Re: [HACKERS] WAL logging problem in 9.4.3?

From: Michael Paquier <michael(at)paquier(dot)xyz>
To: Heikki Linnakangas <hlinnaka(at)iki(dot)fi>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: [HACKERS] WAL logging problem in 9.4.3?
Date: 2018-07-11 03:32:41
Message-ID: 20180711033241.GQ1661@paquier.xyz
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On Tue, Jul 10, 2018 at 05:35:58PM +0300, Heikki Linnakangas wrote:
> 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.

Thanks for the pointer. My tap test has been covering two out of the
three scenarios you have in your script. I have been able to convert
the extra as the attached, and I have added as well an extra test with
TRUNCATE triggers. So it seems to me that we want to disable the
optimization if any type of trigger are defined on the relation copied
to as it could be possible that these triggers work on the blocks copied
as well, for any BEFORE/AFTER and STATEMENT/ROW triggers. What do you
think?

>> 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.

Yes, that sounds right. Fixed as well. I have additionally done more
work on the comments.

Thoughts?
--
Michael

Attachment Content-Type Size
wal-minimal-copy-truncate-v2.patch text/x-diff 10.2 KB

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