Re: How to retain lesser paths at add_path()?

2019年8月1日(木) 1:41 Tom Lane <tgl@sss.pgh.pa.us>:
>
> Robert Haas <robertmhaas@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,

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?