Re: *_collapse_limit, geqo_threshold

On Wed, Jul 08, 2009 at 04:13:11PM -0500, Kevin Grittner wrote:
> Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> > It occurs to me that one way to make GEQO less scary would be to
> > take out the nondeterminism by resetting its random number generator
> > for each query. You might get a good plan or an awful one, but at
> > least it'd be the same one each time. DBAs like predictability.
>
> +1 The biggest reason that I've tended to avoid geqo is that I would
> never know when it might do something really stupid with a query one
> time out of some large number, leading to mysterious complaints which
> could eat a lot of time.
>
> For a moment it seemed logical to suggest a session GUC for the seed,
> so if you got a bad plan you could keep rolling the dice until you got
> one you liked; but my right-brain kept sending shivers down my spine
> to suggest just how uncomfortable it was with that idea....
>
> -Kevin
>

+1 I like the idea of a session GUC for the random number seed. If
we can come up with a way to prune the search space more aggressively,
GECO (or GECO2) will be much less prone to generating a bad plan.
I also think that a session variable would make it easier to test
GECO* by removing the nondeteminism.

Ken