Re: How hints affect the CBO?

If still in doubt, set the 10053 event (CBO Enable optimizer trace) and see for yourself the choices the optimizer makes...

Good luck !

Sur Thu, 27 Jun 2002 15:26:53 +0400, "Vladimir M. Zakharychev" <bob_at_dpsp-yes.com> a écrit:
> Niall,
>
> thanks for responce and doc link.
>
> > > That is, no matter how inneffective a query with FULL hint is, CBO will
> > > always do full scan even if index access yelds better plan, because it is
> > > explicitly instructed to do so and this is valid access path.
> >
> > In this particular case then this is correct (at least on 9.2 but I believe
> > it applies to 817 etc as well. )
> >
>
> Well, FULL hint was just a bold example. What I was really interested in
> is if all other access path hints exhibit the same behavior if they specify
> valid access path, that is, they will necessarily be honored by the CBO if
> the path is still valid (for example, INDEX(t idx) and idx exists and all
> stats are there). Or if there is a clear example of otherwise. I didn't
> manage to create one, but I still think this is not impossible unless valid
> hints are treated as direct orders which CBO must obey (and if it doesn't
> then this is either not a valid hint or a bug in CBO).
>
> --
> Vladimir Zakharychev (bob@dpsp-yes.com) http://www.dpsp-yes.com
> Dynamic PSP(tm) - the first true RAD toolkit for Oracle-based internet applications.
> All opinions are mine and do not necessarily go in line with those of my employer.
>
>

Christian GILBERT
SNPI DTO/SGBD Oracle
01 55 88 41 21 Received on Thu Jun 27 2002 - 09:58:58 CDT