主机:HP LH II
主频:330MHz
内存:128MB
操作系统:Oper server 5.0.4
数据库:Sybase 11.0.3
一、不合理的索引设计
例:表record有620000行,试看在不同的索引下,下面几个SQL的运行情况:
1.在date上建有一个非群集索引
select count(*) from record where date'19991201' and date'19991214'
and amount2000 --(25秒)
select date, sum(amount) from record group by date --(55秒)
select count(*) from record where date'19990901' and place in
('BJ','SH') --(27秒)
2.在date上的一个群集索引
select count(*) from record where date'19991201' and date'19991214'
and amount2000 --(14秒)
select date,sum(amount) from record group by date --(28秒)
select count(*) from record where date'19990901' and place in
('BJ','SH') --(14秒)
3.在place、date、amount上的组合索引
select count(*) from record where date'19991201' and date'19991214'
and amount2000 --(26秒)
select date,sum(amount) from record group by date --(27秒)
select count(*) from record where date'19990901' and place in
('BJ’,'SH') --(1秒)
4.在date、place、amount上的组合索引
select count(*) from record where date'19991201' and date'19991214'
and amount2000 --(1秒)
select date, sum(amount) from record group by date --(11秒)
select count(*) from record where date'19990901' and place in
('BJ','SH') --(1秒)
select * from record where substring(card_no,1,4)='5378' --(13秒)
select * from record where amount/301000 --(11秒)
select * from record where convert(char(10),date,112)='19991201' --
(10秒)
select * from record where card_no like '5378%' --(1秒)
select * from record where amount1000*30 --(1秒)
select * from record wheredate='1999/12/01' --(1秒)
你会发现SQL明显快起来!
例:表stuff有200000行,id_no上有非群集索引,请看下面这个SQL:
select count(*) from stuff where id_no in ('0','1') --(23秒)
分析:where条件中的'in'在逻辑上相当于'or',所以语法分析器会将in('0','1')转化为id_no='0' or id_no='1'来执行。我们期望它会根据每个or子句分别查找,再将结果相加,这样可以利用id_no上的索引;但实际上(根据showplan),它却采用了“OR策略",即先取出满足每个or子句的行,存入临时数据库的工作表中,再建立唯一索引以去掉重复行,最后从这个临时表中计算结果。因此,实际过程没有利用id_no上索引,并且完成时间还要受tempdb数据库性能的影响。
create proc count_stuff as
declare @a int
declare @b int
declare @c int
declare @d char(10)
begin
select @a=count(*) from stuff where id_no='0'
select @b=count(*) from stuff where id_no='1'
end
select @c=@a+@b
select @d=convert(char(10),@c)
print @d