Date: Fri, 24 Mar 2006 09:47:20 -0400
From: Alvaro Herrera <alvherre@commandprompt.com>
To: "Jim C. Nasby" <jnasby@pervasive.com>
Cc: Qingqing Zhou <zhouqq@cs.toronto.edu>, pgsql-performance@postgresql.org
Subject: Re: WAL logging of SELECT ... INTO command
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	Qingqing Zhou <zhouqq@cs.toronto.edu>,
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Jim C. Nasby wrote:
> On Fri, Mar 24, 2006 at 08:39:02AM -0400, Alvaro Herrera wrote:
> > Jim C. Nasby wrote:
> > 
> > > Why would the content of the old_table be unreliable? If we've replayed
> > > logs up to the point of the CTAS then any data that would be visible to
> > > the CTAS should be fine, no?
> > > 
> > > Though, the way Tom put it in one of his replies it sounds like WAL
> > > doesn't do any kind of statement logging, only data logging. If that's
> > > the case I'm not sure that the CTAS would actually get replayed. But I
> > > suspect I'm just misunderstanding...
> > 
> > The CTAS doesn't get logged (nor replayed obviously).  What happens is
> > that the involved files are fsync'ed before transaction commit, AFAIR.
> 
> Ahh, yes, that sounds right. Might be a nice gain to be had if there was
> some way to log the statement, but I suspect getting WAL to support that
> would be extremely non-trivial.

None at all, at least in the current incarnation, I think, because said
query execution is dependent on the contents of the FSM, which is itself
dependent on the timing of VACUUM and other stuff.  Such an action,
running with a different FSM content, can very trivially cause data
corruption.

-- 
Alvaro Herrera                                http://www.CommandPrompt.com/
The PostgreSQL Company - Command Prompt, Inc.