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

On Thu, Jul 16, 2009 at 04:27:39PM +0100, Greg Stark wrote:
> On Thu, Jul 16, 2009 at 4:16 PM, Tom Lane<tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> > However, I do observe that this seems a sufficient counterexample
> > against the theory that we can just remove the collapse limits and let
> > GEQO save us on very complex queries. ?On my machine, the example query
> > takes about 22 seconds to plan using CVS HEAD w/ all default settings.
> > If I set both collapse_limit variables to very high values (I used 999),
> > it takes ... um ... not sure; I gave up waiting after half an hour.
>
> What's the point of GEQO if it doesn't guarantee to produce the
> optimal plana and *also* doesn't guarantee to produce some plan, any
> plan, within some reasonable amount of time? Either we need to fix
> that or else I don't see what it's buying us over our regular planner
> which also might not produce a plan within a reasonable amount of time
> but at least if it does it'll be the right plan.
>

I do agree that we should have an actually time limit cap for
GEQO that would have it return the best plan so far at that time.
Then you can at least bound your planning time.

Regards,
Ken