On Thu, Jun 01, 2006 at 03:15:09PM -0400, Tom Lane wrote: > These would all be nice things to know, but I'm afraid it's pie in the > sky. We have no reasonable way to get those numbers. (And if we could > get them, there would be another set of problems, namely plan instability: > the planner's choices would become very difficult to reproduce.)
Speaking of plan instability, something that's badly needed is the ability to steer away from query plans that *might* be the most optimal, but also will fail horribly should the cost estimates be wrong. People generally don't care about getting the absolutely most optimal plan; they do care about NOT getting a plan that's horribly bad. One possible way to do this would be to have the estimator calculate a worst-case cost to go along with the best case cost, or maybe even 3 numbers: ideal, what we think will happen, and worst-case. I know that index scans already have that information, at least in the form of cost for a plan with 0 correlation and one with perfect correlation. Does anyone have any experience developing genetic algorithms? I think coming up with cost estimators might be an ideal use for that technology... -- Jim C. Nasby, Sr. Engineering Consultant [EMAIL PROTECTED] Pervasive Software http://pervasive.com work: 512-231-6117 vcard: http://jim.nasby.net/pervasive.vcf cell: 512-569-9461 ---------------------------(end of broadcast)--------------------------- TIP 5: don't forget to increase your free space map settings