Date: Sat, 4 Jul 2015 01:25:23 +0200
From: Andres Freund <andres@anarazel.de>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Fujii Masao <masao.fujii@gmail.com>,
 Martijn van Oosterhout <kleptog@svana.org>,
 PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: WAL logging problem in 9.4.3?
Message-ID: <20150703232523.GP3291@awork2.anarazel.de>
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On 2015-07-03 18:38:37 -0400, Tom Lane wrote:
> > 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.

Consider:

BEGIN;
CREATE TABLE;
INSERT largeval;
TRUNCATE;
INSERT 1;
COPY;
INSERT 2;
COMMIT;

INSERT 1 is going to be WAL logged. For that to work correctly TRUNCATE
has to be WAL logged, as otherwise there'll be conflicting/overlapping
tuples on the target page.

But:

The truncation itself is not fully wal logged, neither is the COPY. Both
rely on heap_sync()/immedsync(). For that to be correct the current
relfilenode's truncation may *not* be wal-logged, because the contents
of the COPY or the truncation itself will only be on-disk, not in the
WAL.

Only being on-disk but not in the WAL is a problem if we crash and
replay the truncate record.

> 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.

Unless we're prepared to make everything that potentially WAL logs
something do the rel->rd_createSubid == mySubid && dance, I can't see
that working.


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