Re: Hints and RBO?

Actually, if there are not statistics for all the tables in a query, Oracle uses Rule based optimizer. Guessing comes into play only when the optimizer has statistics with which to base an "educated" guess (OR7)!!

-Kevin
"oralap" <nospam_at_nospam.com> wrote in message news:lBIF5.66$B81.2839_at_iad-read.news.verio.net...
> If you have the optimizer mode set to CHOOSE, the CBO is in effect. If
 the
> tables have not been analyzed, then Oracle guesses at the statistics.
 It's
> not going to guess well, but it will respond to hints.
>
> For tips related to the CBO, see the Tuning section of
 http://www.lonyx.com.
>
> http://www.lonyx.com - the search engine that knows what a CBO is.
>
>
>
>
> Bill Buchan <wbuchan_at_uk.noha-systems.com> wrote in message
> news:39e73af8.52266_at_nntp.edi.noha-systems.com...
> > I'm confused. Please help me resolve the flaw in my logic (I have
> > RTFM but I'm missing the bit I want to understand!):
> >
> > I have set the optimizer mode to CHOOSE but have not gathered any
> > statistics. Therefore the optimizer will default to RULE based?
> >
> > However if I put HINTs in my SQL the EXPLAIN PLAN changes; therefore
> > the optimizer is responding to the hints.
> >
> > I thought that you could only use hints with CBO?
> >
> > Where have I gone wrong?
> >
> > Also, I presume that if the COST column in PLAN_TABLE is not null,
> > then statistics are being used by the CBO. My problem is that I
> > deleted all the statistics (ANALYZE all indexs,tables DELETE
> > STATISTICS) and there are still values appearing in this column when I
> > do a new EXPLAIN.
> >
> > Many thanks for any enlightenment (PS. v.8.0.6 on Solaris)
> > - Bill.
> >
>
>
Received on Fri Oct 13 2000 - 15:54:40 CDT