Message-ID: <5fbbebb78a9e5ecea8adbbc1ba720dd0ca6d5205.camel@j-davis.com>
Subject: Re: Document NULL
From: Jeff Davis <pgsql@j-davis.com>
To: "David G. Johnston" <david.g.johnston@gmail.com>
Cc: Yugo NAGATA <nagata@sraoss.co.jp>, Tom Lane <tgl@sss.pgh.pa.us>, Peter
 Eisentraut <peter@eisentraut.org>, David Rowley <dgrowleyml@gmail.com>,
 PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: Tue, 10 Dec 2024 10:52:53 -0800
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On Mon, 2024-12-09 at 15:27 -0700, David G. Johnston wrote:
> I have not done this.=C2=A0 This is already a large patch and this kind o=
f
> example doesn't seem like our norm.=C2=A0 I'm not opposed to more content
> like this but for now would leave considering it as something an
> interested party can propose once this goes in.

Fair enough

Though I think it's a great example and I'd like to find some place to
put it.

>=20
> =C2=A0 =C2=A0<para>
> =C2=A0 =C2=A0 The presence of null values in the system results in three-=
valued
> logic.
> =C2=A0 =C2=A0 In conventional two-valued (binary) logic every outcome is =
either
> true or false.
> =C2=A0 =C2=A0 In three-valued logic the concept of unknown, represented u=
sing
> the null value, is
> =C2=A0 =C2=A0 also an outcome.=C2=A0 This results in falsifying the commo=
n-sense
> notion
> =C2=A0 =C2=A0 that "p OR NOT p" is always true.
> =C2=A0 =C2=A0</para>

Thank you.

I might reword the final sentence as more of an example, like: "Unknown
values can lead to surprising behavior, for instance "NULL OR NOT NULL"
evaluates to the null value."

>=20
> When executing an aggregate or window function the state tracking
> component
> =C2=A0 =C2=A0(which may be initialized to a non-null value, e.g., 0 for t=
he
> count function)
> =C2=A0 =C2=A0will remain unchanged even if the underlying processing
> =C2=A0 =C2=A0function returns a null value, whether from being defined st=
rict
> =C2=A0 =C2=A0or it simply returns a null value upon execution.

Thank you.

> Since count doesn't have an input function to check the only way to
> see zero such rows is if the underlying thing being counted is empty.

While true for COUNT(*), technically that's incorrect for COUNT(x),
which counts the rows for which x is non-null. That doesn't invalidate
your point, though: the initial state is unchanged either way.

Regards,
	Jeff Davis