Re: CBO Predicate selectivity

Quoting amit poddar <amit.poddar_at_yale.edu>:

> 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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