Re: Re: [COMMITTERS] pgsql: Rewrite GEQO's gimme_tree function so that it always finds a - Mailing list pgsql-hackers

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.


Andres