April 7th, 2013, 11:41 PM
Join Table Performance
Is it a big performance issue when i am joining my table to "virtual table"?
Select * From TableA as A
LEFT JOIN (Select * FROM TABLE A as B
LEFT JOIN TABLEC as C on B.ID =C.ID)as JoinedBC on A.ID=JoinedBC.ID
April 8th, 2013, 01:16 AM
Relatively speaking, yes it is, because the database engine doesn't have any indexes on the virtual table.
April 8th, 2013, 02:08 AM
you need a better example, because JoinedBC.ID is ambiguous
besides, in general the optimizer is smarter than you think