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 so2. 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