On Thu, Jul 26, 2012 at 9:42 AM, Marcus Engene <mengpg2@xxxxxxxxx> wrote: > Hi, > > I've read a little bit about join_collapse_limit and from_collapse_limit and > I see their reason to exist. > > A stupid question: in algorithms 101 you're usually told to make a chess > program and then you usually do a width first min max tree. A low level > opponent would interrupt this possibly infinite traversal early, thus > returning a possibly bad move, and if it's on a higher level it's allowed to > work longer and it will likely present a better path in the tree. > > I understood it as that the *_collapse_limits are to stop a worst case join > making the optimizer going haywire, but it feels sad that trivial big joins > are cut off even if they're not too nasty. > > Why would it not make some sense to have some time/space constraint on the > join heuristics instead of/in combination to how the limit presently work? > If we hit the ceiling, the best produced plan so far is used. The chess > analogy would obviously be a handful chess pieces left but the min-max-tree > traversal constraint is on a low depth (rather than time/memory) so it would > quickly traverse the few options and then be constrained. Well, isn't it the point of the genetic optimizer to solve exactly that problem? I do find it interesting though that there is a window between collapse limit and geqo. merlin -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
