On Thu, Jul 2, 2009 at 12:48 PM, Euler Taveira de
Oliveira<euler@timbira.com> wrote:
> I know that it didn't solve the estimation problem but ... IMHO the
> [under|over]estimation should be treated by an external tool (autoexplain?).
> So when we enable the query progress and some node reports a difference
> between estimated and real more than x%, log the plan. Doing it, we will be
> helping DBAs to investigate the bad plans.
Keep in mind that it is frequently the case that the estimates are
substantially off but the plan still works OK. I just put a dirty
hack into one of my apps to improve the selectivity estimates by a
factor of 200, but they're still off by a factor of 5. Even when they
were off by 1000x the bad plan happened only intermittently. You
notice the cases where the estimates are off and it makes for a bad
plan, but there are lots of other cases where the estimates are off
but the plan is still OK.
...Robert
