Re: WAL logging problem in 9.4.3?

Andres Freund <andres(at)anarazel(dot)de> writes:
> On 2015-07-03 19:26:05 +0200, Andres Freund wrote:
>> commit cab9a0656c36739f59277b34fea8ab9438395869
>> Author: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)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