Re: incorrect information in documentation
| От | David G. Johnston |
|---|---|
| Тема | Re: incorrect information in documentation |
| Дата | |
| Msg-id | CAKFQuwaV87e4sdFdZab4+zcUdza+EpveQi_98=f9wJ1+QLUALw@mail.gmail.com обсуждение исходный текст |
| Ответ на | Re: incorrect information in documentation (Bruce Momjian <bruce@momjian.us>) |
| Ответы |
Re: incorrect information in documentation
|
| Список | pgsql-docs |
On Mon, Aug 9, 2021 at 11:05 AM Bruce Momjian <bruce@momjian.us> wrote:
> selectivity = (1 - null_frac1) * (1 - null_frac2) * min(1/
> num_distinct1,
> 1/num_distinct2)
> = (1 - 0) * (1 - 0) / max(10000, 10000)
> = 0.0001
Nice, can you provide a patch please?
Change the line:
selectivity = (1 - null_frac1) * (1 - null_frac2) * min(1/num_distinct1, 1/num_distinct2)
to be:
selectivity = (1 - null_frac1) * (1 - null_frac2) / max(num_distinct1, num_distinct2)
The wording already talks about "divide by max".
Though:
"so we use an algorithm that relies only on the number of distinct values for both relations together with their null fractions:"
maybe adds a parenthetical note:
"so we use an algorithm that relies only on the number of distinct values (the row count estimate for the whole table, not the -1 in the column statistics) for both relations together with their null fractions:"
Just note I haven't tried to absorb that whole page, let alone the implementation, and am not all that familiar with this part of PostgreSQL. Its seems right, though, in isolation.
David J.
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