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From: Michael Paquier <michael@paquier.xyz>
To: Andres Freund <andres@anarazel.de>
Cc: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Dilip Kumar <dilipbalaut@gmail.com>
Cc: pgsql-hackers <pgsql-hackers@postgresql.org>
Cc: Robert Haas <robertmhaas@gmail.com>
Subject: Re: Move pg_attribute.attcompression to earlier in struct for reduced size?
Date: Thu, 27 May 2021 11:07:53 +0900
Message-ID: <YK7++YoPYbsHOb+W@paquier.xyz> (raw)
In-Reply-To: <20210527015415.ctuj4yrwnjip5kve@alap3.anarazel.de>
References: <3962395.1621609263@sss.pgh.pa.us>
	<20210521211929.pcehg6f23icwstdb@alap3.anarazel.de>
	<YKsm7qkz+Cq3C3mr@paquier.xyz>
	<CAFiTN-uLQCRLatapZhEaNGH9njY=LndyXJux7H9YCfT9RcE7gA@mail.gmail.com>
	<YKtpZI4PBpVG3ryH@paquier.xyz>
	<CAFiTN-vpHrCzdeyKxpoPViyJCZZx-h8KghQJFyxZd7+Dys6Cow@mail.gmail.com>
	<YKyPM6UC+tBHEuxo@paquier.xyz>
	<20210527003144.xxqppojoiwurc2iz@alap3.anarazel.de>
	<1677691.1622075746@sss.pgh.pa.us>
	<20210527015415.ctuj4yrwnjip5kve@alap3.anarazel.de>

On Wed, May 26, 2021 at 06:54:15PM -0700, Andres Freund wrote:
> Oh, it'll definitely be more expensive in that case - but that seems
> fair game. What I was wondering about was whether VACUUM FULL would be
> measurably slower, because we'll now call toast_get_compression_id() on
> each varlena datum. It's pretty easy for VACUUM FULL to be CPU bound
> already, and presumably this'll add a bit.

This depends on the number of attributes, but I do see an extra 0.5%
__memmove_avx_unaligned_erms in reform_and_rewrite_tuple() for a
normal VACUUM FULL with a 1-int-column relation on a perf profile,
with rewrite_heap_tuple eating most of it as in the past, so that's
within the noise bandwidth if you measure the runtime.  What would be
the worst case here, a table with one text column made of non-NULL
still very short values?
--
Michael

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