为什么在此查询中使用聚簇索引扫描？

Question

该查询（SQL2012）执行计划显示我，聚集索引扫描在内部子查询中使用对PK指数：

SELECT n3.id as node_id,x.id as id, 
    (select xv.value from xv 
    --with(forceseek) 
    where xv.id=x.id) as [value] 
    FROM x 
    INNER JOIN n3 
    ON x.[obj_id]=n3.id 
    AND n3.parent_id = '975422E0-5630-4545-8CF7-062D7DF72B6B' 

的表格x和XV是主 - >细节的表格。

当我使用提示forceseek时，它显示聚集索引查找和查询执行速度很快。 为什么有Scan而不是Seek？ 如何更改查询以使索引搜索没有提示FORCESEEK？

UPD： 完整的演示脚本：

SET ANSI_NULLS ON 
GO 

SET QUOTED_IDENTIFIER ON 
GO 
/* 
DROP TABLE [dbo].[xv] 
DROP TABLE [dbo].[x] 
DROP TABLE [dbo].[n3] 
*/ 

CREATE TABLE [dbo].[n3](
    [id] [uniqueidentifier] NOT NULL, 
    [parent_id] [uniqueidentifier] NOT NULL, 
CONSTRAINT [PK_n3] PRIMARY KEY CLUSTERED 
(
    [id] ASC 
) 
) 

GO 

CREATE TABLE [dbo].[x](
    [obj_id] [uniqueidentifier] NOT NULL, 
    [id] [int] IDENTITY(1,1) NOT NULL, 
CONSTRAINT [PK_x] PRIMARY KEY CLUSTERED 
(
    [id] ASC 
)) 

GO 

ALTER TABLE [dbo].[x] WITH CHECK ADD CONSTRAINT [FK_x_n3] FOREIGN KEY([obj_id]) 
REFERENCES [dbo].[n3] ([id]) 
ON DELETE CASCADE 
GO 

ALTER TABLE [dbo].[x] CHECK CONSTRAINT [FK_x_n3] 
GO 

CREATE TABLE [dbo].[xv](
    [id] [int] NOT NULL, 
    [value] [sql_variant] NOT NULL, 
CONSTRAINT [PK_xv] PRIMARY KEY CLUSTERED 
(
    [id] ASC 
)) 

GO 

ALTER TABLE [dbo].[xv] WITH CHECK ADD CONSTRAINT [FK_xv_x] FOREIGN KEY([id]) 
REFERENCES [dbo].[x] ([id]) 
ON DELETE CASCADE 
GO 

ALTER TABLE [dbo].[xv] CHECK CONSTRAINT [FK_xv_x] 
GO 

INSERT INTO n3(id,parent_id) 
select newid(), '975422E0-5630-4545-8CF7-062D7DF72B6B' 
GO 10 

INSERT INTO n3(id,parent_id) 
select newid(), '805422E0-5630-4545-8CF7-062D7DF72B6B' 
GO 5 

INSERT INTO x([obj_id]) 
select id from n3 where parent_id='975422E0-5630-4545-8CF7-062D7DF72B6B'; 


insert into xv (id, value) 
select id, cast(RAND(1) as sql_variant) from x 

--select * from x 
--select * from n3 

    SELECT n3.id as node_id,x.id as id, 
    (select xv.value from dbo.xv 
    --with(forceseek) 
    where xv.id=x.id 
) as [value] 
    FROM dbo.x 
    INNER JOIN dbo.n3 
    ON x.[obj_id]=n3.id 
    AND n3.parent_id = '975422E0-5630-4545-8CF7-062D7DF72B6B' 


/* 
DROP TABLE [dbo].[xv] 
DROP TABLE [dbo].[x] 
DROP TABLE [dbo].[n3] 
*/ 

--Update statistics xv with fullscan 

Answer 1

我怀疑statisticsxv的表可能是过时的。更新xv的统计信息并尝试再次运行查询。

Update statistics xv with fullscan 

更新：

看数据建立和查询后，对于给定的parent_id输入是很清楚，在这两种x和xv匹配的所有记录，这样很明显，优化选择索引扫描，而不是寻找，因为它已经从x和xv获取所有记录表

此外，记录数量更少，所以优化器将更喜欢扫描而不是查找