建索引（尤其是主键）时请注意索引列顺序

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。