From: | Álvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org> |
---|---|
To: | Amit Langote <amitlangote09(at)gmail(dot)com> |
Cc: | Justin Pryzby <pryzby(at)telsasoft(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, "pgsql-hackers(at)lists(dot)postgresql(dot)org" <pgsql-hackers(at)lists(dot)postgresql(dot)org> |
Subject: | Re: ALTER TABLE .. DETACH PARTITION CONCURRENTLY |
Date: | 2021-05-06 17:13:47 |
Message-ID: | 20210506171347.GA27028@alvherre.pgsql |
Views: | Raw Message | Whole Thread | Download mbox | Resend email |
Thread: | |
Lists: | pgsql-hackers |
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)
From | Date | Subject | |
---|---|---|---|
Next Message | Tom Lane | 2021-05-06 17:22:27 | Re: Multiple hosts in connection string failed to failover in non-hot standby mode |
Previous Message | Stephen Frost | 2021-05-06 17:12:24 | Re: Asynchronous Append on postgres_fdw nodes. |