Cheers for you help guys. Having filtered and then joined has substantially reduced the run time.
Much obliged, Sebastian On Mon, Nov 10, 2008 at 12:32 PM, Richard Huxton <[EMAIL PROTECTED]> wrote: > Sebastian Ritter wrote: > > Could it have something > > to do with the fact that it is a subquery and thus the planner can not > > deduce filtering conditions from the outer query against it? My > apologises > > if that made no sense. > > Could make a difference. > > > In summary, what im trying to understand is the following: Will there be > a > > performance difference between filtering query sets first and then > joining > > them together as opposed to joining first and then filtering? Does the > > opitmiser not choose the best course of action either way yielding the > same > > result? > > There obviously is a performance difference between joining all of the > issues table versus joining 1% of it to followups. > > In most cases the planner can push the condition into the subquery, but > not in all cases because: > 1. It's not provably correct to do so > 2. The planner isn't smart enough to figure out that it can > > It's impossible to say which applies to you without knowing the full query. > > -- > Richard Huxton > Archonet Ltd >
