Re: Vagaries of the CBO: Out-of-Range Predicates

It depends :( On version and type of query (and column stats). For an RTFB look at pages 50ff of Jonathan's CBO book (I assume that is the Bible to which you refer).

I do like your introduction with 'this is a seemingly simple situation'...

On 12/18/06, Anand Rao <panandrao_at_gmail.com> wrote:
>
> adding one more question,
>
> 3) How does CBO calculate the density in cases of bind variables when the
> value is out of range?
>
> or is this a RTFM / RTFB ... ??
>
> thanks
> anand
>
> On 18/12/06, Anand Rao <panandrao_at_gmail.com> wrote:
> >
> > Hi,
> >
> > Here is a seemingly simple situation. Database is 10.2.0.1.0 on AIX 5.3.
> >
> > i have a query. Join of two tables (parent/child). CBO chooses the wrong
> > index into the child table.
> >
> > Parent table had 4 distinct tables.
> > Child table has 1 distinct value.
> >
> > The first execution of this query passes a value that is not present
> > (out-of-range value) in the child table.
> >
> > The next execution passes a value that exists in the table, and as
> > expected, oracle doesn't re-optimise the query - thanks to Shared SQL and
> > probably Bind Peeking too.
> >
> > there are no histograms on the table. only normal stats.
> >
> > I found Bug No: 3663924, 5140413 which are the closest matches.
> >
> > It says i need histograms for the column AND also setting the following
> > event (if it is 9.2.x database),
> >
> > SQL> alter session set events '38071 trace name context forever';
> >
> > <https://metalink.oracle.com/metalink/plsql/ml2_documents.showDocument?p_id=3663924&p_database_id=BUG>
> > But my table has an NDV of 1, so an histogram is not going to help.
> >
> > So, i hacked the NDV to be greater than 2 and got CBO to use the right
> > index.
> >
> > My question is,
> >
> > 1) Is hacking the NDV the only way out in this situation where my NDV is
> > 1 and there are no histograms?
> >
> > Reading Jonathan's CBO bible, i find some good pointers spread across
> > chapter 4 and 7. still pouring over it...
> >
> > Oracle has some smart capability to compare the bind variables with
> > those used to determine the execution plan. If the bind variables are too
> > different, a new plan is calculated.
> >
> > 2) How does the CBO determine that a bind variable is different to
> > another for the same query? is it just the datatype and size? there has to
> > be something more than that.
> >
> > thanks,
> > anand
> >
>
>

-- 
Niall Litchfield
Oracle DBA
http://www.orawin.info

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