SQL优化—SQL查询效率，100w数据，查询只要1秒

https://www.cnblogs.com/ShaYeBlog/p/3227244.html

其他 测试

-- setp 1.
-- 建表
create table t_userinfo
(
userid int identity(1,1) primary key nonclustered,
nick varchar(50) not null default '',
classid int not null default 0,
writetime datetime not null default getdate()
)
go

-- 建索引
create clustered index ix_userinfo_classid on t_userinfo(classid)
go

-- step 2.

declare @i int 
declare @k int
declare @nick varchar(10)
set @i = 1
while @i<1000000
begin
set @k = @i % 10
set @nick = convert(varchar,@i)
insert into t_userinfo(nick,classid,writetime) values(@nick,@k,getdate())
set @i = @i + 1
end
-- 耗时 08:27 ，需要耐心等待

-- step 3.
select top 20 userid,nick,classid,writetime from t_userinfo 
where userid not in
(
select top 900000 userid from t_userinfo order by userid asc
)

-- 耗时 8 秒 ,够长的
-- step 4.
select a.userid,b.nick,b.classid,b.writetime from
(
    select top 20 a.userid from 
    (
    select top 900020 userid from t_userinfo order by userid asc
    ) a order by a.userid desc

) a 
inner join 
t_userinfo b 
on 
a.userid = b.userid 
order by a.userid asc

-- 耗时 1 秒，太快了吧，不可以思议

-- step 5 where 查询
select top 20 userid,nick,classid,writetime from t_userinfo where 1=1
and classid = 1 and userid not in
(
    select top 90000 userid from t_userinfo 
    where classid = 1
    order by userid asc
)
-- 耗时 2 秒
-- step 6 where 查询
select a.userid,b.nick,b.classid,b.writetime from
(
    select top 20 a.userid from 
    (
    select top 90000 userid from t_userinfo
    where classid = 1
    order by userid asc
    ) a order by a.userid desc
) a
 inner join 
 t_userinfo b 
on a.userid = b.userid 
order by a.userid asc

-- 查询分析器显示不到 1 秒.

--查询数据个数
select count(*) from t_userinfo     --999999


--测试 
select top 20 userid,nick,classid,writetime from t_userinfo 
where userid not in
(
select top 900000 userid from t_userinfo order by userid asc
)

-- setp 1.
-- 建表
create table t_userinfo
(
userid int identity(1,1) primary key nonclustered,
nick varchar(50) not null default '',
classid int not null default 0,
writetime datetime not null default getdate()
)
go

-- 建索引
create clustered index ix_userinfo_classid on t_userinfo(classid)
go

-- step 2.

declare @i int 
declare @k int
declare @nick varchar(10)
set @i = 1
while @i<1000000
begin
set @k = @i % 10
set @nick = convert(varchar,@i)
insert into t_userinfo(nick,classid,writetime) values(@nick,@k,getdate())
set @i = @i + 1
end
-- 耗时 08:27 ，需要耐心等待

-- step 3.
select top 20 userid,nick,classid,writetime from t_userinfo 
where userid not in
(
select top 900000 userid from t_userinfo order by userid asc
)

-- 耗时 8 秒 ,够长的
-- step 4.
select a.userid,b.nick,b.classid,b.writetime from
(
    select top 20 a.userid from 
    (
    select top 900020 userid from t_userinfo order by userid asc
    ) a order by a.userid desc

) a 
inner join 
t_userinfo b 
on 
a.userid = b.userid 
order by a.userid asc

-- 耗时 1 秒，太快了吧，不可以思议

-- step 5 where 查询
select top 20 userid,nick,classid,writetime from t_userinfo where 1=1
and classid = 1 and userid not in
(
    select top 90000 userid from t_userinfo 
    where classid = 1
    order by userid asc
)
-- 耗时 2 秒
-- step 6 where 查询
select a.userid,b.nick,b.classid,b.writetime from
(
    select top 20 a.userid from 
    (
    select top 90000 userid from t_userinfo
    where classid = 1
    order by userid asc
    ) a order by a.userid desc
) a
 inner join 
 t_userinfo b 
on a.userid = b.userid 
order by a.userid asc

-- 查询分析器显示不到 1 秒.

--查询数据个数
select count(*) from t_userinfo     --999999


--测试 
select top 20 userid,nick,classid,writetime from t_userinfo 
where userid not in
(
select top 900000 userid from t_userinfo order by userid asc
)