RE: alter table xxx set unlogged take long time

Does "wal_level=minimal" help reducing wal emitting a lot for COPY and CREATE INDEX? We plan to remove "set unlogged/log" , instead , just set "wal_level=minimal" ,then COPY data in parallel, then create index.

Thanks,

James
-----Original Message-----
From: Joe Conway <mail(at)joeconway(dot)com>
Sent: Wednesday, July 27, 2022 11:02 PM
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: James Pang (chaolpan) <chaolpan(at)cisco(dot)com>; Jim Mlodgenski <jimmy76(at)gmail(dot)com>; pgsql-performance(at)lists(dot)postgresql(dot)org
Subject: Re: alter table xxx set unlogged take long time

On 7/27/22 10:46, Tom Lane wrote:
> Joe Conway <mail(at)joeconway(dot)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