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Re: WAL Log Size

From: Greg Smith <gsmith(at)gregsmith(dot)com>
To: Sam Mason <sam(at)samason(dot)me(dot)uk>
Cc: pgsql-general(at)postgresql(dot)org
Subject: Re: WAL Log Size
Date: 2008-02-29 07:08:13
Message-ID: Pine.GSO.4.64.0802290200230.2395@westnet.com (view raw or flat)
Thread:
Lists: pgsql-general
On Fri, 29 Feb 2008, Sam Mason wrote:

> Just out of interest, why doesn't it do the following?
>
>  BEGIN;
>  create table xlog_switch as
>    select '0123456789ABCDE' from generate_series(1,1000000);
>  ROLLBACK;

I'm not 100% sure here what happens when you do the above, and it depends 
on version, but there are cases where creating a new or empty table in a 
transaction is optimized to not create any WAL as a performance 
improvement.  This has become a common idiom for that reason:

BEGIN;
truncate table t;
copy t from '/fdsa/fds/afds.csv' with csv;
COMMIT;

To take advantage of this loading without WAL feature.

I wish I had more details here, never have found a more formal definition 
of how this works than suggestions on the list.

--
* Greg Smith gsmith(at)gregsmith(dot)com http://www.gregsmith.com Baltimore, MD

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