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
repeatedlytruncated. Per a performance gripe a couple weeks ago from Todd Cook.
to me the reasoning here looks flawed.