Date: Thu, 10 Jun 2021 11:09:52 +0900
From: Michael Paquier <michael@paquier.xyz>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Alvaro Herrera <alvherre@alvh.no-ip.org>,
	Robert Haas <robertmhaas@gmail.com>,
	Dilip Kumar <dilipbalaut@gmail.com>,
	Andres Freund <andres@anarazel.de>,
	pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: Move pg_attribute.attcompression to earlier in struct for
 reduced size?
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On Tue, Jun 08, 2021 at 11:32:21PM -0400, Tom Lane wrote:
> I can imagine sometime in the future where we need to get rid of all
> instances of pglz so we can reassign that compression code to something
> else.  But would we insist on a mass VACUUM FULL to make that happen?
> Doubt it.  You'd want a tool that would make that happen over time,
> in the background; like the mechanisms that have been discussed for
> enabling checksums on-the-fly.

Well, I can imagine that some people could afford being more
aggressive here even if it implies some downtime and if they are not
willing to afford the deployment of a second instance for a
dump/restore or a logirep setup.

(The parallel with data checksums is partially true, as you can do a
switch of checksums with physical replication as the page's checksums
are only written when pushed out of shared buffers, not when they are
written into WAL.  This needs a second instance, of course.)

> In the meantime I'm +1 for dropping this logic from VACUUM FULL.
> I don't even want to spend enough more time on it to confirm the
> different overhead measurements that have been reported.

Agreed.  It looks like we are heading toward doing just that for this
release.
--
Michael

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