Re: Multi-pass planner

On Aug 20, 2009, at 11:18 PM, Josh Berkus wrote:
>> I don't think it's a bad idea, I just think you have to set your
>> expectations pretty low. If the estimates are bad there isn't really
>> any plan that will be guaranteed to run quickly.
>
> Well, the way to do this is via a risk-confidence system. That is,
> each
> operation has a level of risk assigned to it; that is, the cost
> multiplier if the estimates are wrong. And each estimate has a
> level of
> confidence attached. Then you can divide the risk by the confidence,
> and if it exceeds a certain level, you pick another plan which has a
> lower risk/confidence level.
>
> However, the amount of extra calculations required for even a simple
> query are kind of frightning.

Would it? Risk seems like it would just be something along the lines
of the high-end of our estimate. I don't think confidence should be
that hard either. IE: hard-coded guesses have a low confidence.
Something pulled right out of most_common_vals has a high confidence.
Something estimated via a bucket is in-between, and perhaps adjusted
by the number of tuples.
--
Decibel!, aka Jim C. Nasby, Database Architect decibel(at)decibel(dot)org
Give your computer some brain candy! www.distributed.net Team #1828