Cheers for you help guys. Having filtered and then joined has substantially
reduced the run time.
On Mon, Nov 10, 2008 at 12:32 PM, Richard Huxton <dev(at)archonet(dot)com> 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
> > if that made no sense.
> Could make a difference.
> > In summary, what im trying to understand is the following: Will there be
> > performance difference between filtering query sets first and then
> > them together as opposed to joining first and then filtering? Does the
> > opitmiser not choose the best course of action either way yielding the
> > 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
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