Re: Optimizer estimated cardinality very low

Hello,

I have replied privately to Jonathan regarding the 10053 traces.

Nenad,

Thanks for sharing your case. The scenario itself with the estimation is very similar and as you have stated "a one-row estimation is often a cue for a completely wrong calculation." when in my case should have been in the order of millions. My thought is there is an issue on the join calculation (stats, optimizer code, combination of both, etc.)

For my environment, the version is 12.1 , directives are not in use and issue comes up with or without DS. I tried the workaround just in case it would be helpful and results were the same 1 row estimation.

Thanks again!

On Wed, Oct 14, 2020 at 9:17 AM Noveljic Nenad <nenad.noveljic_at_vontobel.com> wrote:

> A background information for my ask:
>
>
>
> I hit a similar problem once where the join cardinality=1 was a
> consequence of a wrong adjustment after dynamic sampling. The wrong
> adjustment bug was introduced with the bug fix 22817465 : AUTODOP
> CALCULATES DOP OF 1 FOR SPECIFIC QUERY. Therefore, the cardinality was
> correct after switching off this bug fix.
>
>
>
> The following unpublished bug was raised for this problem Bug 27989222 :
> WRONG CARDINALITY ESTIMATION WITH STALE DIRECTIVES WHEN FIX 21802552 IS
> ENABLED.
>
>
>
> I elaborated on this issue here:
> https://nenadnoveljic.com/blog/join-cardinality-misestimate/
>
>
>
> You might be hitting the same bug, as your plan is generated by using plan
> directives and dynamic sampling.
>
>
>
> Best regards,
>
>
>
> Nenad
>
>
>
>
>
> *From:* Noveljic Nenad
> *Sent:* Mittwoch, 14. Oktober 2020 10:49
> *To:* 'jlewisoracle_at_gmail.com' <jlewisoracle_at_gmail.com>; Daniel Coello <
> coello.daniel_at_gmail.com>
> *Cc:* Oracle L <Oracle-L_at_freelists.org>
> *Subject:* RE: Optimizer estimated cardinality very low
>
>
>
> Could you try with
>
>
>
> alter session set "_fix_control"='21802552:off';
>
> Best regards,
>
>
>
> Nenad
>
>
>
> https://nenadnoveljic.com/blog
>
>
>
>
>
>
>
> *From:* oracle-l-bounce_at_freelists.org <oracle-l-bounce_at_freelists.org> *On
> Behalf Of *Jonathan Lewis
> *Sent:* Mittwoch, 14. Oktober 2020 10:44
> *To:* Daniel Coello <coello.daniel_at_gmail.com>
> *Cc:* Oracle L <Oracle-L_at_freelists.org>
> *Subject:* Re: Optimizer estimated cardinality very low
>
>
>
>
>
> Daniel,
>
>
>
> I've lost count of the number of times I've said: "you hardly ever need to
> read the 10053 (CBO) trace file"; but in this case I think I need to see
> the 10053 trace file.
>
> I suspect the issue is something to do with the hybrid histogram and the
> fact that the July data is the top partition of the table but I can't think
> of anything that would make the cardinality estimate drop to 1.
>
>
>
> If you could enable event 10053 ('alter session set events '10053 trace
> name context forever'; and execute the june and july statements and send me
> the trace files (zip'ed would be good). Please start a fresh session for
> each query so that they are in different files, and to make sure that the
> optimizer does actually optimize them afresh (which is when the 10053 trace
> is generated) add small comment to each statement.
>
>
>
> Regards
>
> Jonathan Lewis
>
>
>
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