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

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