On Thu, Aug 20, 2009 at 12:55 PM, Kevin Grittner<kevin.gritt...@wicourts.gov> wrote: > Robert Haas <robertmh...@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 -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
