Re: Optimizer estimated cardinality very low

Hi Nenand,

Thanks for the suggestion. Unfortunately, for our case, setting this parameter to false did not change the estimations.

Thanks,

On Fri, Oct 23, 2020 at 11:53 AM Noveljic Nenad <nenad.noveljic_at_vontobel.com> wrote:

> Daniel,
>
>
>
> I observed a problem similar to yours. The difference in my case is that
> I’m joining a partitioned table with a non-partitioned table. The column
> group is on the non-partitioned table. The NDV on a join columns in the
> partitioned table is significantly lower than in the non-partitioned table.
>
>
>
> selectivity differs by orders of magnitude depending on how much the
> column group NDV deviates from the product of selectivities of its member
> columns probably normalized by the correlation factor.
>
>
>
> Calculation for a low deviation: selectivity ~ 1/NDV_of_ColGroup =>
> wrongly results in a very low selectivity
>
> Calculation for a high deviation: selectivity =
> 1/NDV_of_join_column_in_partition (no nulls here) => correctly results in a
> higher selectivity
>
>
>
> With _optimizer_join_sel_sanity_check = false I’m always getting a good
> selectivity. Could you try it for your case?
>
>
>
> I’m still trying to figure out the denominator in the selectivity formula
> with partitioned tables.
>
>
>
> Best regards,
>
>
>
> Nenad
>
>
>
> https://nenadnoveljic.com/blog
>
>
>
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