT TOP 5000000 x.n, 'HR', 'Unix'
FROM (SELECT ROW_NUMBER() OVER (ORDER BY @@ROWCOUNT) AS n
        FROM sys.messages AS m
        CROSS JOIN sys.messages AS m2) AS x


INSERT dbo.Whatever ( MessageId, MessageCategory, MessagePlatform )
SELECT TOP 1000000 x.n, 'Accounting', 'Mac'
FROM (SELECT ROW_NUMBER() OVER (ORDER BY @@ROWCOUNT) AS n
        FROM sys.messages AS m
        CROSS JOIN sys.messages AS m2) AS x

INSERT dbo.Whatever ( MessageId, MessageCategory, MessagePlatform )
SELECT TOP 1000000 x.n, 'IT', 'Windows'
FROM (SELECT ROW_NUMBER() OVER (ORDER BY @@ROWCOUNT) AS n
        FROM sys.messages AS m
        CROSS JOIN sys.messages AS m2) AS x

INSERT dbo.Whatever ( MessageId, MessageCategory, MessagePlatform )
SELECT TOP 1000000 x.n, 'IT', 'Linux'
FROM (SELECT ROW_NUMBER() OVER (ORDER BY @@ROWCOUNT) AS n
        FROM sys.messages AS m
        CROSS JOIN sys.messages AS m2) AS x

INSERT dbo.Whatever ( MessageId, MessageCategory, MessagePlatform )
SELECT TOP 1000000 x.n, 'HR', 'Mac'
FROM (SELECT ROW_NUMBER() OVER (ORDER BY @@ROWCOUNT) AS n
        FROM sys.messages AS m
        CROSS JOIN sys.messages AS m2) AS x

INSERT dbo.Whatever ( MessageId, MessageCategory, MessagePlatform )
SELECT TOP 1000000 x.n, 'HR', 'Windows'
FROM (SELECT ROW_NUMBER() OVER (ORDER BY @@ROWCOUNT) AS n
        FROM sys.messages AS m
        CROSS JOIN sys.messages AS m2) AS x

ALTER TABLE dbo.Whatever ADD CONSTRAINT pk_thoughtful PRIMARY KEY CLUSTERED (Id)

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第一种策略： 由于您正在查询 a MAX()，因此对索引中的MessageId列进行排序可能更有意义DESC。这可以让您避免Sort在计划中出现不需要的情况。

CREATE INDEX ix_love_and_rockets
    ON dbo.Whatever
(
    MessagePlatform,
    MessageId DESC,
    MessageCategory );

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第二种策略： 如果您最关心MessagePlatformWindows 和 Linux，则对这些值进行过滤索引可能是有意义的。我将坚持使用的降序排列MessageId。

CREATE INDEX ix_bauhaus_was_better
    ON dbo.Whatever
(
    MessageCategory,
    MessageId DESC )
    INCLUDE ( MessagePlatform )
    WHERE MessagePlatform IN ( 'Windows', 'Linux' );

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结果： 当运行一些示例查询时，索引使用是混合的。

SELECT MAX(w.MessageId) AS MaxId
FROM   dbo.Whatever AS w
WHERE  w.MessagePlatform = 'Linux'
       AND w.MessageCategory = 'Accounting'
       AND 1 = ( SELECT 1 );

SELECT MAX(w.MessageId) AS MaxId
FROM   dbo.Whatever AS w
WHERE  w.MessagePlatform = 'Windows'
       AND w.MessageCategory = 'Accounting'
       AND 1 = ( SELECT 1 );

SELECT MAX(w.MessageId) AS MaxId
FROM   dbo.Whatever AS w
WHERE  w.MessagePlatform = 'Windows'
       AND 1 = ( SELECT 1 );

SELECT MAX(w.MessageId) AS MaxId
FROM   dbo.Whatever AS w
WHERE  w.MessagePlatform = 'Linux'
       AND 1 = ( SELECT 1 );

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这是统计时间和 I/O 结果：

查询 1

SQL Server 执行时间：CPU 时间 = 0 毫秒，已用时间 = 0 毫秒。表“随便”。扫描计数 1，逻辑读取 3，物理读取 0，预读读取 0，lob 逻辑读取 0，lob 物理读取 0，lob 预读读取 0。

SQL Server 执行时间：CPU 时间 = 0 毫秒，已用时间 = 0 毫秒。
查询 2

SQL Server 执行时间：CPU 时间 = 0 毫秒，已用时间 = 0 毫秒。表“随便”。扫描计数1，逻辑读10938，物理读0，预读0，lob逻辑读0，lob物理读0，lob预读0。

