我们做个试验创建一个表，其中主键上的聚集索引按照id倒叙排列，然后分别倒叙顺序select数据，比较select的时间：
＋测试代码
if object_id('test_indexorder','U') is not null
begin
truncate table test_indexorder
drop table test_indexorder
end
go
create table test_indexorder
(
id int identity(1,1) not null,
name varchar(20) not null,
content varchar(50) not null,
co1 varchar(50),
co2 varchar(50),
co3 varchar(50),
co4 varchar(50),
co5 varchar(50),
constraint pk_testorder primary key clustered(
id desc
)
)
go
--insert 1000000 条数据
set nocount on;
declare @t datetime;
set @t = getdate();
DECLARE @cn int;
set @cn = 1000000;
while(@cn > 0)
begin
insert into test_indexorder(name,content,co1,co2,co3,co4,co5)
VALUES(
'name' + cast(@cn as varchar(10)),
cast(newid() as varchar(50)),
cast(newid() as varchar(50)),
cast(newid() as varchar(50)),
cast(newid() as varchar(50)),
cast(newid() as varchar(50)),
cast(newid() as varchar(50)));
set @cn = @cn -1;
end
print '插入时间（毫秒）：';
print datediff(millisecond,@t,getdate());
set nocount off;
GO
checkpoint
dbcc freeproccache
dbcc dropcleanbuffers
GO
go
set nocount on;
declare @t datetime;
set @t = getdate();
with t_rn as (
select *,rn = ROW_NUMBER() OVER (ORDER BY id desc) FROM test_indexorder
)
SELECT id,name,content,co1,co2,co3,co4,co5 from t_rn WHERE rn between 19007 and 19057;
print '查询时间(毫秒)'
print datediff(millisecond,@t,getdate())
set @t = getdate();
with t_rn as (
select *,rn = ROW_NUMBER() OVER (ORDER BY id asc) FROM test_indexorder
)
SELECT id,name,content,co1,co2,co3,co4,co5 from t_rn WHERE rn between 17007 and 17057;
print '查询时间(毫秒)'
print datediff(millisecond,@t,getdate())
set nocount off;
以下是查询时间结果
查询时间(毫秒)
393
查询时间(毫秒)
606
按照和索引相同顺序从100万条数据中取50条时需要393毫秒，相反顺序时需要606毫秒。造成的性能影响还是挺大的。
结论：
在建索引时要考虑常用查询的排序方式，在建主键时要特别注意，因为sql server会自动按照升序来建，这时候如果您的查询多数用主键列倒叙排列，记得要修改一下默认的设置。
