Re: [HACKERS] WAL logging problem in 9.4.3?

From: Heikki Linnakangas <hlinnaka(at)iki(dot)fi>
To: Michael Paquier <michael(at)paquier(dot)xyz>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: [HACKERS] WAL logging problem in 9.4.3?
Date: 2018-07-10 14:35:58
Message-ID: df32e286-ae2b-f45a-8f2e-4fa02684300b@iki.fi
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Thanks for picking this up!

(I hope this gets through the email filters this time, sending a shell
script seems to be difficult. I also trimmed the CC list, if that helps.)

On 04/07/18 07:59, Michael Paquier wrote:
> Hence I propose the patch attached which disables the TRUNCATE and COPY
> optimizations for two cases, which are the ones actually causing
> problems. One solution has been presented by Simon here for COPY, which
> is to disable the optimization when there are no blocks on a relation
> with wal_level = minimal:
> https://www.postgresql.org/message-id/CANP8+jKN4V4MJEzFN_iEtdZ+1oM=YETxvmuu1YK4UMXQY2gaGw@mail.gmail.com
> For back-patching, I find that really appealing.

This fails in the case that there are any WAL-logged changes to the
table while the COPY is running. That can happen at least if the table
has an INSERT trigger, that performs operations on the same table, and
the COPY fires the trigger. That scenario is covered by the little bash
script I posted earlier in this thread
(https://www.postgresql.org/message-id/55AFC302.1060805%40iki.fi).
Attached is a new version of that script, updated to make it work with v11.

> The second thing that the patch attached does is to tweak
> ExecuteTruncateGuts so as the TRUNCATE optimization never runs for
> wal_level = minimal.

If we go down that route, let's at least keep the TRUNCATE optimization
for temporary and unlogged tables.

- Heikki

Attachment Content-Type Size
test-wal-minimal-2-bash-script text/plain 2.6 KB

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