sql Merge Join

if object_id('dbo.Table1') is not null drop table Table1 GO CREATE TABLE Table1 (Table1_id int primary key CLUSTERED, name char(10)) GO if object_id('dbo.Table2') is not null drop table Table2 GO CREATE TABLE Table2 ( Table2_id int primary key NONCLUSTERED, Table1_id int, name char(10)) GO CREATE CLUSTERED INDEX indTable2 ON Table2 (Table1_id) GO DECLARE @i int SELECT @i = 1 WHILE @i < 1000 BEGIN INSERT INTO Table1 VALUES (@i, LTRIM(str(@i))) SELECT @i = @i + 1 END GO DECLARE @i int SELECT @i = 1 WHILE @i < 1000 BEGIN INSERT INTO Table2 VALUES (@i, @i, LTRIM(str(@i))) SELECT @i = @i + 1 END GO SET SHOWPLAN_TEXT ON GO SELECT a.Table1_id, b.Table1_id FROM Table1 a INNER JOIN Table2 b ON a.Table1_id = b.Table1_id GO SET SHOWPLAN_TEXT OFF GO

This is the algorithm of the Merge join (the description of its work in general case, for many-to-many relationship):

while (not Table1.eof) and (not Table2.eof) do begin while Table2.Table1_id > Table1.Table1_id do Table1.MoveToNextRecord(); value = Table1.Table1_id; while Table2.Table1_id < value do Table2.MoveToNextRecord(); RID = Table1.RowID(); while Table2.Table1_id = value do begin while Table1.Table1_id = value do begin < SELECT Table1.Table1_id, Table2.Table1_id > Table1.MoveToNextRecord(); end Table1.MoveTo(RID); Table2.MoveToNextRecord(); end end

Note. If the joined tables are small (contain only one data page, for example), and at least one of the joined tables have index on the column that joins the tables, then SQL Server will use Nested-Loop join instead of Merge join or Hash join (usually).

Because the query optimizer usually selects the best execution plan for a given select statement, it is not necessary to enforce the desirable join type, but sometimes it can be useful. You can enforce the desirable join type by using the OPTION clause.

This is the example to enforce Merge join:

USE pubs GO SET SHOWPLAN_TEXT ON GO SELECT a.au_id FROM authors a JOIN titleauthor b ON a.au_id = b.au_id OPTION (MERGE JOIN) GO SET SHOWPLAN_TEXT OFF GO

This is the result:

StmtText ------------------------------------------------------------------------------------------------ SELECT a.au_id FROM authors a JOIN titleauthor b ON a.au_id = b.au_id OPTION (MERGE JOIN) (1 row(s) affected) StmtText ------------------------------------------------------------------------------------------------ |--Merge Join(Inner Join, MERGE:([a].[au_id])=([b].[au_id]), RESIDUAL:([a].[au_id]=[b].[au_id])) |--Clustered Index Scan(OBJECT:([pubs].[dbo].[authors].[UPKCL_auidind] AS [a]), ORDERED) |--Index Scan(OBJECT:([pubs].[dbo].[titleauthor].[auidind] AS [b]), ORDERED) (3 row(s) affected)