Feedback-ID: ia2694551:Fastmail
Date: Tue, 26 Mar 2024 16:25:42 +0100
From: Alvaro Herrera <alvherre@alvh.no-ip.org>
To: Tender Wang <tndrwang@gmail.com>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: Can't find not null constraint, but \d+ shows that
Message-ID: <202403261525.ciahntfgj25o@alvherre.pgsql>
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On 2024-Mar-26, Tender Wang wrote:

> postgres=# CREATE TABLE t1(c0 int, c1 int);
> postgres=# ALTER TABLE  t1 ADD CONSTRAINT Q PRIMARY KEY(c0, c1);
> postgres=# ALTER TABLE  t1 DROP c1;
> 
> postgres=# ALTER TABLE  t1  ALTER c0 DROP NOT NULL;
> ERROR:  could not find not-null constraint on column "c0", relation "t1"

Ooh, hah, what happens here is that we drop the PK constraint
indirectly, so we only go via doDeletion rather than the tablecmds.c
code, so we don't check the attnotnull flags that the PK was protecting.

> The attached patch is my workaround solution.  Look forward your apply.

Yeah, this is not a very good approach -- I think you're just guessing
that the column is marked NOT NULL because a PK was dropped in the
past -- but really what this catalog state is, is corrupted contents
because the PK drop was mishandled.  At least in theory there are other
ways to drop a constraint other than dropping one of its columns (for
example, maybe this could happen if you drop a collation that the PK
depends on).  The right approach is to ensure that the PK drop always
does the dance that ATExecDropConstraint does.  A good fix probably just
moves some code from dropconstraint_internal to RemoveConstraintById.

-- 
Álvaro Herrera         PostgreSQL Developer  —  https://www.EnterpriseDB.com/
<inflex> really, I see PHP as like a strange amalgamation of C, Perl, Shell
<crab> inflex: you know that "amalgam" means "mixture with mercury",
       more or less, right?
<crab> i.e., "deadly poison"