Date: Thu, 3 Jun 2021 12:04:48 -0400
From: Alvaro Herrera <alvherre@alvh.no-ip.org>
To: Michael Paquier <michael@paquier.xyz>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, 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 2021-Jun-02, Michael Paquier wrote:

> After 12 runs of VACUUM FULL on my laptop, I have removed the two
> highest and the two lowest to remove some noise, and did an average of
> the rest:
> - HEAD, 100 text columns: 5720ms
> - REL_13_STABLE, 100 text columns: 4308ms
> - HEAD, 200 text columns: 10020ms
> - REL_13_STABLE, 200 text columns:  8319ms
> 
> So yes, that looks much visible to me, and an argument in favor of the
> removal of the forced recompression on HEAD when rewriting tuples.

Just to be clear -- that's the time to vacuum the table without changing
the compression algorithm, right?  So the overhead is just the check for
whether the recompression is needed, not the recompression itself?

If the check for recompression is this expensive, then yeah I agree that
we should take it out.  If recompression is actually occurring, then I
don't think this is a good test :-)

-- 
Álvaro Herrera       Valdivia, Chile