为什么我将Int / Smallint隐式转换为Varchar，这真的会影响基数估计吗？

11

我正在尝试使用实际执行计划上的“显示计划分析”（SSMS）来执行性能缓慢的查询。分析工具指出，行数的估计值与计划中某些位置的返回结果不符，并进一步给了我一些隐式转换警告。

我不理解这些从int到Varchar的隐式转换-引用的字段不是查询中任何参数/过滤器的一部分，并且在涉及的所有表中，列数据类型都是相同的：

我收到以下CardinalityEstimate警告：

表达式中的类型转换（CONVERT_IMPLICIT（varchar（12），[ccd]。[profileid]，0））可能会影响查询计划选择中的“ CardinalityEstimate”-该字段在我的数据库中到处都是整数

表达式中的类型转换（CONVERT_IMPLICIT（varchar（6），[ccd]。[nodeid]，0））可能会影响查询计划选择中的“ CardinalityEstimate”-该字段在我数据库中的每个地方都是smallint

表达式中的类型转换（CONVERT_IMPLICIT（varchar（6），[ccd]。[sessionseqnum]，0））可能会影响查询计划选择中的“ CardinalityEstimate”-该字段在我的数据库中到处都是smallint

表达式中的类型转换（CONVERT_IMPLICIT（varchar（41），[ccd]。[sessionid]，0））可能会影响查询计划选择中的“ CardinalityEstimate”-该字段在我的数据库中到处都是小数

