Re: hash join cardinality missestimate

Insufficient information

However you have a combination of two important factors in the join

1. All the columns explicitly referenced in the fragment of trace file have frequency histograms
2. 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_at_gmail.com> 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.
>

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