Re: CBO Predicate selectivity

Quoting amit poddar <amit.poddar@xxxxxxxx>:

> Hi,
> 
> "You say you can't change the query, so if you want to fake this query, 
> you probably need to use dbms_stats.set_table_stats
> to tell Oracle that the table has a very small number of rows -
> but don't change the block count - so that a properly calculated
> selectivity is fooled into getting the right cardinality."
> 
> Sorry for being dense, but how does changing num_rows change the 
> selectivity ?
> 

It doesn't. Note that Jonathan said "so that a properly calculated selectivity
is fooled into getting the right cardinality" i.e. the selectivity calculation
(5% or 5% of 5%) isn't changed but the resulting cardinality estimate (num_rows
* selectivity) is.

-- 
regards

Wolfgang Breitling
Oracle 7,8,8i,9i OCP DBA
Centrex Consulting Corporation
www.centrexcc.com

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