Re: Review remove {join,from}_collapse_limit, add enable_join_ordering

On Thu, Jul 16, 2009 at 06:49:08PM +0200, Andres Freund wrote:
> On Thursday 16 July 2009 17:59:58 Tom Lane wrote:
> > Andres Freund <andres@anarazel.de> writes:
> > > The default settings currently make it relatively hard to trigger geqo at
> > > all.
> >
> > Yes, and that was intentional.  One of the implications of what we're
> > discussing here is that geqo would get used a lot more for "typical
> > complex queries" (if there is any such thing as a typical one).  So
> > it's fully to be expected that the fallout would be pressure to improve
> > geqo in various ways.
> >
> > Given that we are at the start of the development cycle, that prospect
> > doesn't scare me --- there's plenty of time to fix whatever needs
> > fixing.  However, I am leaning to the feeling that I don't want to be
> > putting people in a position where they have no alternative but to use
> > geqo.  So adjusting rather than removing the collapse limits is seeming
> > like a good idea.
> Hm. I see a, a bit more fundamental problem with geqo:
> I tried several queries, and I found not a single one, where the whole 
> genetical process did any significant improvments to the 'worth'.
> It seems that always the best variant out of the pool is either the path 
> choosen in the end, or at least the cost difference is _really_ low.
> 
> 
> Andres
> 

Hi Andres,

From some of my reading of the literature on join order
optimization via random sampling, such as what would establish
the initial GEQO pool, there is a very good possibility of having
a "pretty good" plan in the first pool, especially for our larger
initial pool sizes of 100-1000. And in fact, the final plan has
a good chance of being of approximately the same cost as a member
of the initial pool. Uniform sampling alone can give you a close
to optimum plan 80% of the time with an initial sample size of
100. And using biased sampling raises that to 99% or better.

Regards,
Ken