From: Chao Li <li.evan.chao@gmail.com>
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Subject: doc: Clarify ALTER CONSTRAINT enforceability wording
Message-Id: <711B1ED3-1781-4B6C-A573-B58AF20770E5@gmail.com>
Date: Mon, 25 May 2026 17:31:07 +0800
Cc: Andrew Dunstan <andrew@dunslane.net>,
 jian he <jian.universality@gmail.com>
To: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
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 <https://www.postgresql.org/message-id/711B1ED3-1781-4B6C-A573-B58AF20770E5%40gmail.com>
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Hi,

I just started to test =E2=80=9CAdd support for altering CHECK =
constraint enforceability=E2=80=9D. I read the related ALTER TABLE doc =
first, and found it a bit confusing:
```
ALTER CONSTRAINT=20
    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 =E2=80=9CCHECK" 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=3D"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 =E2=80=9Cbut see below=E2=80=9D is quite =
confusing. I read the doc several times up and down, and finally my best =
guess is that =E2=80=9Csee below=E2=80=9D refers to the immediately =
following paragraph=E2=80=99s sentence =E2=80=9COnly not-null =
constraints may be altered in this fashion at present=E2=80=9D, but the =
sentence does not say anything about enforceability.
```
ALTER CONSTRAINT ... INHERIT
ALTER CONSTRAINT ... NO INHERIT=20
      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/


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