在执行计划中，“常量扫描”是什么意思？

[TheGodOfGods]

有分区视图sales_fact，执行删除数据操作

delete from sales_fact
 where date_key > 20001024 

后消息提示
表 'sales_fact_19990101'。扫描计数 0，逻辑读取 0 次...
表 'sales_fact_20000101'。扫描计数 1，逻辑读取 2 次...

其中，sales_fact_19990101保存date_key在19990101 和 19991231之间的数据
sales_fact_20000101保存date_key在20000101 和 20001231之间的数据
现在问题是：删除19991231以后的数据，为什么会扫描sales_fact_19990101， 而且扫描计数为0？
执行计划里边显示对表sales_fact_19990101扫描时为“常量扫描”，这是什么意思？


表，分区视图，测试代码如下

if	object_id('[dbo].[sales_fact_19990101]') is not null
drop table [dbo].[sales_fact_19990101]
if	object_id('[dbo].[sales_fact_20000101]') is not null
drop table [dbo].[sales_fact_20000101]
if	object_id('[dbo].[sales_fact_20010101]') is not null
drop table [dbo].[sales_fact_20010101]
CREATE TABLE [dbo].[sales_fact_19990101] (
   [date_key] [int] NOT NULL 
CHECK ([date_key] BETWEEN 19990101 AND 19991231),
   [product_key] [int] NOT NULL ,
   [customer_key] [int] NOT NULL ,
   [promotion_key] [int] NOT NULL ,
   [store_key] [int] NOT NULL ,
   [store_sales] [money] NULL ,
   [store_cost] [money] NULL ,
   [unit_sales] [float] NULL 
) 
ALTER TABLE [sales_fact_19990101]
ADD PRIMARY KEY (
[date_key], [product_key], [customer_key], [promotion_key], [store_key])
;
-- 创建 2000 年事实表
CREATE TABLE [dbo].[sales_fact_20000101] (
   [date_key] [int] NOT NULL 
CHECK ([date_key] BETWEEN 20000101 AND 20001231),
   [product_key] [int] NOT NULL ,
   [customer_key] [int] NOT NULL ,
   [promotion_key] [int] NOT NULL ,
   [store_key] [int] NOT NULL ,
   [store_sales] [money] NULL ,
   [store_cost] [money] NULL ,
   [unit_sales] [float] NULL 
) 
ALTER TABLE [sales_fact_20000101]
ADD PRIMARY KEY (
[date_key], [product_key], [customer_key], [promotion_key], [store_key])
;
-- 创建 2001 年事实表
CREATE TABLE [dbo].[sales_fact_20010101] (
   [date_key] [int] NOT NULL 
CHECK ([date_key] BETWEEN 20010101 AND 20011231),
   [product_key] [int] NOT NULL ,
   [customer_key] [int] NOT NULL ,
   [promotion_key] [int] NOT NULL ,
   [store_key] [int] NOT NULL ,
   [store_sales] [money] NULL ,
   [store_cost] [money] NULL ,
   [unit_sales] [float] NULL 
) 
ALTER TABLE [sales_fact_20010101]
ADD PRIMARY KEY (
[date_key], [product_key], [customer_key], [promotion_key], [store_key])
;

--创建视图
IF  EXISTS (SELECT * FROM sys.views WHERE object_id = OBJECT_ID(N'[dbo].[sales_fact]'))
DROP VIEW [dbo].[sales_fact]
go
Create VIEW [dbo].[sales_fact]
as
select * from [dbo].[sales_fact_20010101]
union all 
select * from [dbo].[sales_fact_20000101]
union all 
select * from [dbo].[sales_fact_19990101]
go


INSERT INTO [sales_fact]
VALUES (19990125, 347, 8901, 0, 13, 5.3100, 1.8585, 3.0)

INSERT INTO [sales_fact]
VALUES (19990324, 576, 7203, 0, 13, 2.1000, 0.9450, 3.0)

INSERT INTO [sales_fact]
VALUES (19990604, 139, 7203, 0, 13, 5.3700, 2.2017, 3.0)

INSERT INTO [sales_fact]
VALUES (20000914, 396, 8814, 0, 13, 6.4800, 2.0736, 2.0)

INSERT INTO [sales_fact]
VALUES (20001113, 260, 8269, 0, 13, 5.5200, 2.4840, 3.0)
select * from sales_fact
 where date_key > 20000324 
delete from sales_fact
 where date_key > 20001024 

[--小F--]

常量扫描会引入一个或者多个常量行到一个查询中；通常情况下紧跟常量扫描的是计算标量运算符，计算标量运算符会为常量扫描运算符产生的行添加列。

[TheGodOfGods]

多谢小F,不过还是不清除为什么删除数据时会有对表sales_fact_19990101的常量扫描操作
当前删除操作也不会删除表sales_fact_19990101中任何数据。

---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  |--Sequence
       |--Clustered Index Delete(OBJECT:([YMW].[dbo].[sales_fact_20000101].[PK__sales_fact_20000__52593CB8]), WHERE:([YMW].[dbo].[sales_fact_20000101].[date_key] > (20001024)))
       |--Clustered Index Delete(OBJECT:([YMW].[dbo].[sales_fact_19990101].[PK__sales_fact_19990__4F7CD00D]))
            |--Constant Scan