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

On Thu, Jul 16, 2009 at 12:49 PM, Andres Freund<andres@anarazel.de> 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.

Ouch.  Did you insert some debugging code to get that information, or
how did you come to that conclusion?

...Robert