Re: Move pg_attribute.attcompression to earlier in struct for reduced size?

On Thu, May 27, 2021 at 1:29 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Yeah.  My belief here is that users might bother to change
> default_toast_compression, or that we might do it for them in a few
> years, but the gains from doing so are going to be only incremental.
> That being the case, most DBAs will be content to allow the older
> compression method to age out of their databases through routine row
> updates.  The idea that somebody is going to be excited enough about
> this to run a downtime-inducing VACUUM FULL doesn't really pass the
> smell test.

That was my original understanding of your position, FWIW. I agree
with all of this.

> That doesn't make LZ4 compression useless, by any means, but it does
> suggest that we shouldn't be adding overhead to VACUUM FULL for the
> purpose of easing instantaneous switchovers.

Right. More generally, there often seems to be value in
under-specifying what a compression option does. Or in treating them
as advisory.

You mentioned the history of SET STORAGE, which seems very relevant. I
am reminded of the example of B-Tree deduplication with unique
indexes, where we selectively apply the optimization based on
page-level details. Deduplication isn't usually useful in unique
indexes (for the obvious reason), though occasionally it is extremely
useful. I think that there might be a variety of things that work a
little like that. It can help with avoiding unnecessary dump and
reload hazards, too.

I am interested in hearing the *principle* behind Robert's position.
This whole area seems like something that might have at least a couple
of different schools of thought. If it is then I'd sincerely like to
hear the other side of the argument.

-- 
Peter Geoghegan