Re: Histogram question.

On Tue, Apr 5, 2022 at 7:35 AM Jian He <hejian.mark@gmail.com> wrote:

> Queries in PostgreSQL: 2. Statistics : Postgres Professional
> <https://postgrespro.com/blog/pgsql/5969296>
>
>
>
> SELECT sum(s.most_common_freqs[ array_position((s.most_common_vals::text::
> text[]),v) ]) FROM pg_stats s, unnest(s.most_common_vals::text::text[]) v
> WHERE s.tablename = 'boarding_passes' AND s.attname = 'seat_no';
>
> *return 0.6762. *
>
> SELECT sum(s.most_common_freqs[ array_position((s.most_common_vals::text::
> text[]),v) ]) FROM pg_stats s, unnest(s.most_common_vals::text::text[]) v
> WHERE s.tablename = 'boarding_passes' AND s.attname = 'seat_no' AND v >
> '30C';
>
> return *0.2127*
>
> SELECT round( reltuples * ( 0.2127 -- from most common values + (1 -
> 0.6762 - 0) * (49 / 100.0) -- from histogram )) FROM pg_class WHERE
> relname = 'boarding_passes';
>
> the above mentioned query, the part I don't understand is *49/100.*
>
>
I believe the exercise is intended to create a set of histograms based on
data values over a series of intervals. The 49/100 (if I'm reading the
source material correctly) refers to finding all the boarding passes in the
lower 49 of 100 intervals. I didn't bother to read what the interval
definition is, but I think that's what the "49" is referring to..