Re: Optimizer estimated cardinality very low

Nenand,

I am curious about the statement " The extremely low cardinality with extended statistics is actually expected in this case". Sounds like having extended stats on column groups where the NDV will be very high, like these tables which are highly correlated, would be detrimental for the optimizer's calculation; is that correct?

Using the hint to deactivate the extended stats did not change the outcome of very low cardinality estimation, although it seems to use a different method to calculate the selectivity of the semi join. I have tested not using extended stats, also removing histograms for the join columns and deactivating the extended stats without getting a more accurate result out of the semi join estimation. The histograms/extended stats help getting more accurate single table estimated cardinalities but they don't have the same impact on the join estimates in this case.

Would there be a way to force the sanity checks to kick in? Wondering if it can be tested.

Thanks again for your feedback,

On Sat, Oct 24, 2020 at 3:59 PM Noveljic Nenad <nenad.noveljic_at_vontobel.com> wrote:

> Daniel,
>
>
>
> The extremely low cardinality with extended statistics is actually
> expected in this case - NDVs(ORDR_RQST_NUMBER,ORDR_RQST_DATE) on both
> tables are very high resulting in extreme low selectivity.
>
>
>
> For two cases, i.e. "_optimizer_enable_extended_stats=false" and "June
> partition", sanity checks annulated this calculation to produce reasonable
> selectivites: 0.473 and 1, respectively.
>
>
>
> The open question is, why doesn't this sanity check kick in for also the
> July partition. Maybe some deviation threshold hasn't been reached.
>
>
>
> Generally, I'm not keen on hints, but, in this case, if order_reqst and
> order_detl are always highly correlated, it might be a good idea to
> deactivate extended stats per hint for this query rather than depend on
> sanity checks.
>
>
>
> Best regards,
>
>
>
> Nenad
>
> https://nenadnoveljic.com/blog
>
>
>
>
>
>
>
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