Re: incorrect information in documentation

On Mon, Aug 9, 2021 at 11:05 AM Bruce Momjian <bruce(at)momjian(dot)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.