Re: Re: [COMMITTERS] pgsql: Rewrite GEQO's gimme_tree function so that it always finds a
От | Robert Haas |
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Тема | Re: Re: [COMMITTERS] pgsql: Rewrite GEQO's gimme_tree function so that it always finds a |
Дата | |
Msg-id | 603c8f070911090728k7854623cg1e961027c5294064@mail.gmail.com обсуждение исходный текст |
Ответ на | Re: Re: [COMMITTERS] pgsql: Rewrite GEQO's gimme_tree function so that it always finds a (Andres Freund <andres@anarazel.de>) |
Ответы |
Re: Re: [COMMITTERS] pgsql: Rewrite GEQO's gimme_tree function so that it always finds a
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Список | pgsql-hackers |
On Mon, Nov 9, 2009 at 10:27 AM, Andres Freund <andres@anarazel.de> wrote: > On Monday 09 November 2009 16:23:52 Robert Haas wrote: >> On Mon, Nov 9, 2009 at 10:21 AM, Andres Freund <andres@anarazel.de> wrote: >> > On Monday 09 November 2009 16:18:10 Robert Haas wrote: >> >> On Sun, Jul 19, 2009 at 4:00 PM, Tom Lane <tgl@postgresql.org> wrote: >> >> > Log Message: >> >> > ----------- >> >> > Rewrite GEQO's gimme_tree function so that it always finds a legal >> >> > join sequence, even when the input "tour" doesn't lead directly to >> >> > such a sequence. The stack logic that was added in 2004 only supported >> >> > cases where relations that had to be joined to each other (due to join >> >> > order restrictions) were adjacent in the tour. However, relying on a >> >> > random search to figure that out is tremendously inefficient in large >> >> > join problems, and could even fail completely (leading to "failed to >> >> > make a valid plan" errors) if >> >> > random_init_pool ran out of patience. It seems better to make the >> >> > tour-to-plan transformation a little bit fuzzier so that every tour >> >> > can form a legal plan, even though this means that apparently >> >> > different tours will sometimes yield the same plan. >> >> > >> >> > In the same vein, get rid of the logic that knew that tours >> >> > (a,b,c,d,...) are the same as tours (b,a,c,d,...), and therefore >> >> > insisted the latter are invalid. The chance of generating two tours >> >> > that differ only in this way isn't that high, and throwing out 50% of >> >> > possible tours to avoid such duplication seems more likely to waste >> >> > valuable genetic- refinement generations than to do anything useful. >> >> > >> >> > This leaves us with no cases in which geqo_eval will deem a tour >> >> > invalid, so get rid of assorted kluges that tried to deal with such >> >> > cases, in particular the undocumented assumption that DBL_MAX is an >> >> > impossible plan cost. >> >> > >> >> > This is all per testing of Robert Haas' >> >> > lets-remove-the-collapse-limits patch. That idea has crashed and >> >> > burned, at least for now, but we still got something useful out of it. >> >> > >> >> > It's possible we should back-patch this change, since the "failed to >> >> > make a valid plan" error can happen in existing releases; but I'd >> >> > rather not until it has gotten more testing. >> >> >> >> I think I just ran smack dab into this bug on 8.3.8 (RPM: >> >> postgresql-8.3.8-1.fc10.i386). I had a query that wasn't coming out >> >> very well with the default settings so I raised the collapse limits >> >> and let GEQO have a crack at it. This was not a rousing success. >> >> It didn't actually fail, but it did this sort of thing for a real long >> >> time. >> > >> > Yea. Seeing those backtraces all the time was what lead me to use 64bit >> > bitmapsets... >> > >> > The problem with that change is that it might change existing queries >> > that work well today to get very slow - I have one such case. Its just a >> > happenstance, but... >> Wait, which change can make existing queries slow? My original >> change, this fix by Tom, or 64-bit bitmapsets? > The fix by Tom - it completely changes which plans will get produced (Oh, well. > Your change did as well, but nobody thought of backpatching those) > > Although even the old plans were not really reproducable, so I guess my > argument isnt that strong. Well, we might want to look at your example then - this wasn't backpatched, but it's in HEAD. ...Robert
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