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From: Jeff Davis <pgsql@j-davis.com>
To: David G. Johnston <david.g.johnston@gmail.com>
Cc: Yugo NAGATA <nagata@sraoss.co.jp>
Cc: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Peter Eisentraut <peter@eisentraut.org>
Cc: David Rowley <dgrowleyml@gmail.com>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: Document NULL
Date: Tue, 10 Dec 2024 10:52:53 -0800
Message-ID: <5fbbebb78a9e5ecea8adbbc1ba720dd0ca6d5205.camel@j-davis.com> (raw)
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	<505f9dfcc6e84a442c6fbdf600ab1df1e5030603.camel@j-davis.com>
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On Mon, 2024-12-09 at 15:27 -0700, David G. Johnston wrote:
> I have not done this.  This is already a large patch and this kind of
> example doesn't seem like our norm.  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.

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

> 
> When executing an aggregate or window function the state tracking
> component
>    (which may be initialized to a non-null value, e.g., 0 for the
> count function)
>    will remain unchanged even if the underlying processing
>    function returns a null value, whether from being defined strict
>    or 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






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