SQL Server 执行时间：CPU 时间 = 625 毫秒，已用时间 = 614 毫秒。
查询 3

SQL Server 执行时间：CPU 时间 = 0 毫秒，已用时间 = 0 毫秒。表“随便”。扫描计数 1，逻辑读 5，物理读 0，预读 0，lob 逻辑读 0，lob 物理读 0，lob 预读 0。

SQL Server 执行时间：CPU 时间 = 0 毫秒，已用时间 = 0 毫秒。
查询 4

SQL Server 执行时间：CPU 时间 = 0 毫秒，已用时间 = 0 毫秒。表“随便”。扫描计数 1，逻辑读取 4，物理读取 0，预读读取 0，lob 逻辑读取 0，lob 物理读取 0，lob 预读读取 0。

SQL Server 执行时间：CPU 时间 = 0 毫秒，已用时间 = 0 毫秒。

我的博客上讲述两个过滤和降序索引一点点在这里，如果你想多一点信息。

为什么查询 2 需要更长的时间？答案是“字母表”。

如果您MessagePlatform按DESC顺序创建索引，则顺序将翻转。

只考虑这两个索引：

CREATE INDEX ix_love_and_rockets
    ON dbo.Whatever
(
    MessagePlatform,
    MessageId DESC,
    MessageCategory );


CREATE INDEX ix_rockets_and_love
    ON dbo.Whatever
(
    MessagePlatform DESC,
    MessageId DESC,
    MessageCategory );

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我们正在改变的顺序MessagePlatform。现在，如果我们运行相同的两个查询来暗示这些索引，性能差异将不会翻转。

SELECT MAX(w.MessageId) AS MaxId
FROM   dbo.Whatever AS w WITH (INDEX = ix_love_and_rockets)
WHERE  w.MessagePlatform = 'Linux'
       AND w.MessageCategory = 'Accounting'
       AND 1 = ( SELECT 1 );

SELECT MAX(w.MessageId) AS MaxId
FROM   dbo.Whatever AS w WITH (INDEX = ix_love_and_rockets)
WHERE  w.MessagePlatform = 'Windows'
       AND w.MessageCategory = 'Accounting'
       AND 1 = ( SELECT 1 );

SELECT MAX(w.MessageId) AS MaxId
FROM   dbo.Whatever AS w WITH (INDEX = ix_rockets_and_love)
WHERE  w.MessagePlatform = 'Linux'
       AND w.MessageCategory = 'Accounting'
       AND 1 = ( SELECT 1 );

SELECT MAX(w.MessageId) AS MaxId
FROM   dbo.Whatever AS w WITH (INDEX = ix_rockets_and_love)
WHERE  w.MessagePlatform = 'Windows'
       AND w.MessageCategory = 'Accounting'
       AND 1 = ( SELECT 1 );

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查询 1

表“随便”。扫描计数 1，逻辑读取 4，物理读取 0，预读读取 0，lob 逻辑读取 0，lob 物理读取 0，lob 预读读取 0。

SQL Server 执行时间：CPU 时间 = 0 毫秒，已用时间 = 0 毫秒。
查询 2

表“随便”。扫描计数 1，逻辑读取 9338，物理读取 0，预读读取 0，lob 逻辑读取 0，lob 物理读取 0，lob 预读读取 0。

SQL Server 执行时间：CPU 时间 = 687 毫秒，已用时间 = 684 毫秒。
查询 3

表“随便”。扫描计数 1，逻辑读 5，物理读 0，预读 0，lob 逻辑读 0，lob 物理读 0，lob 预读 0。

SQL Server 执行时间：CPU 时间 = 0 毫秒，已用时间 = 0 毫秒。
查询 4

SQL Server 执行时间：CPU 时间 = 0 毫秒，已用时间 = 0 毫秒。表“随便”。扫描计数 1，逻辑读取 9337，物理读取 0，预读读取 0，lob 逻辑读取 0，lob 物理读取 0，lob 预读读取 0。

SQL Server 执行时间：CPU 时间 = 672 毫秒，已用时间 = 678 毫秒。

根本原因：在两个查询中，我们都寻求MessagePlatform，但是我们在上有一个残差谓词MessageCategory。

区别在于数据分布。在我的测试数据中，要过滤掉的Windows行要MessageCategory多得多Linux——特别是Windows/HR组合的行要多 1 毫米。

希望这可以帮助！

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