Date: Wed, 28 Dec 2022 13:23:21 -0800
From: Nathan Bossart <nathandbossart@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: 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: <20221228212321.GB4009813@nathanxps13>
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On Wed, Dec 28, 2022 at 04:20:19PM -0500, Tom Lane wrote:
> Nathan Bossart <nathandbossart@gmail.com> writes:
>> I think the main difference between your patch and mine is that I've
>> exposed vac_update_datfrozenxid() via a function instead of a VACUUM
>> option.  IMHO that feels a little more natural, but I can't say I feel too
>> strongly about it.
> 
> I thought about that but it seems fairly unsafe, because that means
> that vac_update_datfrozenxid is executing inside a user-controlled
> transaction.  I don't think it will hurt us if the user does a
> ROLLBACK afterward --- but if he sits on the open transaction,
> that would be bad, if only because we're still holding the
> LockDatabaseFrozenIds lock which will block other VACUUMs.
> There might be more hazards besides that; certainly no one has ever
> tried to run vac_update_datfrozenxid that way before.

That's a good point.

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