[编辑]这是查询和实际执行计划，以供参考 https://www.brentozar.com/pastetheplan/?id=SysYt0NzN

和表定义

/****** Object:  Table [dbo].[agentconnectiondetail]    Script Date: 1/10/2019 9:10:04 AM ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[agentconnectiondetail](
    [sessionid] [decimal](18, 0) NOT NULL,
    [sessionseqnum] [smallint] NOT NULL,
    [nodeid] [smallint] NOT NULL,
    [profileid] [int] NOT NULL,
    [resourceid] [int] NOT NULL,
    [startdatetime] [datetime2](7) NOT NULL,
    [enddatetime] [datetime2](7) NOT NULL,
    [qindex] [smallint] NOT NULL,
    [gmtoffset] [smallint] NOT NULL,
    [ringtime] [smallint] NULL,
    [talktime] [smallint] NULL,
    [holdtime] [smallint] NULL,
    [worktime] [smallint] NULL,
    [callwrapupdata] [varchar](40) COLLATE SQL_Latin1_General_CP1_CI_AS NULL,
    [callresult] [smallint] NULL,
    [dialinglistid] [int] NULL,
    [convertedStartDatetimelocal] [datetime2](7) NULL,
    [convertedEndDatetimelocal] [datetime2](7) NULL,
 CONSTRAINT [PK_agentconnectiondetail] PRIMARY KEY CLUSTERED 
(
    [sessionid] ASC,
    [sessionseqnum] ASC,
    [nodeid] ASC,
    [profileid] ASC,
    [resourceid] ASC,
    [startdatetime] ASC,
    [qindex] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
/****** Object:  Table [dbo].[contactcalldetail]    Script Date: 1/10/2019 9:10:04 AM ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[contactcalldetail](
    [sessionid] [decimal](18, 0) NOT NULL,
    [sessionseqnum] [smallint] NOT NULL,
    [nodeid] [smallint] NOT NULL,
    [profileid] [int] NOT NULL,
    [contacttype] [smallint] NOT NULL,
    [contactTypeDescription] [varchar](20) COLLATE Latin1_General_CI_AS NULL,
    [contactdisposition] [smallint] NOT NULL,
    [contactdispositionDescription] [varchar](20) COLLATE Latin1_General_CI_AS NULL,
    [dispositionreason] [varchar](100) COLLATE Latin1_General_CI_AS NULL,
    [originatortype] [smallint] NOT NULL,
    [originatorTypeDescription] [varchar](20) COLLATE Latin1_General_CI_AS NULL,
    [originatorid] [int] NULL,
    [originatordn] [varchar](30) COLLATE Latin1_General_CI_AS NULL,
    [destinationtype] [smallint] NULL,
    [destinationTypeDescription] [varchar](20) COLLATE Latin1_General_CI_AS NULL,
    [destinationid] [int] NULL,
    [destinationdn] [varchar](30) COLLATE Latin1_General_CI_AS NULL,
    [startdatetimeUTC] [datetime2](7) NOT NULL,
    [enddatetimeUTC] [datetime2](7) NOT NULL,
    [gmtoffset] [smallint] NOT NULL,
    [callednumber] [varchar](30) COLLATE Latin1_General_CI_AS NULL,
    [origcallednumber] [varchar](30) COLLATE Latin1_General_CI_AS NULL,
    [applicationtaskid] [decimal](18, 0) NULL,
    [applicationid] [int] NULL,
    [applicationname] [varchar](30) COLLATE Latin1_General_CI_AS NULL,
    [connecttime] [smallint] NULL,
    [customvariable1] [varchar](40) COLLATE Latin1_General_CI_AS NULL,
    [customvariable2] [varchar](40) COLLATE Latin1_General_CI_AS NULL,
    [customvariable3] [varchar](40) COLLATE Latin1_General_CI_AS NULL,
    [customvariable4] [varchar](40) COLLATE Latin1_General_CI_AS NULL,
    [customvariable5] [varchar](40) COLLATE Latin1_General_CI_AS NULL,
    [customvariable6] [varchar](40) COLLATE Latin1_General_CI_AS NULL,
    [customvariable7] [varchar](40) COLLATE Latin1_General_CI_AS NULL,
    [customvariable8] [varchar](40) COLLATE Latin1_General_CI_AS NULL,
    [customvariable9] [varchar](40) COLLATE Latin1_General_CI_AS NULL,
    [customvariable10] [varchar](40) COLLATE Latin1_General_CI_AS NULL,
    [accountnumber] [varchar](40) COLLATE Latin1_General_CI_AS NULL,
    [callerentereddigits] [varchar](40) COLLATE Latin1_General_CI_AS NULL,
    [badcalltag] [char](1) COLLATE Latin1_General_CI_AS NULL,
    [transfer] [bit] NULL,
    [NextSeqNum] [smallint] NULL,
    [redirect] [bit] NULL,
    [conference] [bit] NULL,
    [flowout] [bit] NULL,
    [metservicelevel] [bit] NULL,
    [campaignid] [int] NULL,
    [origprotocolcallref] [varchar](32) COLLATE Latin1_General_CI_AS NULL,
    [destprotocolcallref] [varchar](32) COLLATE Latin1_General_CI_AS NULL,
    [convertedStartDatetimelocal] [datetime2](7) NULL,
    [convertedEndDatetimelocal] [datetime2](7) NULL,
    [AltKey]  AS (concat([sessionid],[sessionseqnum],[nodeid],[profileid]) collate database_default) PERSISTED NOT NULL,
    [PrvSeqNum] [smallint] NULL,
 CONSTRAINT [PK_contactcalldetail] PRIMARY KEY CLUSTERED 
(
    [sessionid] ASC,
    [sessionseqnum] ASC,
    [nodeid] ASC,
    [profileid] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
/****** Object:  Table [dbo].[contactqueuedetail]    Script Date: 1/10/2019 9:10:04 AM ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[contactqueuedetail](
    [sessionid] [decimal](18, 0) NOT NULL,
    [sessionseqnum] [smallint] NOT NULL,
    [profileid] [int] NOT NULL,
    [nodeid] [smallint] NOT NULL,
    [targetid] [int] NOT NULL,
    [targettype] [smallint] NOT NULL,
    [targetTypeDescription] [varchar](10) COLLATE Latin1_General_CI_AS NULL,
    [qindex] [smallint] NOT NULL,
    [queueorder] [smallint] NOT NULL,
    [disposition] [smallint] NULL,
    [dispositionDescription] [varchar](50) COLLATE Latin1_General_CI_AS NULL,
    [metservicelevel] [bit] NULL,
    [queuetime] [smallint] NULL,
 CONSTRAINT [PK_contactqueuedetail] PRIMARY KEY CLUSTERED 
(
    [sessionid] ASC,
    [sessionseqnum] ASC,
    [profileid] ASC,
    [nodeid] ASC,
    [targetid] ASC,
    [targettype] ASC,
    [qindex] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
/****** Object:  Index [<Name of Missing Index, sysname,>]    Script Date: 1/10/2019 9:10:04 AM ******/
CREATE NONCLUSTERED INDEX [<Name of Missing Index, sysname,>] ON [dbo].[contactcalldetail]
(
    [convertedStartDatetimelocal] ASC
)
INCLUDE (   [sessionid],
    [sessionseqnum],
    [nodeid],
    [profileid]) 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
/****** Object:  Index [idx_CCD_ContactType_DestType_StDtLocal]    Script Date: 1/10/2019 9:10:04 AM ******/
CREATE NONCLUSTERED INDEX [idx_CCD_ContactType_DestType_StDtLocal] ON [dbo].[contactcalldetail]
(
    [destinationtype] ASC,
    [contacttype] ASC,
    [convertedStartDatetimelocal] ASC
)
INCLUDE (   [sessionid],
    [sessionseqnum],
    [nodeid],
    [profileid],
    [convertedEndDatetimelocal]) 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
SET ANSI_PADDING ON
GO
/****** Object:  Index [idx_CQD_Profile_Traget_TargetType]    Script Date: 1/10/2019 9:10:04 AM ******/
CREATE NONCLUSTERED INDEX [idx_CQD_Profile_Traget_TargetType] ON [dbo].[contactqueuedetail]
(
    [profileid] ASC,
    [targetid] ASC,
    [targettype] ASC
)
INCLUDE (   [targetTypeDescription],
    [queueorder],
    [disposition],
    [dispositionDescription],
    [queuetime]) 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
sql-server  execution-plan  sql-server-2017  type-conversion  cardinality-estimates 
— Voysinmyhead
source

