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

On Thursday 16 July 2009 17:59:58 Tom Lane wrote:
> Andres Freund <andres(at)anarazel(dot)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