Date: Wed, 23 Sep 2020 12:23:21 -0300
From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Amit Langote <amitlangote09@gmail.com>
Cc: Pg Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: ALTER TABLE .. DETACH PARTITION CONCURRENTLY
Message-ID: <20200923152321.GA26342@alvherre.pgsql>
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On 2020-Sep-23, Amit Langote wrote:

Hi Amit, thanks for reviewing this patch!

> On Tue, Aug 4, 2020 at 8:49 AM Alvaro Herrera <alvherre@2ndquadrant.com> wrote:

> I suspect that we don't really need this defensive constraint.  I mean
> even after committing the 1st transaction, the partition being
> detached still has relispartition set to true and
> RelationGetPartitionQual() still returns the partition constraint, so
> it seems the constraint being added above is duplicative.

Ah, thanks for thinking through that.  I had vague thoughts about this
being unnecessary in the current mechanics, but hadn't fully
materialized the thought.  (The patch was completely different a few
unposted iterations ago).

> Moreover, the constraint is not removed as part of any cleaning up
> after the DETACH process, so repeated attach/detach of the same
> partition results in the constraints piling up:

Yeah, I knew about this issue (mentioned in my self-reply on Aug 4) and
didn't worry too much about it because I was thinking I'd rather get rid
of the constraint addition in the first place.

> Noticed a bug/typo in the patched RelationBuildPartitionDesc():
> 
> -   inhoids = find_inheritance_children(RelationGetRelid(rel), NoLock);
> +   inhoids = find_inheritance_children(RelationGetRelid(rel), NoLock,
> +                                       include_detached);
> 
> You're passing NoLock for include_detached which means you never
> actually end up including detached partitions from here.

I fixed this in the version I posted on Sept 10.  I think you were
reading the version posted at the start of this thread.

Thanks,

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services