Date: Wed, 8 Jan 2025 18:09:47 -0500
From: Bruce Momjian <bruce@momjian.us>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: jbe-mlist@magnetkern.de,
	PostgreSQL-development <pgsql-hackers@lists.postgresql.org>
Subject: Re: Parameter NOT NULL to CREATE DOMAIN not the same as CHECK (VALUE
 IS NOT NULL)
Message-ID: <Z38FuyGMdLv0oLKI@momjian.us>
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On Wed, Jan  8, 2025 at 04:24:34PM -0500, Tom Lane wrote:
> Bruce Momjian <bruce@momjian.us> writes:
> > I think this needs some serious research.
> 
> We've discussed this topic before.  The spec's definition of IS [NOT]
> NULL for composite values is bizarre to say the least.  I think
> there's been an intentional choice to keep most NOT NULL checks
> "simple", that is we look at the overall value's isnull bit and
> don't probe any deeper than that.
> 
> If the optimizations added in v17 changed existing behavior,
> I agree that's bad.  We should probably fix it so that those
> are only applied when argisrow is false.

Okay, makes sense.  Do we have any sense of whether the docs can be
improved in this area, based on the original email report?  Here is a
proposed patch for that.

-- 
  Bruce Momjian  <bruce@momjian.us>        https://momjian.us
  EDB                                      https://enterprisedb.com

  Do not let urgent matters crowd out time for investment in the future.


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diff --git a/doc/src/sgml/ref/create_domain.sgml b/doc/src/sgml/ref/create_domain.sgml
index ce555203486..c111285a69c 100644
--- a/doc/src/sgml/ref/create_domain.sgml
+++ b/doc/src/sgml/ref/create_domain.sgml
@@ -283,7 +283,8 @@ CREATE TABLE us_snail_addy (
   <para>
    The syntax <literal>NOT NULL</literal> in this command is a
    <productname>PostgreSQL</productname> extension.  (A standard-conforming
-   way to write the same would be <literal>CHECK (VALUE IS NOT
+   way to write the same for non-composite data types would be
+   <literal>CHECK (VALUE IS NOT
    NULL)</literal>.  However, per <xref linkend="sql-createdomain-notes"/>,
    such constraints are best avoided in practice anyway.)  The
    <literal>NULL</literal> <quote>constraint</quote> is a

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