From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andres Freund <andres@anarazel.de>
Cc: Fujii Masao <masao.fujii@gmail.com>
Cc: Martijn van Oosterhout <kleptog@svana.org>
Cc: PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: WAL logging problem in 9.4.3?
Date: Fri, 03 Jul 2015 18:38:37 -0400
Message-ID: <21533.1435963117@sss.pgh.pa.us> (raw)
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Andres Freund <andres@anarazel.de> writes:
> On 2015-07-03 19:26:05 +0200, Andres Freund wrote:
>> commit cab9a0656c36739f59277b34fea8ab9438395869
>> Author: Tom Lane <tgl@sss.pgh.pa.us>
>> Date: Sun Aug 23 19:23:41 2009 +0000
>>
>> Make TRUNCATE do truncate-in-place when processing a relation that was created
>> or previously truncated in the current (sub)transaction. This is safe since
>> if the (sub)transaction later rolls back, we'd just discard the rel's current
>> physical file anyway. This avoids unreasonable growth in the number of
>> transient files when a relation is repeatedly truncated. Per a performance
>> gripe a couple weeks ago from Todd Cook.
>>
>> to me the reasoning here looks flawed.
> It looks to me we need to re-neg on this a bit. I think we can still be
> more efficient than the general codepath: We can drop the old
> relfilenode immediately. But pg_class.relfilenode has to differ from the
> old after the truncation.
Why exactly? The first truncation in the (sub)xact would have assigned a
new relfilenode, why do we need another one? The file in question will
go away on crash/rollback in any case, and no other transaction can see
it yet.
I'm prepared to believe that some bit of logic is doing the wrong thing in
this state, but I do not agree that truncate-in-place is unworkable.
regards, tom lane
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To: pgsql-hackers@postgresql.org
Cc: tgl@sss.pgh.pa.us, andres@anarazel.de, masao.fujii@gmail.com, kleptog@svana.org
Subject: Re: WAL logging problem in 9.4.3?
In-Reply-To: <21533.1435963117@sss.pgh.pa.us>
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