Re: CBO Predicate selectivity
- From: amit poddar <amit.poddar@xxxxxxxx>
- To: breitliw@xxxxxxxxxxxxx
- Date: Thu, 13 Jul 2006 09:50:47 -0400
Yes by changing num_rows we can get right cardinality.
But for single table access path oracle would still use a FTS.
Won't we have to adjust clustering factor and/or leaf_blocks of the
index on that table to make the CBO use the index
amit
Wolfgang Breitling wrote:
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.
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