Re: GSoC proposal - "make an unlogged table logged"

On 2014-04-03 13:38:29 +0300, Heikki Linnakangas wrote:
> On 04/01/2014 08:58 PM, Andres Freund wrote:
> >On 2014-04-01 12:56:04 -0500, Jim Nasby wrote:
> >>On 3/4/14, 8:50 AM, Andres Freund wrote:
> >>>Can't that be solved by just creating the permanent relation in a new
> >>>relfilenode? That's equivalent to a rewrite, yes, but we need to do that
> >>>for anything but wal_level=minimal anyway.
> >>
> >>Maybe I'm missing something, but doesn't this actually involve writing the data twice? Once into WAL and again into the relation itself?
> >
> >Yes. But as I said, that's unavoidable for anything but
> >wal_level=minimal.
>
> Ideally, you would *only* write the data to WAL, when you do ALTER TABLE ...
> SET LOGGED. There's no fundamental reason you need to rewrite the
> heap, too.

As another point: What's the advantage of that? The amount of writes
will be the same, no? It doesn't seem to be all that interesting that
a second filenode exists temporarily?

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services