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From: Joe Conway <mail@joeconway.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: James Pang (chaolpan) <chaolpan@cisco.com>
Cc: Jim Mlodgenski <jimmy76@gmail.com>
Cc: pgsql-performance@lists.postgresql.org <pgsql-performance@lists.postgresql.org>
Subject: Re: alter table xxx set unlogged take long time
Date: Wed, 27 Jul 2022 11:01:33 -0400
Message-ID: <ffb85e8c-cf2d-0f5a-c805-ad66a5a23601@joeconway.com> (raw)
In-Reply-To: <2883168.1658933183@sss.pgh.pa.us>
References: <PH0PR11MB5191F1E9F55101897E453814D6949@PH0PR11MB5191.namprd11.prod.outlook.com>
	<CAB_5SRdsCDx8Mq974S=WxWBG+vSD7axZVtAoTvMvzQ+6ghffuA@mail.gmail.com>
	<PH0PR11MB51919A15ED33C356120145F7D6949@PH0PR11MB5191.namprd11.prod.outlook.com>
	<2240967.1658839372@sss.pgh.pa.us>
	<PH0PR11MB51917F811DDACDBC4B0D9AEAD6949@PH0PR11MB5191.namprd11.prod.outlook.com>
	<CAB_5SRf6JmebYvcVXmzRtpDB1e9+KsGHLJGznjmWzpg2LxcPAQ@mail.gmail.com>
	<PH0PR11MB51918C3648CF7FECAC90CABED6949@PH0PR11MB5191.namprd11.prod.outlook.com>
	<e1920272-57a6-7c5a-924d-be26065ad9ad@joeconway.com>
	<2883168.1658933183@sss.pgh.pa.us>

On 7/27/22 10:46, Tom Lane wrote:
> Joe Conway <mail@joeconway.com> writes:
>> Then (completely untested) I *think* you could create the "partition" 
>> initially as a free standing unlogged table, load it, index it, switch 
>> to logged, and then attach it to the partitioned table.
> 
> I'm still of the opinion that this plan to load the data unlogged
> and switch to logged later is a loser.  Sooner or later you have
> got to write the data to WAL, and this approach doesn't eliminate
> that cost.  What it does do is create one whole extra cycle of
> writing the data to disk and reading it back.  I don't think
> it's an oversight that no such thing is suggested in our standard
> tips for bulk-loading data:

Yeah, agreed. I was mostly responding to the OP desire to use unlogged 
and not taking a stance on that.

> https://www.postgresql.org/docs/current/populate.html
> 
> What perhaps *is* an oversight is that we don't suggest
> use of COPY FREEZE there.  AFAIK that doesn't reduce the initial
> data loading cost directly, but it would save overhead later.

Oh, yes, very good point.


-- 
Joe Conway
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com





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