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

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Kohei KaiGai <kaigai(at)heterodb(dot)com>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: How to retain lesser paths at add_path()?
Date: 2019-07-31 16:41:39
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Robert Haas <robertmhaas(at)gmail(dot)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).

regards, tom lane

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