JOIN Hints: Why SQL Works As Follows?

Hi,

Why SQL server dose work as follows when I dose not provide any join hints?
It looks like HASH join is the best plan, but SQL dose not. What kind of
JOIN method is used by SQL optimizer?

Thanks in advance,
Wonhyuk William Chung
wonhyukc@usa.net
MCSE/ MCT



-----------
use northwind
go
select orderid, CompanyName --productname,
from orders o inner join customers c on o.customerID = c.CustomerID
/*
Table `Orders`. Scan count 91, logical reads 184, physical reads 0,
read-ahead reads 0.
Table `Customers`. Scan count 1, logical reads 1, physical reads 0,
read-ahead reads 0.
.0553
*/

select orderid, CompanyName --productname,
from orders o inner hash join customers c on o.customerID = c.CustomerID
/*
hash
Table `Customers`. Scan count 1, logical reads 1, physical reads 0,
read-ahead reads 0.
Table `Orders`. Scan count 1, logical reads 4, physical reads 0, read-ahead
reads 0.
.115
*/

select orderid, CompanyName --productname,
from orders o inner merge join customers c on o.customerID = c.CustomerID
/*
merge
Table `Customers`. Scan count 1, logical reads 4, physical reads 1,
read-ahead reads 3.
Table `Orders`. Scan count 1, logical reads 4, physical reads 0, read-ahead
reads 0.
.115
*/

select orderid, CompanyName --productname,
from orders o inner loop join customers c on o.customerID = c.CustomerID
/*
loop
Table `Customers`. Scan count 830, logical reads 1681, physical reads 0,
read-ahead reads 0.
Table `Orders`. Scan count 1, logical reads 5, physical reads 0, read-ahead
reads 0.
.116
*/