Date: Sun, 21 Mar 2021 12:54:53 -0500
From: Justin Pryzby <pryzby@telsasoft.com>
To: Alvaro Herrera <alvherre@alvh.no-ip.org>
Cc: Amit Langote <amitlangote09@gmail.com>,
	pgsql-hackers@lists.postgresql.org
Subject: Re: ALTER TABLE .. DETACH PARTITION CONCURRENTLY
Message-ID: <20210321175453.GB11765@telsasoft.com>
References: <20210317184534.GB11765@telsasoft.com>
 <20210319135737.GA28771@alvherre.pgsql>
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On Fri, Mar 19, 2021 at 10:57:37AM -0300, Alvaro Herrera wrote:
> > Also, it "fails to avoid" adding duplicate constraints:
> > 
> > Check constraints:
> >     "c" CHECK (i IS NOT NULL AND i > 1 AND i < 2)
> >     "cc" CHECK (i IS NOT NULL AND i >= 1 AND i < 2)
> >     "p1_check" CHECK (true)
> >     "p1_i_check" CHECK (i IS NOT NULL AND i >= 1 AND i < 2)
> 
> Do you mean the "cc" and "p1_i_check" one?  I mean, if you already have

No, I started with c and cc, and it added the broken constraint p1_check (which
you say you've fixed) and the redundant constraint p1_i_check.  I guess that's
what you meant.

> a constraint in the partition that duplicates the partition constraint,
> then during attach we still create our new constraint?  I guess a
> solution to this would be to scan all constraints and see if any equals
> the expression that the new one would have.  Sounds easy enough now that
> write it out loud.

But it looks like DetachAddConstraintIfNeeded already intended to do that:

+            if (equal(constraintExpr, thisconstr))                                                                                                                                                                     
+                    return;                                                                                                                                                                                            

Actually, it appears your latest notpatch resolves both these issues.
But note that it doesn't check if an existing constraint "implies" the new
constraint - maybe it should.

-- 
Justin