Re: Hints and RBO?

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 - 13:34:44 CDT