On Monday, March 17, 2014, Tom Lane <
tgl@sss.pgh.pa.us> wrote:
Claudio Freire <klaussfreire@gmail.com> writes:
> On Mon, Mar 17, 2014 at 7:01 PM, Jim Nasby <jim@nasby.net> wrote:
>> Even better would be if the planner could estimate how bad a plan will
>> become if we made assumptions that turn out to be wrong.
> That's precisely what risk estimation was about.
Yeah. I would like to see the planner's cost estimates extended to
include some sort of uncertainty estimate, whereupon risk-averse people
could ask it to prefer low-uncertainty plans over high-uncertainty ones
(the plans we typically choose for ORDER BY ... LIMIT queries being great
examples of the latter). But it's a long way from wishing that to making
it so. Right now it's not even clear (to me anyway) how we'd measure or
model such uncertainty.
Most of the cases where I've run into horrible estimates, it seemed like the same level of knowledge/reasoning that could allow us to know it was risky, would allow us to just do a better job in the first place.
The exception I can think of is in an antijoin between two huge rels. It is like subtracting two large measurements to get a much smaller result. We should know the uncertainty will be large.
Cheers,
Jeff