Re: WAL logging problem in 9.4.3?

On 2015-07-03 19:26:05 +0200, Andres Freund wrote:
> On 2015-07-03 19:02:29 +0200, Andres Freund wrote:
> > Maybe I'm just daft right now (35C outside, 32 inside, so ...), but I'm
> > right now missing how the whole "skip wal logging if relation has just
> > been truncated" optimization can ever actually be crashsafe unless we
> > use a new relfilenode (which we don't!).
> 
> We actually used to use a different relfilenode, but optimized that
> away: cab9a0656c36739f59277b34fea8ab9438395869
> 
> 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.