Date: Wed, 22 Mar 2006 06:47:32 -0600
From: "Jim C. Nasby" <jnasby@pervasive.com>
To: Simon Riggs <simon@2ndquadrant.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: WAL logging of SELECT ... INTO command
Message-ID: <20060322124732.GA15742@pervasive.com>
References: <20060321122211.GU15742@pervasive.com>
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On Tue, Mar 21, 2006 at 08:33:50PM +0000, Simon Riggs wrote:
> On Tue, 2006-03-21 at 06:22 -0600, Jim C. Nasby wrote:
> > Currently, it appears that SELECT * INTO new_table FROM old_table logs
> > each page as it's written to WAL. Is this actually needed? Couldn't the
> > database simply log that the SELECT ... INTO statement was executed
> > instead? Doing so would likely result in a large performance improvement
> > in most installs. Is there no provision for writing anything but data
> > page changes (or whole pages) to WAL?
> 
> AFAIK it takes the same code path as CREATE TABLE AS SELECT, which
> already does exactly what you suggest (except when using PITR).

Ok, I saw disk activity on the base directory and assumed it was pg_xlog
stuff. Turns out that both SELECT INTO and CREATE TABLE AS ignore
default_tablepsace and create the new tables in the base directory. I'm
guessing that's a bug... (this is on 8.1.2, btw).

Also, why do we log rows for CTAS/SELECT INTO when PITR is in use for
simple SELECTs (ones that don't call non-deterministic functions)? The
data should alread be available AFAICS...
-- 
Jim C. Nasby, Sr. Engineering Consultant      jnasby@pervasive.com
Pervasive Software      http://pervasive.com    work: 512-231-6117
vcard: http://jim.nasby.net/pervasive.vcf       cell: 512-569-9461