我們做個試驗創建一個表，其中主鍵上的聚集索引按照id倒敘排列，然后分別倒敘順序select數據，比較select的時間：

＋測試代碼

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 條數據

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 '插入時間（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 '查詢時間(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 '查詢時間(毫秒)'

print datediff(millisecond,@t,getdate())

set nocount off;

以下是查詢時間結果

查詢時間(毫秒)

393

查詢時間(毫秒)

606

按照和索引相同順序從100萬條數據中取50條時需要393毫秒，相反順序時需要606毫秒。造成的性能影響還是挺大的。

結論：

在建索引時要考慮常用查詢的排序方式，在建主鍵時要特別注意，因為sql server會自動按照升序來建，這時候如果您的查詢多數用主鍵列倒敘排列，記得要修改一下默認的設置。