On Thu, Aug 1, 2019 at 2:12 PM Kohei KaiGai <kai...@heterodb.com> wrote:
> 2019年8月1日(木) 1:41 Tom Lane <t...@sss.pgh.pa.us>: > > > > Robert Haas <robertmh...@gmail.com> writes: > > > Yeah, but I have to admit that this whole design makes me kinda > > > uncomfortable. Every time somebody comes up with a new figure of > > > merit, it increases not only the number of paths retained but also the > > > cost of comparing two paths to possibly reject one of them. A few > > > years ago, you came up with the (good) idea of rejecting some join > > > paths before actually creating the paths, and I wonder if we ought to > > > try to go further with that somehow. Or maybe, as Peter Geoghegan, has > > > been saying, we ought to think about planning top-down with > > > memoization instead of bottom up (yeah, I know that's a huge change). > > > It just feels like the whole idea of a list of paths ordered by cost > > > breaks down when there are so many ways that a not-cheapest path can > > > still be worth keeping. Not sure exactly what would be better, though. > > > > Yeah, I agree that add_path is starting to feel creaky. I don't > > know what to do instead though. Changing to a top-down design > > sounds like it would solve some problems while introducing others > > (not to mention the amount of work and breakage involved). > > > Hmm... It looks the problem we ought to revise about path construction > is much larger than my expectation, and uncertain for me how much works > are needed. > > Although it might be a workaround until fundamental reconstruction, > how about to have a margin of estimated cost to reject paths? > Current add_path() immediately rejects lesser paths if its cost is > even a little more expensive than the compared one. One the other hands, > Hmm.. I don't think so. Currently add_path() uses fuzzy comparisons on costs of two paths, although the fuzz factor (1%) is hard coded and not user-controllable. > I understand it is not an essential re-design of path-construction logic, > and > may have limitation. However, amount of works are reasonable and no side- > effect. (current behavior = 0% threshold). > How about your opinions? > > How's about Tom's suggestion on adding another dimension in add_path() to be considered, just like how it considers paths of better sort order or parallel-safe? Thanks Richard
