SQL Server Merge Join Operator – Part1

Hi Friends,

SQL Server Merge Join operator performs the inner join, left outer join, left semi join, left anti semi join, right outer join, right semi join, right anti semi join, and union logical operations and is a physical operator. A merge operator can be used only when both sets of rows are pre-sorted according to the join expression(s).

Let’s take an example of merge join operator.

USE [AdventureWorks2012]

FROM Sales.SalesOrderHeader SH
	INNER JOIN Sales.SalesOrderDetail SD
	ON SH.SalesOrderID = SD.SalesOrderID


According to the above execution plan, SQL Server query optimizer performed a Clustered Index Scan on Sales.SalesOrderHeader and Sales.SalesOrderDetail tables as we did not specify a WHERE clause. Query statement in example used clustered index to join two tables and since clustered index sorted in the order of clustered keys, it covers all queries and used to retrieve any column in the table that are specified in SELECT statement.

Key difference between a Nested Loop join and Merge join is, in a merge join both input operators are executed only once and we can verify this using ToolTip as shown below.




Bear in mind, when we are selecting all columns, both the tables needs to be loaded in memory and needs to send over network so there could be a performance overhead. This operator combines the advantage of hash match and nested loops which results in low CPU consumption and enables fast output of matched rows for further processing.

We are going discuss more on this operator tomorrow, stay tuned.



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About Kanchan Bhattacharyya

Kanchan is an astute IT professional, a seasoned SQL Database Administrator with 13+ years of industry experience. A calculated risk taker with deep technical knowledge and has gained proficiency in database consulting and solutions across different environments. Kanchan holds MBA degree in IT and an International Executive MBA in Project Management. He is very passionate about SQL Server and holds MCSE (Data Platform), MCSA – SQL 2012, MCITP – SQL 2008 certifications. Currently he is focusing on cloud and latest releases of SQL Server. When not working, Kanchan likes to spend his time reading on new technical developments specifically on SQL Server and other related technologies.

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