On Wed, 26 Mar 2003, Tom Lane wrote: > Stephan Szabo <[EMAIL PROTECTED]> writes: > > On 26 Mar 2003, Greg Stark wrote: > >> Can you show an example where the join order would affect the result set? I > >> can't think of any. > > > I can think of a few somewhat degenerate cases. > > I don't think you need a degenerate case. Consider > > from (G left join L on (G.SELID = L.SELID)) > right join C on (L.SELID = C.SELID) > > versus > > from G left join > (L right join C on (L.SELID = C.SELID)) > on (G.SELID = L.SELID)
I'd noticed that too, but I was trying to do it without changing the type of join since I wasn't sure whether he'd meant one using only left joins or outer joins in general. > The difference between this and the query we were discussing is just use > of "right" rather than "left" in the second JOIN. AIUI, in the Oracle > syntax this difference would be expressed by moving the (*) from one > side to the other of the L.SELID = C.SELID clause. Unless they have > strange restrictions on the combinations of clauses you can mark with > (*), I don't see how they can assume that join order is insignificant. I'd guess that they might check that the conditions are in a specific set of limited conditions in order to allow the reordering and disallow the reordering otherwise (well, I'd hope that they do this if they ever allow reordering). ---------------------------(end of broadcast)--------------------------- TIP 1: subscribe and unsubscribe commands go to [EMAIL PROTECTED]
