Re: Multi-pass planner

On Thu, Aug 20, 2009 at 12:55 PM, Kevin
Grittner<Kevin.Grittner@wicourts.gov> wrote:
> Robert Haas <robertmhaas@gmail.com> wrote:
>
>> I think one of the problems with the planner is that all decisions
>> are made on the basis of cost.  Honestly, it works amazingly well in
>> a wide variety of situations, but it can't handle things like "we
>> might as well materialize here, because it doesn't cost much and
>> there's a big upside if our estimates are off".  The estimates are
>> the world, and you live and die by them.
>
> ["thinking out loud"]
>
> If there were some reasonable way to come up with a *range* for cost
> at each step, a reasonable heuristic might be to cost the plan using
> minimum values and maximum values, and use the root mean square of the
> two for comparisons to other plans.  I don't know that we have a good
> basis to come up with ranges rather than absolute numbers, though.

Maybe.  The problem is that we have mostly two cases: an estimate that
we think is pretty good based on reasonable statistics (but may be way
off if there are hidden correlations we don't know about), and a wild
guess.  Also, it doesn't tend to matter very much when the estimates
are off by, say, a factor of two.  The real problem is when they are
off by an order of magnitude or more.

...Robert