Date: Thu, 6 May 2021 13:13:47 -0400
From: =?iso-8859-1?Q?=C1lvaro?= 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>,
	"pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>
Subject: Re: ALTER TABLE .. DETACH PARTITION CONCURRENTLY
Message-ID: <20210506171347.GA27028@alvherre.pgsql>
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On 2021-Apr-30, Amit Langote wrote:

> The case I was looking at is when a partition detach appears as
> in-progress to a serializable transaction.

Yeah, I was exceedingly sloppy on my reasoning about this, and you're
right that that's what actually happens rather than what I said.

> If the caller wants to omit detached partitions, such a partition ends
> up in rd_partdesc_nodetached, with the corresponding xmin being set to
> 0 due to the way find_inheritance_children_extended() sets
> *detached_xmin.  The next query in the transaction that wants to omit
> detached partitions, seeing rd_partdesc_nodetached_xmin being invalid,
> rebuilds the partdesc, again including that partition because the
> snapshot wouldn't have changed, and so on until the transaction ends.
> Now, this can perhaps be "fixed" by making
> find_inheritance_children_extended() set the xmin outside the
> snapshot-checking block, but maybe there's no need to address this on
> priority.

Hmm.  See below.

> Rather, a point that bothers me a bit is that we're including a
> detached partition in the partdesc labeled "nodetached" in this
> particular case.  Maybe we should avoid that by considering in this
> scenario that no detached partitions exist for this transactions and
> so initialize rd_partdesc, instead of rd_partdesc_nodetached.  That
> will let us avoid the situations where the xmin is left in invalid
> state.  Maybe like the attached (it also fixes a couple of
> typos/thinkos in the previous commit).

Makes sense -- applied, thanks.

> Note that we still end up in the same situation as before where each
> query in the serializable transaction that sees the detach as
> in-progress to have to rebuild the partition descriptor omitting the
> detached partitions, even when it's clear that the detach-in-progress
> partition will be included every time.

Yeah, you're right that there is a performance hole in the case where a
partition pending detach exists and you're using repeatable read
transactions.  I didn't see it as terribly critical since it's supposed
to be very transient, but I may be wrong.

-- 
Álvaro Herrera       Valdivia, Chile
"Hay quien adquiere la mala costumbre de ser infeliz" (M. A. Evans)