Thanks a lot Jonathan and Lothar for the useful information. Appreciate it.
În mar., 5 oct. 2021 la 11:13, Jonathan Lewis <jlewisoracle@xxxxxxxxx> a
scris:
Your assumption is correct IF:
It's a single column join with equality
There are no other predicates on the two tables
There are no histograms on the two columns
There are no nulls for either column
There are no SQL Plan Directives in place for the columns individually or
the join
You have histograms and at least one SQL Plan Directive.
Regards
Jonathan Lewis
On Tue, 5 Oct 2021 at 08:43, Laurentiu Oprea <laurentiu.oprea06@xxxxxxxxx>
wrote:
Thanks a lot Jonathan.
My understanding was that for join selectivity the computed sel value was
1/greatest(NDVs) where in my case I was expecting to be 1/(greatest(6,8),
, but looks like computed sel is 2.1058e-04 . Any level of dynamic
sampling(from 0 to 8) looks like it is not influencing this value.
So my assumption is wrong?
În mar., 5 oct. 2021 la 10:27, Jonathan Lewis <jlewisoracle@xxxxxxxxx> a
scris:
Insufficient information
However you have a combination of two important factors in the join
a) All the columns explicitly referenced in the fragment of trace file
have frequency histograms
b) There's an sql_plan_directive in force which is going to adjust any
other calculations the optimizer will do.
For the impact of a frequency histograms on joins you could look at a
series I wrote time ago. Here's a link to part 2, it refers back to part 1
and has pingbacks from parts 3, 4, and 5.:
https://jonathanlewis.wordpress.com/2018/10/05/join-cardinality-2/
For an understanding (though there's a lot of version dependency to
worry about) of SQL Plan Directives you could start with Stefan Koehler's
article, which includes several links to material by Mauro Pagano and
Franck Pachot.
https://blogs.sap.com/2015/06/01/oracle-db-optimizer-part-xii-revealing-sql-plan-directive-details-for-existingloaded-cursor-from-cbo-and-sql-dynamic-sampling-services-trace/
Regards
Jonathan Lewis
On Mon, 4 Oct 2021 at 20:14, Laurentiu Oprea <
laurentiu.oprea06@xxxxxxxxx> wrote:
Hello,
I have a big cardinality misestimate for a HJ operation:
Join cardinality for HJ/SMJ (no post filters): 28073805.777726, outer:
42.720832, inner: 7801742.651163, sel: 2.1058e-04
Join Card - Rounded: 70185 Computed: 70184.514444
Can someone help me understand how optimizer came with that sel value?
(attached 10053 extract)
Thank you.
