我正在查看以下链接中的过滤统计信息。
http://blogs.msdn.com/b/psssql/archive/2010/09/28/case-of-using-filtered-statistics.aspx
数据严重倾斜，一个区域有 0 行，其余都来自不同的区域。以下是重现该问题的完整代码
create table Region(id int, name nvarchar(100))
go
create table Sales(id int, detail int)
go
create clustered index d1 on Region(id)
go
create index ix_Region_name on Region(name)
go
create statistics ix_Region_id_name on Region(id, name)
go
create clustered index ix_Sales_id_detail on Sales(id, detail)
go
-- only two values in this table as lookup or dim table
insert Region values(0, 'Dallas')
insert Region values(1, 'New York')
go
set nocount on
-- Sales is skewed
insert Sales values(0, 0)
declare @i int
set @i = 1
while @i <= 1000 begin
insert Sales values (1, @i)
set @i = @i + 1
end
go
update statistics Region with fullscan
update statistics Sales with fullscan
go
set statistics profile on
go
--note that this query will over estimate
-- it estimate there will be 500.5 rows
select detail from Region join Sales on Region.id = Sales.id where name='Dallas' option (recompile)
--this query will under estimate
-- this query will also estimate 500.5 rows in fact 1000 rows returned
select detail from Region join Sales on Region.id = Sales.id where name='New York' option (recompile)
go
set statistics profile off
go
create statistics Region_stats_id on Region (id)
where name = 'Dallas'
go
create statistics Region_stats_id2 on Region (id)
where name = 'New York'
go
set statistics profile on
go
--now the estimate becomes accurate (1 row) because
select detail from Region join Sales on Region.id = Sales.id where name='Dallas' option (recompile)
--the estimate becomes accurate (1000 rows) because stats Region_stats_id2 is used to evaluate
select detail from Region join Sales on Region.id = Sales.id where name='New York' option (recompile)
go
set statistics profile off
我的问题是我们在两张桌子上都有以下统计数据
sp_helpstats 'region','all'
sp_helpstats 'sales','all'
表区域：
statistics_name statistics_keys
d1 id
ix_Region_id_name id, name
ix_Region_name name
餐桌销售：
statistics_name statistics_keys
ix_Sales_id_detail id, detail
1.为什么下面这些查询的估计出错了
select detail from Region join Sales on Region.id = Sales.id where name='Dallas' option (recompile)
--the estimate becomes accurate (1000 rows) because stats Region_stats_id2 is used to evaluate
select detail from Region join Sales on Region.id = Sales.id where name='New York' option (recompile)
2.当我按照作者创建过滤统计信息时，我可以正确看到估计，但是为什么我们需要创建过滤统计信息，我怎么能说我需要过滤统计信息来进行查询，因为即使我创建了简单的统计信息，我也得到了相同的结果。
迄今为止我遇到的最好的 1.Kimberely tripp 歪曲统计视频
2.Technet 统计白皮书
但仍然无法理解为什么过滤的统计数据在这里有所作为
提前致谢。 更新：7/4
在马丁和詹姆斯回答之后改写问题：
1.除了kimberely脚本还有什么方法可以避免数据偏斜
，另一种估计方法是计算一个值的行数。
2.您是否遇到过任何数据偏斜问题。我认为这取决于大表。但我正在寻找一些详细的答案
3.我们必须为 sql 扫描表以及某些阻塞有时会在触发更新统计信息时下降的查询承担 IO 成本。在维护统计信息时，您是否看到除此之外的任何开销。
原因是我也在考虑基于 DTA 输入的几个条件创建过滤统计信息。
再次感谢