Date: Tue, 27 Apr 2021 11:47:33 -0400
From: Alvaro Herrera <alvherre@alvh.no-ip.org>
To: Amit Langote <amitlangote09@gmail.com>
Cc: Justin Pryzby <pryzby@telsasoft.com>, Tom Lane <tgl@sss.pgh.pa.us>,
	PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: ALTER TABLE .. DETACH PARTITION CONCURRENTLY
Message-ID: <20210427154733.GA28178@alvherre.pgsql>
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On 2021-Apr-27, Amit Langote wrote:

> On Tue, Apr 27, 2021 at 4:34 PM Amit Langote <amitlangote09@gmail.com> wrote:

> I think we may need a separate context for partdesc_nodetached, likely
> with the same kludges as rd_pdcxt.  Maybe the first problem will go
> away with that as well.

Ooh, seems I completely misunderstood what RelationClose was doing.  I
thought it was deleting the whole rd_pdcxt, *including* the "current"
partdesc.  But that's not at all what it does: it only deletes the
*children* memcontexts, so the partdesc that is currently valid remains
valid.  I agree that your proposed fix appears to be promising, in that
a separate "context tree" rd_pddcxt (?) can be used for this.  I'll try
it out now.

> Few other minor things I noticed:
> 
> +    * Store it into relcache.  For snapshots built excluding detached
> +    * partitions, which we save separately, we also record the
> +    * pg_inherits.xmin of the detached partition that was omitted; this
> +    * informs a future potential user of such a cached snapshot.
> 
> The "snapshot" in the 1st and the last sentence should be "partdesc"?

Doh, yeah.

> + * We keep two partdescs in relcache: rd_partdesc_nodetached excludes
> + * partitions marked concurrently being detached, while rd_partdesc includes
> + * them.
> 
> IMHO, describing rd_partdesc first in the sentence would be better.
> Like: rd_partdesc includes all partitions including any that are being
> concurrently detached, while rd_partdesc_nodetached excludes them.

Makes sense.

-- 
Álvaro Herrera       Valdivia, Chile