Answers:

11

隐式转换是由计算列引起的AltKey：

CREATE TABLE dbo.Test
(
    [sessionid] [decimal](18, 0) NOT NULL,
    [sessionseqnum] [smallint] NOT NULL,
    [nodeid] [smallint] NOT NULL,
    [profileid] [int] NOT NULL,
    [AltKey] AS 
        CONCAT
        (
            [sessionid],
            [sessionseqnum],
            [nodeid],
            [profileid]
        ) PERSISTED NOT NULL,
);

给定上面的简化表，下面的简单语句生成问题中给出的相同的隐式转换警告：

SELECT T.*
FROM dbo.Test AS T;

带有警告的计划

从文档（添加重点）：

CONCAT 串联前将所有参数隐式转换为字符串类型。

当SQL Server考虑不使用持久值而是显式计算值的计划替代方案时，将添加警告。如果最终计划使用持久值，则不会删除该警告。

在这种情况下，可以安全地忽略警告。据我所知，这也适用于您的执行计划-涉及的隐式转换CONCAT不会对计划选择产生不利影响。

使用未记录且不受支持的跟踪标志176可以防止持久化计算列扩展并删除警告：

SELECT * 
FROM dbo.Test AS T
OPTION (QUERYTRACEON 176);

与tf 176

有关更多详细信息，请参见我的文章“ 适当地保留计算列 ”。

— 保罗·怀特9
source
5

这些是您收到有关以下内容的隐式转换警告的字段：

  • [ccd].[profileid] （int到varchar（12））
  • [ccd].[nodeid] （smallint到varchar（6））
  • [ccd].[sessionseqnum] （smallint到varchar（6））
  • [ccd].[sessionid] （十进制为varchar（41））

引用的字段不是查询中任何参数/过滤器的一部分

当然可以，在您加入的条件下。ccd.profileid用作过滤器的位置（以及与agentconnectiondetail的连接中）：

FROM contactcalldetail ccd 
    INNER JOIN contactqueuedetail csqd 
        ON ccd.sessionID=csqd.sessionid 
            AND ccd.sessionSeqNum=csqd.sessionSeqNum 
            AND ccd.nodeID=csqd.nodeID 
            AND ccd.profileid=csqd.profileid -- Right here

并且在所有涉及的表中列数据类型都相同

您可能需要仔细检查以下表的定义：

  • contactcalldetail.profileid
  • contactqueuedetail.profileid
  • agentconnectiondetail .profileid

听起来他们没有使用您认为正在使用的数据类型。

这真的会影响基数估算吗？

根据您提供的信息，我的答案中有些猜测。我鼓励您将实际的执行计划和表定义添加到您的问题中，以便我们掌握可能涉及这些隐式转换问题的所有详细信息。

通常，对联接条件的隐式转换可能会导致严重的估计问题。如果不查看实际的执行计划，很难说这是否在您的情况下发生。

— 乔什·达内尔
source
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