Dne 26.4.2011 07:35, Robert Haas napsal(a): > On Apr 13, 2011, at 6:19 PM, Tomas Vondra <tv@xxxxxxxx> wrote: >> Yes, I've had some lectures on non-linear programming so I'm aware that >> this won't work if the cost function has multiple extremes (walleys / >> hills etc.) but I somehow suppose that's not the case of cost estimates. > > I think that supposition might turn out to be incorrect, though. Probably > what will happen on simple queries is that a small change will make no > difference, and a large enough change will cause a plan change. On > complex queries it will approach continuous variation but why > shouldn't there be local minima? Aaaah, damn! I was not talking about cost estimates - those obviously do not have this feature, as you've pointed out (thanks!). I was talking about the 'response time' I mentioned when describing the autotuning using real workload. The idea is to change the costs a bit and then measure the average response time - if the overall performance improved, do another step in the same direction. Etc. I wonder if there are cases where an increase of random_page_cost would hurt performance, and another increase would improve it ... And I'm not talking about individual queries, I'm talking about overall performance. regards Tomas -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
