一週間悩んで、やっと分かったのでメモ。 こういうSQLの場合、前者よりも後者の方が速いです。 SELECT * FROM FooTable LEFT OUTER JOIN BarTable1 ON FooTable.ID = BarTable1.ID LEFT OUTER JOIN BarTable2 ON BarTable1.F_ID = BarTable2.ID LEFT OUTER JOIN BarTable3 ON BarTable2.F_ID = BarTable3.ID LEFT OUTER JOIN BarTable4 ON BarTable3.F_ID = BarTable4.ID LEFT OUTER JOIN BarTable5 ON BarTable4.F_ID = BarTable5.ID WHERE FooTable.Name = ? OR BarTable5.N