Merge join IO cost.

USE tempdb
GO
CREATE TABLE #t1 (id INT NOT NULL PRIMARY KEY CLUSTERED, val INT NOT NULL)
 
CREATE TABLE #t2 (id INT NOT NULL, id2 INT NOT NULL PRIMARY KEY CLUSTERED (id, id2), val INT NOT NULL)
 
UPDATE STATISTICS #t1 WITH ROWCOUNT = 1000000
 
UPDATE STATISTICS #t2 WITH ROWCOUNT = 1000000
 
SELECT * FROM #t1 a INNER JOIN #t2 b ON a.id = b.id
 
SELECT * FROM #t1 a INNER JOIN #t1 b ON a.id = b.id
 
SELECT * FROM #t2 a INNER JOIN #t2 b ON a.id = b.id