Re: [HACKERS] Re: [HACKERS] GSoC 2015 proposal: Improve the performance of “ALTER TABLE .. SET LOGGED / UNLOGGED” statement

On Wed, Jul 8, 2015 at 11:37 AM, Andres Freund <andres@anarazel.de> wrote:
>
> On 2015-07-08 10:58:51 -0300, Fabrízio de Royes Mello wrote:
> > Think in an ETL job that can be use an unlogged table to improve the load
> > performance, but this job create a "large table" and to guarantee the data
> > consistency you need to transform it into a regular table, and with the
> > current implementation rewrite the entire heap, toast and indexes.
>
> Don't buy that. The final target relation will usually already have
> content. Also everything but wal_level=minimal will force you to WAL log
> the contents anyway.

If the "wal_level=minimal" we don't need to force the wal log of the contents. If the "wal_level != minimal" we need just to xlog all the pages, but in both cases we don't need the extra job to create a new datafiles and copy the contents between them. So we'll improve performance, or am I missing something?

Regards,

--
Fabrízio de Royes Mello
Consultoria/Coaching PostgreSQL
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