SQL join-nested join

Overview

There are three main physical connection methods for databases: Nested join, merge join, and hash join. This article mainly talks about the nested join (NL). In NL, the two connected sets are respectively outertable and innertable.

 
Select * FromOrderdetailsAsOdInner Join [Order] AsOOnOD. order_id=O. ID
 

Here, the Order table is outertable and the orderdetaiils table is innertable. However, the database optimizer selects a small table as the outertable

SimpleAlgorithmDescription

First, retrieve a row in outertable and compare it with each row in innertable. If the row meets the conditions, return. Then outertable continues to fetch the next row, and compares it with each row in innertable...

PseudoCode(From http://blogs.msdn.com/ B /craigfr/archive/2006/07/26/nested-loops-join.aspx)

For each row R1 in the outer table
For each row R2 in the inner table
If R1 joins with R2
Return (R1,
R2)

In the most primitive mode, the cost of the algorithm = the number of outertable rows * The number of innertable rows

SQL ServerNo optimized nested join applied in

Test Data

View code

 If    Exists ( Select  *   From SYS. Objects Where   Object_id   =   Object_id (N '  [DBO]. [order]  '  ))  Drop   Table   [  DBO  ] . [  Order ]  Go  --  Order table  Create   Table DBO. [  Order  ]  (ID  Int  , Cus_name  Nvarchar ( 50  ));  Insert   Into  [  Order  ]   Select   1 , '  Mike  '  Union   All  Select   2 , '  Ben  '  Union   All Select   3 , '  Clare  '  If    Exists ( Select   *   From SYS. Objects Where   Object_id   =   Object_id (N '  [DBO]. [orderdetails] '  ))  Drop   Table   [  DBO  ] . [  Orderdetails  ]  Go  --  Order Details  Create   Table DBO. [  Orderdetails ]  (ID  Int  , Order_id  Int  , Good_name  Nvarchar ( 50  ));  Insert   Into  Orderdetails  Select   1 , 1 ,'  Hard Disk  '  Union   All  Select   2 , 1 , '  Display  '  Union   All  Select   3 , 2 ,'  IPod  '  Union   All  Select   4 , 4 , '  Food  ' 

Execute SQL

 --  Set statistics profile on: displays the time required for analysis, compilation, and query execution.  -- For more information, see http://www.cnblogs.com/qanholas/archive/2011/05/06/2038543.html.  Set   Statistics Profile On  Select  O. ID, O. cus_name, OD. good_name  From Orderdetails As  Od  Inner   Join   [  Order  ]   As OOn O. ID =  OD. order_id  Option (Loop Join ) --  Force optimizer to use nested join 

Result

We can see that

1> when running SQL Server (SQL 2008 is used for testing), the system automatically selects a small table as the outertable and a large table as the innertable;

2> the third row displays the outertable information, indicating that outertalbe only needs to be scanned once. The fourth row displays the innertable information, indicating that innertable needs to run n (n = the number of outertable rows) times, and returns N * m (M = innertalbe rows)

SQL ServerApply the nested join after INDEX OPTIMIZATION IN

Test data: continue to use the previous test data

Create an index: Create clustered index odoid on [orderdetails] (order_id)

Execute SQL:

Set StatisticsProfileOnSelectO. ID, O. cus_name, OD. good_nameFromOrderdetailsAsOdInner Join [Order] AsOOnO. ID=OD. order_idOption(LoopJoin)--Force optimizer to use nested join
 

Result:

We can see that

After innertable adds an index to the connected field, although outertable executes n times, innertable returns only the valid row for each outertable loop. Before the index is added, innertable needs to return all rows for each outertable loop, and then compare whether the join conditions are met.

Summary

Nested join applies to a small set (less than 2000 rows... As you can see on the Internet, you can practice it on your own.) As an outertable, a large collection (which can be larger than millions, and a large collection must be indexed on the connection conditions ), when these two sets are connected, you can use nested join.

Refer:

Http://www.cnblogs.com/RicCC/archive/2007/06/26/SQL-Server-Performance-Tuning-Nested-Loop-Merge-Hash-Join.html

Http://blogs.msdn.com/ B /craigfr/archive/2006/07/26/nested-loops-join.aspx