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help / color / mirror / Atom feedFrom: Steve Midgley <[email protected]>
To: Jian He <[email protected]>
Cc: pgsql-sql <[email protected]>
Subject: Re: Histogram question.
Date: Tue, 5 Apr 2022 08:14:57 -0700
Message-ID: <CAJexoS+NPAb4BLRSRYP=qNwaq3A591D6run1Qwa85mV_10ehYA@mail.gmail.com> (raw)
In-Reply-To: <CAMV54g17LBTqs069dE5c1EmnGnJXmMtgCNxH5vesjfbJY8_D0g@mail.gmail.com>
References: <CAMV54g17LBTqs069dE5c1EmnGnJXmMtgCNxH5vesjfbJY8_D0g@mail.gmail.com>
On Tue, Apr 5, 2022 at 7:35 AM Jian He <[email protected]> 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..
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To: [email protected]
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Subject: Re: Histogram question.
In-Reply-To: <CAJexoS+NPAb4BLRSRYP=qNwaq3A591D6run1Qwa85mV_10ehYA@mail.gmail.com>
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