子查詢，連線查詢，在SQL2008R2中效能如何？

以前或多或少的聽說過，能用子查詢的地方改用連線查詢，效能有提公升，如下的偽**

select *  from tab1 where col1 in (select col1 from tab2)

select * from tab1 inner
join tab2 on tab1.col1=tab2.col2

首先看看獨立子查詢的邏輯讀:

set

statistics io on;
use adventureworks
godbcc freeproccache
checkpoint
dbcc dropcleanbuffers
select
count(*) from sales.salesorderheader as soh
where soh.salesorderid in
(select salesorderid from sales.salesorderdetail as sod
where orderqty>1
and sod.productid in
(select productid from production.product as p
where p.color=n'red'))

(1 row(s) affected)

table 'worktable'. scan count 0, logical reads 0, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

table 'salesorderheader'. scan count 1,logical reads 45, physical reads 2, read-ahead reads 43, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

table 'salesorderdetail'. scan count 1, logical reads 1238, physical reads 35, read-ahead reads 1233, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

table 'product'. scan count 1, logical reads 15, physical reads 4, read-ahead reads 20, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

dbcc freeproccache

checkpoint
dbcc dropcleanbuffers
select
count(distinct soh.salesorderid)
from sales.salesorderheader as soh
join sales.salesorderdetail as sod
on soh.salesorderid=sod.salesorderid
and sod.orderqty>1
join production.product as p on
sod.productid=p.productid
and p.color=n'red'

(1 row(s) affected)

table 'worktable'. scan count 0, logical reads 0, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

table 'salesorderheader'. scan count 1,logical reads 45, physical reads 2, read-ahead reads 43, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

table 'salesorderdetail'. scan count 1,logical reads 1238, physical reads 35, read-ahead reads 1233, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

table 'product'. scan count 1,logical reads 15, physical reads 4, read-ahead reads 20, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

dbcc freeproccache

checkpoint
dbcc dropcleanbuffers
select
count(distinct soh.salesorderid)
from sales.salesorderheader as soh
join sales.salesorderdetail as sod
on soh.salesorderid=sod.salesorderid
and sod.orderqty>1
join production.product as p 
on sod.productid=p.productid
where p.color=n'red'

(1 row(s) affected)

table 'worktable'. scan count 0, logical reads 0, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

table 'salesorderheader'. scan count 1,logical reads 45, physical reads 2, read-ahead reads 43, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

table 'salesorderdetail'. scan count 1,logical reads 1238, physical reads 34, read-ahead reads 1233, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

table 'product'. scan count 1,logical reads 15, physical reads 4, read-ahead reads 20, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

從上可以得到的結論：

2：對於查詢是這樣，對於update/delete也是如此，子查詢和連線查詢在sql2008中沒有任何的區別！！

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