Re: The CBO Lie Detector
- From: Wolfgang Breitling <breitliw@xxxxxxxxxxxxx>
- To: rshamsud@xxxxxxxxxxxx
- Date: Fri, 19 May 2006 07:24:08 -0700
Quoting Riyaj Shamsudeen <rshamsud@xxxxxxxxxxxx>:
>
> I don't think it is a good idea to detect the false information, without
> correcting some of the fundamental CBO discrepancies. In few cases, We
> have deliberately fed false information to get acceptable access plan
> and so, at least, there should be a way to override the lie detector.
I fully agree. Statistics are aggregates, averages, and as such do not always,
maybe even rarely, portray the correct picture (people have drowned in bodies of
water which were on average only a few inches deep). If the CBO draws wrong
conclusions from the "correct" statistics, maybe you can get the CBO to draw the
right conclusions from "doctored" statistics. Sometimes two wrongs do make a
right.
>
> One nagging gripe I have is that there is no way to tell CBO that there
> is a strong correlation exists between two different column predicates,
> at least that I know of. [J.Lewis has excellent presentation exploring
> this issue in detail ]. We have few such queries and we tried to use sql
> profiles in conjunction with cursor_sharing. Every time we hit one or
> other issue that we can not deal with it and finally resorted to feed
> "lies" to CBO.
The CBO is able to do that in many cases since Oracle 9i by setting dynamic
sampling to >= 4 (quoting from the 9.2 Performance Tuning Guide):
"..., plus all tables that have single-table predicates that reference 2 or more
columns."
See also Jeff Moss' blog at
http://oramossoracle.blogspot.com/2005/11/scd2s-and-their-affect-on-cbo-part-ii.html
--
regards
Wolfgang Breitling
Oracle 7,8,8i,9i OCP DBA
Centrex Consulting Corporation
www.centrexcc.com
--
//www.freelists.org/webpage/oracle-l
Other related posts:
