Re: doc: Clarify ALTER CONSTRAINT enforceability wording

> On May 25, 2026, at 17:31, Chao Li <li.evan.chao@gmail.com> wrote:
> 
> Hi,
> 
> I just started to test “Add support for altering CHECK constraint enforceability”. I read the related ALTER TABLE doc first, and found it a bit confusing:
> ```
> ALTER CONSTRAINT 
>    This form alters the attributes of a constraint that was previously created. Currently FOREIGN KEY and CHECK constraints may be altered in this fashion, but see below.
> ```
> 
> In the feature commit 342051d73, only “CHECK" was added:
> ```
> --- a/doc/src/sgml/ref/alter_table.sgml
> +++ b/doc/src/sgml/ref/alter_table.sgml
> @@ -578,8 +578,8 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
>     <listitem>
>      <para>
>       This form alters the attributes of a constraint that was previously
> -      created. Currently only foreign key constraints may be altered in
> -      this fashion, but see below.
> +      created. Currently <literal>FOREIGN KEY</literal> and <literal>CHECK</literal>
> +      constraints may be altered in this fashion, but see below.
>      </para>
> ```
> 
> However, the last phrase “but see below” is quite confusing. I read the doc several times up and down, and finally my best guess is that “see below” refers to the immediately following paragraph’s sentence “Only not-null constraints may be altered in this fashion at present”, but the sentence does not say anything about enforceability.
> ```
> ALTER CONSTRAINT ... INHERIT
> ALTER CONSTRAINT ... NO INHERIT 
>      These forms modify an inheritable constraint so that it becomes not inheritable, or vice-versa. **Only not-null constraints may be altered in this fashion at present.** In addition to changing the inheritability status of the constraint, in the case where a non-inheritable constraint is being marked inheritable, if the table has children, an equivalent constraint will be added to them. If marking an inheritable constraint as non-inheritable on a table with children, then the corresponding constraint on children will be marked as no longer inherited, but not removed.
> ```
> 
> As I understand it, the current behavior is:
> * Deferrability attributes can only be altered for FOREIGN KEY constraints
> * Enforceability attributes can be altered for both FOREIGN KEY and CHECK constraints, and CHECK support was newly added by 342051d73 for v19
> 
> To remove the confusion, I updated the documentation. See the attached patch for details.
> 
> Best regards,
> --
> Chao Li (Evan)
> HighGo Software Co., Ltd.
> https://www.highgo.com/
> 
> 
> 
> 
> <v1-0001-doc-Clarify-ALTER-CONSTRAINT-enforceability-behav.patch>

As I found a bug of the feature and proposed a fix in [1], let's merge this thread into that one, as they are for the same feature.

[1] https://postgr.es/m/E74C57FA-1DD0-4C8E-8FB1-538034752592@gmail.com

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/