On Mon, Jan 09, 2006 at 09:54:44PM -0500, Tom Lane wrote: > Extrapolating from the observation that the heuristics don't work well > on your data to the conclusion that they don't work for anybody is not > good logic. Replacing that code with a flat 50% is not going to happen > (or if it does, I'll be sure to send the mob of unhappy users waving > torches and pitchforks to your door not mine ;-)). I'm not convinced but of course I cannot exclude that some people may be depending on this very heuristic. But I consider this being bug-compatible, I've an hard time to be convinced that such heuristic isn't going to bite other people like it did with me. > I did just think of something we could improve though. The pattern > selectivity code doesn't make any use of the statistics about "most > common values". For a constant pattern, we could actually apply the > pattern test with each common value and derive answers that are exact > for the portion of the population represented by the most-common-values > list. If the MCV list covers a large fraction of the population then > this would be a big leg up in accuracy. Dunno if that applies to your > particular case or not, but it seems worth doing ... Fixing this with proper stats would be great indeed. What would be the most common value for the kernel_version? You can see samples of the kernel_version here http://klive.cpushare.com/2.6.15/ . That's the string that is being searched against both PREEMPT and SMP. BTW, I also run a LIKE '%% SMP %%' a NOT LIKE '%% SMP %%' but that runs fine, probably because as you said in the first email PREEMPT is long but SMP is short. Thanks!
