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From: "David G. Johnston" <david.g.johnston@gmail.com>
Date: Mon, 9 Aug 2021 09:06:16 -0700
Message-ID: 
 <CAKFQuwbHsQ14v3i4oEwPOmgEBqEFjG4i=WsuOMmP2q82myvdQQ@mail.gmail.com>
Subject: Re: incorrect information in documentation
To: ivanmulhin@gmail.com, Pg Docs <pgsql-docs@lists.postgresql.org>
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On Mon, Aug 9, 2021 at 8:02 AM PG Doc comments form <noreply@postgresql.org>
wrote:

> Hello, on page
> https://www.postgresql.org/docs/current/row-estimation-examples.html -
> there
> is a example:
> selectivity = (1 - null_frac1) * (1 - null_frac2) * min(1/num_distinct1,
> 1/num_distinct2)
>             = (1 - 0) * (1 - 0) / max(10000, 10000)
>             = 0.0001
> in the first string  " * min" and in the second " / max"
> as I understand it isn't correct.
>

Division is just multiplication by the reciprocal so while the presentation
here is inconsistent it is correct.  Likewise, the larger a number the
smaller its reciprocal, so the change from min to max also works.

David J.

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<div dir=3D"ltr"><div dir=3D"ltr"><div class=3D"gmail_default" style=3D"fon=
t-family:arial,helvetica,sans-serif"><span style=3D"font-family:Arial,Helve=
tica,sans-serif">On Mon, Aug 9, 2021 at 8:02 AM PG Doc comments form &lt;<a=
 href=3D"mailto:noreply@postgresql.org">noreply@postgresql.org</a>&gt; wrot=
e:</span><br></div></div><div class=3D"gmail_quote"><blockquote class=3D"gm=
ail_quote" style=3D"margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,=
204,204);padding-left:1ex">Hello, on page<br>
<a href=3D"https://www.postgresql.org/docs/current/row-estimation-examples.=
html" rel=3D"noreferrer" target=3D"_blank">https://www.postgresql.org/docs/=
current/row-estimation-examples.html</a> - there<br>
is a example:<br>
selectivity =3D (1 - null_frac1) * (1 - null_frac2) * min(1/num_distinct1,<=
br>
1/num_distinct2)<br>
=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =3D (1 - 0) * (1 - 0) / max(10000=
, 10000)<br>
=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =3D 0.0001<br>
in the first string=C2=A0 &quot; * min&quot; and in the second &quot; / max=
&quot; <br>
as I understand it isn&#39;t correct.<br></blockquote><div><br></div><div c=
lass=3D"gmail_default" style=3D"font-family:arial,helvetica,sans-serif">Div=
ision is just multiplication by the reciprocal so while the presentation he=
re is inconsistent it is correct.=C2=A0 Likewise, the larger a number the s=
maller its reciprocal, so the change from min to max also works.=C2=A0=C2=
=A0</div><div class=3D"gmail_default" style=3D"font-family:arial,helvetica,=
sans-serif"><br></div><div class=3D"gmail_default" style=3D"font-family:ari=
al,helvetica,sans-serif">David J.</div><div class=3D"gmail_default" style=
=3D"font-family:arial,helvetica,sans-serif"><br></div></div></div>

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