Date: Thu, 29 Dec 2022 10:52:14 -0800
From: Nathan Bossart <nathandbossart@gmail.com>
To: Andres Freund <andres@anarazel.de>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Peter Geoghegan <pg@bowt.ie>,
	postgresql@taljaren.se, pgsql-hackers@lists.postgresql.org,
	Michael Paquier <michael@paquier.xyz>
Subject: Re: BUG #17717: Regression in vacuumdb (15 is slower than 10/11 and
 possible memory issue)
Message-ID: <20221229185214.GA212944@nathanxps13>
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On Wed, Dec 28, 2022 at 07:03:29PM -0800, Andres Freund wrote:
> Separately I wonder if it's worth micro-optimizing vac_update_datfrozenxid() a
> bit. I e.g. see a noticable speedup bypassing systable_getnext() and using
> heap_getnext().  It's really too bad that we want to check for "in the future"
> xids, otherwise we could use a ScanKey to filter at a lower level.

Another thing I'm exploring is looking up the datfrozenxid/datminmxid
before starting the pg_class scan so that the scan can be stopped early if
it sees that we cannot possibly advance the values.  The
overwrite-corrupt-values logic might make this a little more complicated,
but I think it'd be sufficient to force the pg_class scan to complete if we
ever see a value "in the future."  Overwriting the corrupt value might be
delayed, but it would eventually happen once the table ages advance.

-- 
Nathan Bossart
Amazon Web Services: https://aws.amazon.com