On Fri, 2007-09-21 at 13:53 -0400, Tom Lane wrote: > Simon Riggs <simon@xxxxxxxxxxxxxxx> writes: > > On Fri, 2007-09-21 at 12:08 -0400, Tom Lane wrote: > >> The reason you get a bad plan is that this rowcount estimate is so far > >> off: > > > That's true, but its not relevant, > > Yes it is --- the reason it wants to use a hashjoin instead of a > nestloop is exactly that it thinks the loop would iterate too many > times. (Ten is already too many in this case --- if it had estimated > five rows out of the join, it'd have gone with the nestloop, since > the cost estimate difference at the top level is less than 2x.) That's not my perspective. If the LIMIT had been applied accurately to the cost then the hashjoin would never even have been close to the nested join in the first place. It's just chance that the frequency distribution is favourable to us and thus amenable to using the hint of improving stats_target. The improved knowledge of the distribution just hides the fact that the cost model is still wrong: a cost of 45000 per row shows this. -- Simon Riggs 2ndQuadrant http://www.2ndQuadrant.com ---------------------------(end of broadcast)--------------------------- TIP 4: Have you searched our list archives? http://archives.postgresql.org
